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THE SYNTHESIS AND FREE-RADICAL REACTIONS OF 2-CHLOROETHYL SILYL ENGL ETHERS;<br />
A SYNTHESIS OF THE CARBON-9 TO CARBON-21 SUBUNIT OF THE<br />
APLYSIATOXINS AND OSCILLATOXINS<br />
ROBERT RONALD KANE, B.S<br />
A DISSERTATION<br />
CHEMISTRY<br />
Submitted to the Graduate Faculty<br />
of Texas Tech University in<br />
Partial Fulfillment of<br />
the Requirements for<br />
the Degree of<br />
DOCTOR OF PHILOSOPHY<br />
Approved<br />
Accepted<br />
Dean/of the Graduate School<br />
December, 1990
•p'.c<br />
•6ol<br />
• ' 1<br />
i —•'<br />
9^6><br />
© ROBERT RONALD KANE, 1990
ACKNOWLEDGEMENTS<br />
This dissertation, the culmination of four years of research in the laboratory as well<br />
as numerous years of education, would not have been possible without the encouragement<br />
and assistance of numerous individuals.<br />
Heartfelt appreciation goes out to Professors Harold Bier, Preston Reeves, and<br />
David Wasmund of Texas Lutheran CoUege, who patiently provided a first rate<br />
undergraduate education while nurturing the interest in chemistry of one of their least<br />
ccx)perative students. The attainment of the undergraduate degree would not have been<br />
possible without their assistance, as well as that of Dr. Charles Oestreich (president of<br />
Texas Lutheran College).<br />
The Graduate School of Texas Tech, as well as the Department of <strong>Chem</strong>istry, are<br />
appreciated for the providing an opportunity for a postgraduate education to a questionably<br />
motivated candidate. The Department of <strong>Chem</strong>istry has provided a faculty who have<br />
proved to be excellent instructors, both inside and outside of the classroom. Especially<br />
helpful have been the members of the advisory committee, Professors Henry Shine, Bruce<br />
Whittlesey, John Marx, David Bimey, and Richard Bartsch, who have provided advice and<br />
aid, scholarly and personal. Special thanks go to Professor David Bimey, who agreed to<br />
serve as a member of the committee for the dissertation defense upon short notice.<br />
This time at Texas Tech University has been made especially enjoyable by the<br />
friendship of various coUeaeues in the department. Especially notable are the past and<br />
present members of Professor Robert Walkup's research group, who have provided a<br />
professional yet personable environment in which to work.<br />
Professor Robert Walkup has been a patient and insightful research advisor as well<br />
as a personal and professional inspiration. He consistently demonstrates that it is possible<br />
to balance a great dedication to ones work with devotion to ones family.<br />
None of this would have been possible without the patient support of a wonderful<br />
family. The encouragement provided by my parents and sisters has been a motivation<br />
when things seemed the most difficult. My wife Dawn and son Joey have filled the past<br />
four years with love and joy, and to them I dedicate this dissertation.<br />
Financial support, without which this work would not have been possible, was<br />
provided by the Robert A. Welch Foundation, the Petroleum Research Fund, and the<br />
Graduate School of Texas Tech University, as well as by David Close, M.D., Benny<br />
Philips, M.D., and Ali A. ElDomeiri, M.D., and is gratefully adknowledged.<br />
iu
ACKNOWLEDGEMENTS iu<br />
FIGURES X<br />
ABBREVL\TIONS xi<br />
CHAPTER 1 INTRODUCTION 1<br />
PARTI<br />
THE SYNTHESIS AND FREE-RADICAL REACTIONS<br />
OF 2-CHIJOROETHYL SILYL ENOL ETHERS<br />
CHAPTER 2 BACKGROUND 3<br />
OrganosiUcon <strong>Chem</strong>istry 3<br />
Historical Background 3<br />
Properties of OrganosiUcon Compounds 4<br />
The 'P-Effect' in OrganosiUcon Compounds 6<br />
Uses of OrganosUicon Compounds 7<br />
OrganosiUcon Compounds as 'Ferrymen' 8<br />
The Synthesis of Silyl Enol Ethers 9<br />
Reactions of SUyl Enol Ethers 11<br />
'Silicon Functionalized' Silyl Enol Ethers 13<br />
Conclusions 14<br />
Radical Transformations in Synthetic Organic <strong>Chem</strong>istry 14<br />
General 14<br />
Radical Cyclizations 16<br />
The Effect of Silicon Substitution 17<br />
Cyclization of p-Silyl Radicals 21<br />
Conclusions 21<br />
Rationale for this Research 21<br />
CHAPTERS RESULTS AND DISCUSSION 23<br />
General Considerations 23<br />
iv
Initial Studies 25<br />
The Synthesis and Attempted Radical Cyclization<br />
of a Chloromethyl SUyl Enol Ether 25<br />
The Synthesis and Radical Cyclization of a<br />
2-Chloroethyl Silyl Enol Ether 27<br />
Optimizing the Recipe for the Radical Cyclization 31<br />
Oxidative Removal of the Silicon Atom 33<br />
The Synthesis and Free-Radical CycUzations of<br />
2-ChloroethyldimethylsUyl Enol Ethers 34<br />
A Stereoselective Tandem CycUzation 39<br />
Unsymmetrical Dialkyl Silyl Enol Ethers 40<br />
Conclusions 41<br />
CHAPTER 4 EXPERIMENTAL DETAILS 42<br />
General Methods 42<br />
(3,3-Dimethylbutenyl-2-oxy)ethoxymethyl(chloromethyl)silane (80) 43<br />
(3,3-Dimethylbutene-2-oxy)ethoxy-2-chloroethyl-methylsUane (86) 44<br />
3-(tert-Butyl)-l-ethoxy-1-methyl-l-sila-2-oxacyclohexane (90) 45<br />
5,5-Dimethyl-l,4-hexanediol(92) 46<br />
Chloro(2-chloroethyl)dimethylsilane (93) 47<br />
Diethoxydimethylsilane (95) 48<br />
(3,3-Dimethylbutene-2-oxy)(2-chloroethyl)dimethylsilane (99) 48<br />
(1 -Cyclohexylethenyloxy)(2-chloroethyl)dimethy 1 silane (100) 49<br />
(2-Chloroethyl)(l-cyclohexenyloxy)dimethylsUane (101) 50<br />
(l-Phenylethenyloxy)(2-chloroethyl)dimethylsilane (102) 51<br />
(2-Chloroethyl)(l-heptenyl-2-oxy)dimethylsilane (103) 52<br />
(3'S,6'R)-(2-Chloroethyl)(3'-(l"-methylethyl)-5'-methyl-l'cyclohexenyl-2'-oxy)dimetiiylsilane<br />
(104) : 53<br />
l-Cyclopropylethenyloxy(2-chloroethyl)dimethylsilane (105) 54<br />
V
2,2-Dimetiiyl-6-trimethylsilyl-3-hexanol(107) 54<br />
l-Cyclohexyl-4-trimethylsilyl- 1-butanol (108) 55<br />
l-TrimethylsUyl-4-nonanol (109) 56<br />
2-(2'-Trimethylsnylethyl)cyclohexanol (110) 57<br />
(2-Chloroethyl)(l,6-heptadien-2-yloxy)dimethylsilane (115) 58<br />
2-Methyl-l-(3-trimethylsilylpropyl)cyclopentanol (118) 59<br />
Bis-(3,3-dimethylbutene-2-oxy)(2-chloroethyl)methylsUane (119) 60<br />
Bis-(cyclohexen-l-yloxy)(2-chloroethyl)methylsUane (120) 61<br />
Chloro(2-chloroethyl)ethyhnethylsilane(121) 62<br />
Chloro(2-chloroethyl)methyl(l-methylpropyl)silane (122) 63<br />
Ethyl(2-chloroethyl)(l-cyclohexylethenyloxy)methylsUane (123) 63<br />
(3,3-Dimethylbutenyl-2-oxy)ethyl(2-chloroethyl)-methylsUane (124) 64<br />
t-Butyl(2-chloroethyl)methyl(l-phenylethenyloxy)silane (125) 65<br />
t-Butyl(2-chloroethyl)(l-heptenyl-2-oxy)methylsUane (126) 66<br />
REFERENCES 67<br />
PARTE<br />
A SYNTHESIS OF THE CARBON-9 TO CARBON-21<br />
SUBUNIT OF THE APLYSLVTOXESfS AND<br />
OSCILLATOXINS<br />
CHAPTERS BACKGROUND 72<br />
Discovery of the Aplysiatoxins and Oscillatoxins 72<br />
Structure Determination 73<br />
Biological Activity of the Aplysiatoxins and Oscillatoxins 76<br />
Progress Toward the Total Synthesis of Oscillatoxin D 76<br />
Other Synthetic Studies on the Aplysiatoxins 79<br />
vi
Conclusions 81<br />
CHAPTER 6 RESULTS AND DISCUSSION 82<br />
Retrosynthetic Analysis of Target Aldehyde (36) 82<br />
Studies on the Stereoselective Synthesis of a C14-C15 Epoxide 83<br />
Model Studies on the AUcylation of a C14 Anion 86<br />
Stereoselective Aldol Route to the C9-C13 Segment of the<br />
OscUlatoxins and Aplysiatoxins 88<br />
Elaboration of tiie Aldol Product (88) to a C9-C13 Iodide (90) 92<br />
Coupling of C9-C13 Iodide (90) and C14-C21 Imine (69) 97<br />
Asymmetric Reduction of C15 Ketone (101) 98<br />
Elaboration of Alcohol (110) to Aldehyde (112) 102<br />
Recent Progress - Synthesis of a C3-C21 Subunit<br />
of the Aplysiatoxins and Oscillatoxins 103<br />
Conclusions 105<br />
CHAPTER7 EXPERIMENTAL DETAILS 106<br />
General Methods 106<br />
(R)-2'-Hydroxy-2'-(3"-benzyloxy)phenylethyl<br />
4-(methyl)benzenesulfonate (61) 107<br />
(R)-1 -(3'-[Benzyloxy]phenyl)etiiane-1,2-epoxide (62) 107<br />
(R)-l"-(2'"-Napthol)-2"-napthyl 2-(3'-methoxy)-phenylethanoate (63) 108<br />
3-(2'-Trimethylsilylethoxy)methoxyacetophenone (65) 109<br />
(S)-l-(t-Butyldimethyl)silyloxy-2-methylpropyl methanesulfonate (67) 110<br />
(S)-l-Iodo-2-methyl-3-t-butyldimethylsUyloxypentane (68) 111<br />
N-l-(3'-[(2"-Trimethylsilylethoxy)methoxy]phenyl)<br />
ethylidinecyclohexylamine (69) 112<br />
(R)-1 -(3'-[(2"-Trimethy lsilylethoxy)methoxy]pheny l)-<br />
4-metiiyl-5-(t-butyldimethyl)silyloxy- 1-pentanone (71) 113<br />
(S)-3-(4'-Methoxyphenyl)methoxy-2-methylpropanal (82) 114<br />
vu
(S)-Methyl 3-(4'-methoxyphenyl)methoxy-2-methylpropionate (84) 115<br />
(2'R,3'S,4'S,4R,5S)-3-(3'-Hydroxy-5'-[4"-methoxy-phenyl]methoxy-<br />
2',4'-dimethylpentanoyl)-5-phenyl-4-methyl-2-oxazoUdinone (88) 116<br />
(l"S,2'R,3"S,4'R,5S,6"S)-3-(2'-[6"-Methyl-2",4"-dioxa-3"-<br />
(4"methoxyphenyl)cyclohex-1 "-yl]-propanoyl)-5-phenyl-<br />
4-methyl-2-oxazolidinone (89) 117<br />
(2S,3S,4S)-l-Iodo-5-(4'-methoxyphenyl)methoxy-2,4dimetiiyl-3-t-butyldimethylsUoxypentane<br />
(90) 118<br />
(2'R,3'S,4'S,4R,5S)-3-(5'-[4"-Methoxy-phenyl]-methoxy-2',4'dimethyl-3'-t-butyldimethyl-sUyloxy-pentanoyl)-5-phenyl-4methyl-2-oxazolidinone<br />
(91) 119<br />
(2S,3R,4S)-3-t-ButyldimethylsUyloxy-5-(4'-methoxyphenyl)methoxy-2,4-dimethyl-l-pentanol(92)<br />
120<br />
(lR,2R,2'S,3'S,4'S)-N-(l'-Hydroxy-l'-phenylpropan-2'-yl)-5-(4'methoxyphenyl)methoxy-3-t-butyldimethyl-silyloxy-2,4-dimethy<br />
1-1 -<br />
pentylamine (93) 121<br />
(2S,3R,4S)-5-(4'-Methoxyphenyl)methoxy-2,4dimethyl-l,3-pentanediol(94)<br />
122<br />
(2R,3S,4R)-Methyl3-hydroxy-5-(4'-methoxyphenyl)methoxy-2,4-dimethylpentanoate<br />
(95) 123<br />
(2S,3R,4S)-Methyl3-(t-butyldimethyl)sUyloxy-5-(4'methoxyphenyl)methoxy-2,4-dimethylpentanoate<br />
(96) 124<br />
(2S,3R,4S)-3-(t-Butyldimethyl)silyloxy-5-(4'-methoxyphenyl)methoxy-2,4-dimethylpent-1<br />
-yl methanesulfonate (97) 125<br />
(3S,5S)-3,5-Dimethyl-4-t-butyldimethylsUyloxytetrahydropyran (98) 126<br />
(2S,3R,4S)-3-Hydroxy-5-(4'-methoxyphenyl)methoxy-2,4dimethylpent-1-yl<br />
methanesulfonate (99) 127<br />
(2S,3R,4S)-l-Iodo-5-(4'-methoxyphenyl)methoxy-2,4dimethyl-3-pentanol<br />
(100) 128<br />
(4S,5R,6S)-7-(4'-Methoxyphenylmethoxy)-4,6-dimethyl-5-<br />
(t-butyldimethyl)silyloxy-l-(3"-(2'"-trimethylsilylethoxy)methoxy)phenyl-l-heptanone<br />
(101) 129<br />
(lS,4S,5R,6R)-7-(4'-Methoxyphenyl)methoxy-4,6-dimethyll-(3"-[2"'-trimethylsilylethoxy]methoxy)phenyl-5r-butyldimethylsiloxy-1-heptanol<br />
(109) 130<br />
viu
(lS,4S,5R,6S)-7-(4'-Methoxyphenyl)methoxy-l-methoxy-4,6-dimethyl-l-<br />
(3"-(trimethylethoxymethoxy)phenyl)-5-(t-butyldimethyl)silyloxyheptane<br />
(102) 132<br />
(2S,3R,4S,7S)-7-Methoxy-2,4-dimethyl-7-(3'-trimethylsUylethoxymethoxy)phenyl-3-(t-butyldimethyl)sUyloxyl-heptanol(lll)<br />
133<br />
(2R,3R,4S)-7-Methoxy-2,4-dimethyl-7-(3'trimethylsUylethoxymethoxy)phenyl-2-<br />
(t-butyldimethyl)sUyloxyheptanal (112) 134<br />
(2S,8S,9R,10R)-7-Hydroxy-13-methoxy-l-(4'-methoxyphenyl)methoxy-2,4,4,8,10-pentamethyl-13-(3"-(trimethylsilylethoxymethoxy)phenyl-9-(t-butyldimethyl)silyloxy-5-tridecanone<br />
(113) 135<br />
(2S,8S,9R,10R)-13-Methoxy-l-(4'-methoxyphenyl)-methoxy-<br />
2,4,4,8,10-pentamethyl-l 3-(3"-(trimethylsilylethoxymethoxy)phenyl-9-(r-butyldimethyl)-silyloxy-5,7-tridecanedione<br />
(115) 136<br />
REFERENCES 138<br />
IX
FIGURES<br />
Figure 2.1 Hyperconjugative StabUization of a-SUyl Anions 5<br />
Figure 2.2 Orbital Interraction Responsible for the 'p-Effect' 6<br />
Fi gure 2.3 Trajectory for Radical Addition 17<br />
Figure 2.4 Polar Contributions in 5-exo CycUzation of a-SUyl Radical 18<br />
Figure 2.5 Polar Contributions in Reduction of a-Silyl Radical 19
AEBN 2,2'-Azobisisobutyronitrile<br />
BOM Benzyloxymethyl<br />
CD Circular Dichroism<br />
ABBREVL^TIONS<br />
CI-MS <strong>Chem</strong>ical lonization-Mass Spectrometry<br />
d. e. Diastereomeric excess<br />
DDQ 2,3-Dichloro-5,6-dicyanobenzoquinone<br />
DMSO Dimetiiylsulfoxide<br />
e. e. Enantiomeric excess<br />
ether Diethyl ether<br />
GC Gas Chromatography<br />
HMPA Hexamethylphosphorictriamide<br />
HRMS High resolution Mass Spec<br />
Hz Hertz<br />
Ipc2BCl Diisocampheylchloroborane<br />
IR Infra red<br />
LAH Lithium aluminum hydride<br />
IDA Lithium dusopropylamide<br />
MCPBA mera-Chloroperbenzoic acid<br />
MPM Methoxyphenylmethyl<br />
NBS N-Bromosuccinnimide<br />
NMR Nuclear magnetic resonance<br />
NOE Nuclear Overhauser Effect<br />
ppm Parts per mUUon<br />
SEM TrimethylsUylethoxymethyl<br />
TBDMS rerr-ButyldimethylsUyl<br />
TBTH Tri-rt-butyltin hydride<br />
THF Tetrahydrofuran<br />
TIPS TriisopropylsUyl<br />
TLC Thin layer chromatography<br />
TMS Trimethylsilyl<br />
TMSE Trimetiiylsilylethyl<br />
UV Ultra violet<br />
XI
CHAPTER 1<br />
INTRODUCTION<br />
An enormous number of transformations of organic compounds are known. Still,<br />
much of the research in organic chemistry is focused upon tiie study and/or development of<br />
new transformations. This research is often based upon a search for practical synthetic<br />
transformations, or new 'synthetic methodology.' The quest for higher yields, greater<br />
chemoselectivity or stereoselectivity, simpler reaction conditions, more practical reagents,<br />
or truly novel transformations are a few of the motivations for the development of new<br />
'synthetic methodologies.'<br />
Part I of this dissertation describes an attempt to develop a novel synthetic method<br />
for the formation of carbon-carbon bonds based on the free radical cyclization of silyl enol<br />
ethers. A new transformation of this type would be attractive for a number of reasons.<br />
Simple silyl enol ethers are well known, and are often easily derived from ketone enolates.<br />
Thus, a new reaction of silyl enol etiiers could potentially apply to an enormous number of<br />
ketone containing substrates. Also, free radical reactions often occur under mild conditions<br />
withstocxi by many sensitive functional groups. FinaUy, free radical cyclizations are often<br />
efficient and selective, a result of the intramolecular nature of this type reaction.<br />
Synthetic methodologies, tiien, are 'tools' which are utUized by organic chemists in<br />
the construction of complex organic compounds. One class of complex organic molecules<br />
that consistently attracts a large amount of attention is 'natural products,' organic<br />
compounds that are isolated from biological sources. These compounds often have<br />
interesting biological activities, and as such are of great interest. Often, the only feasible<br />
method of obtaining the quantities of these compounds required for thorough study of their<br />
biological behavior is through total synthesis. The study of synthetic analogues of natural<br />
products is also extremely useful in determining and/or modifying thebiological activities of<br />
these compounds.<br />
Part II of this dissertation describes work toward the total synthesis of oscillatoxin<br />
D. This compound, which has been isolated from several bluegreen algae, has been shown<br />
to have interesting biological activity. However, this compound has not been isolated in<br />
sufficient quantities to allow thorough biological testing. Thus die total syntiiesis of this<br />
structurally complex compound is a worthwhile goal.<br />
This dissertation, then, reports the results of two extremely different projects,<br />
unified in that each is concerned with an important facet of synthetic organic chemistry.
PARTI<br />
THE SYNTHESIS AND FREE-RADICAL REACTIONS<br />
OF 2-CHIJOROETHYL SILYL ENOL ETHERS
Historical Background<br />
CHAPTER 2<br />
BACKGROUND<br />
OrganosiUcon <strong>Chem</strong>istry<br />
Silicon, the second most abundant element in the Earth's crust, exists in nature in<br />
combination with oxygen as silica or the metal silicates. No naturally occurring<br />
compounds which possess carbon-sUicon bonds, organosilanes, are known. The study of<br />
OrganosiUcon chemistry began in 1863 with Friedel and Crafts, who prepared the first<br />
organosilane, tetraethylsUane 1 (organosUicon compounds are named by Usting the<br />
Ugands, excluding hydrogen, on sUicon, followed by the word 'silane'). This synthesis<br />
CH3CH2^^ ^CH2CH3<br />
CH3CH2 CH2CH3<br />
was accomplished by reacting diethylzinc with tetrachlorosilane, which was<br />
first prepared by BerzeUus in 1823. However, further progress in the field was slow, and<br />
by 1937 F. S. Kipping, early-on a major investigator of the chemistry of organosUicon<br />
compounds, had concluded that there was Uttle hope that the chemistry of organosUicon<br />
compounds would become useful.^ Modem chemists have come to the glad realization that<br />
Kipping was mistaken, and over the last twenty five years the study and utiUzation of<br />
OrganosiUcon compounds has enjoyed explosive growth.<br />
This boom actually began with the development in 1945 of the 'direct synthesis' of<br />
chloromethylsilanes^ and was given an additional boost in 1957 with the discovery of the<br />
transition-metal catalyzed 'hydrosilylation' of alkenes.^ These two techniques made<br />
available thousands of new organosUanes, and the birth and phenomenal growth of the<br />
silicone industry led to renewed interest in the chemistry of silicon-containing compounds<br />
and insured the availability of a variety of low cost silicon monomers. Armed with these<br />
more accessible starting materials, chemists painstakingly developed organosUicon<br />
chemistry, once a very esoteric field, into a very useful and weU understood one. This has<br />
resulted in organosUicon compounds becoming commonplace in the organic chemistry<br />
laboratory, and in fact many uses of these compounds have become routine. To<br />
understand why these compounds have become so pervasive in modem chemistry, one<br />
3
should first consider the unique physical and chemical properties of organosUicon<br />
compounds.<br />
Properties of OrganosiUcon Compounds<br />
Several characteristics of silicon affect its uniqueness and its utiUty in organic<br />
chemistry. Silicon, with the electron configuration (Ne)3s23p23dO, is often compared to<br />
carbon (electron configuration (He)2s22p2), which sits directly above siUcon in the periodic<br />
table. Both elements are commonly tetravalent, although sUicon, by virtue of its empty 3d<br />
orbitals, can expand its octet. Silicon is larger and more electropositive than carbon, and<br />
carbon-silicon bonds are longer (1.9 A vs. I.SA) and weaker (318 kJ mopi vs. 334 kJ<br />
mopi) than the corresponding carbon-carbon bonds. On the other hand, sUicon-oxygen<br />
and silicon-fluorine bonds are stronger than the corresponding carbon-oxygen and carbon-<br />
fluorine bonds. Although siUcon is formaUy considered a metal (or metalloid), it forms<br />
strong, highly covalent bonds with carbon, hydrogen, and oxygen (among others). As<br />
such, organosUicon compounds can often be handled without the need to resort to<br />
techniques more elaborate than commonly used in modem synthetic organic chemistry.<br />
This property alone has most likely been a significant factor in the widespread acceptance<br />
of the use of organosUicon compounds in synthetic organic chemistry.<br />
The ability of siUcon to expand its (Xtet has been used to explain sUicon's high<br />
reactivity under SN2 conditions, as well as the propensity for certain SN2 reactions at<br />
sUicon to proceed with retention of configuration. A number of hypervalent sUicon<br />
compounds, caUed sUicates, have been weU characterized. Recent examples are the<br />
octahedral hexafluorosUicate ion, SiF^^- and the pentacoordinate siUcates 2."^ Very recently<br />
M=Li, (CH3)4N<br />
R=CH3, CgHs<br />
computer modeling studies of pentacoordinated siUcates SiFs" and SiHs" have<br />
been published.^ These compounds, as well as silicates 2, can be considered as models<br />
for pentacoordinated sUicon intermediates or activated states in SN2 type reactions.
An example of the useful selectivity that is a consequence of the high reactivity of<br />
the chlorosilanes is the synthesis of chloromethylethylmethylpropylsilane 4 by reaction of<br />
trichlorochloromethylsilane 3 with the appropriate Grignard reagents (Scheme 2.1).^ As<br />
one can imagine, the possibility of replacing the chlorines of tetrachlorosilane sequentially<br />
^'\c,i-C' p, 1)CH,MqBr CH3CH2^ CH3<br />
Cl^ ' ^ ^ 2)EtMgBr CH3CH2CH2/^'--"<br />
3 3)n-PrMgBr ^ 2 2 ^<br />
Scheme 2.1<br />
with four different nucleophiles has allowed the syntheses of countless organosilanes of<br />
varied functionality. Thus, the vast availabiUty of chlorosilanes, coupled with these<br />
compounds' high reactivity under SN2 conditions, is another significant factor in the<br />
widespread utiUty of organosUicon compounds in synthetic organic chemistry.<br />
The involvement of the silicon 3d orbitals in other types of reactivity characteristic<br />
of organosUicon compounds has been debated, with the consensus being that the 3d<br />
orbitals have little influence. The widening of the bond angle about the oxygen of silyl<br />
ethers, as opposed to alkyl ethers (dimethyl ether, C-O-C = 112°; methoxy silane, Si-O-C =<br />
121°; disiloxane, Si-O-Si = 144°), as weU as the reduced basicity of the silyl ethers, has in<br />
the past been attributed to the mixing of oxygen lone pair and silicon 3d orbitals.<br />
However, a recent inquiry into this problem using crystallographic and ab initio modeling<br />
techniques suggests that this trend is caused by orbital interactions exclusive of the silicon<br />
3d orbitals.^ The stabilization of organosUyl a-anions (or a-metalloids) can be explained<br />
by a combination of molecular polarization and nc-a*si-c overlap (Figure 2.1), without<br />
0*^<br />
71<br />
Figure 2.1<br />
Hyperconjugative Stabilization<br />
of a-Silyl Anions<br />
resorting to (p-d)^ bonding. These findings are analogous to those for the corresponding<br />
organosulfur a-anion species.^ The relative ease for the formation of a-silyl radicals<br />
5
(relative to aUcyl radicals) has been attributed to a polar transition state having substantial<br />
anionic character a to siUcon.^<br />
The 'B-Effect' in OrganosiUcon Compounds<br />
The propensity for sUicon to stabilize an electron deficient center one atom removed<br />
is commonly called the 'p-effect'. As the 'p-effect' is central to the work we are reporting,<br />
a more thorough description of the explanation for this phenomenon will be provided.<br />
The exceptional reactivity of p-chloroaUcylsilanes (general structure 5) was noted in<br />
1937, and by 1946 it had been shown that these compounds were more reactive than the<br />
respective a- or y- substituted compounds towards heat or aluminum chloride^o, as well as<br />
nucleophilic reagents,ii including Grignard reagents.^^ The general reaction in each case<br />
was die elimination of a sUyl chloride 6 and the formation of an olefin (Scheme 2.2). A<br />
heat,AlC^or ^ ^^.^ -^etiiylene<br />
nucleophUe ^ ^ci<br />
Scheme 2.2<br />
review of the literature concerning the p-effect was pubUshed in 1970.^3 Much<br />
work is still being done in order to understand better the anomalous properties of p-<br />
functional organosilanes, including studies of p-silyl cations^"^, ESR^^ and kinetic^^<br />
studies of P-silyl radicals, an electrochemical and theoretical study of P-silyl cation<br />
radicals^^, and a study of the influence of the other Ugands on silicon upon the 'p-effect'.^^<br />
The 'p-effect' has recently been utiUzed to direct a photolytic radical decarbonylation.^9<br />
The accepted explanation for the stabiUzation of electron deficient atoms p to silicon<br />
is a hyperconjugative interaction between the bonding Oc-Si orbital and the electron<br />
deficient p orbital on the atom p to sUicon (Figure 2.2). This overlap is accentuated by the<br />
Figure 2.2<br />
Orbital Interaction Responsible for<br />
the 'p-Effect'<br />
6
high coefficient on the a-carbon, which is a consequence of the polar silicon-carbon bond.<br />
As expected, electron withdrawing Ugands on silicon were found to enhance the 'p-<br />
effect'i8, as they would increase the electrophilicity of the siUcon atom and thus the polarity<br />
of the carbon-silicon bond and the coefficient on the a-carbon. It has been found that the<br />
p-effect is greatest when the silicon-carbon bond and the electron deficient p orbital can<br />
achieve coplanarity, as would be expected. This hyperconjugation has also been invoked<br />
to explain the weakening of the bonds to the p-carbons in organosUicon compounds.^o<br />
Empirical evidence for this hyperconjugative interaction has been provided in the<br />
electrochemical oxidation of compounds such as T.i^ The relative ease of the electro<br />
chemical oxidation of these compounds was ascribed to an increase in die HOMO energy,<br />
relative to the all-carbon systems, caused by CJsi-c-Po overlap. That homoconjugative<br />
(p-d)7t interactions also play a part in the StabUization of p-silyl radicals was shown by the<br />
ESR studyi^, in which the authors calculated approximately 10% electron density in the 3d<br />
orbitals from the observed hyperfine splitting. One could imagine that this 'back-bonding'<br />
could result in an unusually electrophiUc radical.<br />
Uses of OrganosUicon Compounds<br />
CH3^ ^CH3<br />
SiC ^0CH3<br />
CH3'^ "^ ^<br />
1<br />
A large number of organosUicon compounds are known, and many have unique<br />
properties that make them extremely useful. For example, the ubiquitous organosUicon<br />
compound tetramethylsUane is employed in the majority of ^H NMR experiments as an<br />
intemal chemical shift standard. Its ^H NMR signal, which is found slightly upfield of the<br />
normal range for organic compounds, as well as its inertness, make it an almost perfect<br />
standard. Silicon containing crown ethers have been found to have interesting and unique<br />
properties.^^ The interesting silatranes 8, which have aroused theoretical interest because<br />
7
of the possibiUty of nitrogen-sUicon bond formation, have been found to be extremely toxic<br />
to warm-blooded animals when the substituent 'R' is an aryl group. In fact, the para-<br />
chlorobenzene substituted analogue (8, R =p-Cl-C6H4) has been marketed in the USA as a<br />
rodenticide since 1971. The rapid detoxification of this compound in the bodies of<br />
poisoned rodents has made it especially attractive.22 SiUcon-substituted drugs have been<br />
investigated and in some instances have been found to have useful differences in activities<br />
from the analogous carbon containing drugs. For example, sUa-difenidol (9, E=Si) has<br />
been found to be ten times more potent than difenidol (9, E=C) in its antimuscarinic<br />
activity.23<br />
OrganosUicon Compounds as 'Ferrymen'<br />
Despite these and many other uses of organosUicon compounds as final products<br />
(with the siUcone industry as a notable example), sUicon owes much of its utiUty in the<br />
modem organic chemistry laboratory to its role as a 'ferryman,' a term introduced by<br />
Colvin.2^ This word aptiy describes silicon's role in a large part of practical organosUicon<br />
chemistry—its presence selectively directs the course of a reaction or transformation, yet<br />
sUicon is ultimately absent from the final product. One example of OrganosiUcon<br />
compounds functioning as 'ferrymen' is the drastic change in the product distribution of the<br />
thermal rearrangement of silyl ether 10 as compared to hydroxydiene 11 (Scheme 2.3) In<br />
this case, the trimethylsilyl group has facilitated the formation of a product different than<br />
that found for the unprotected diene.^^ Another common example of the ferryman role for<br />
organosUicon compounds is the use of sUyl ethers as protecting groups. Routinely,<br />
chemists will 'mask' a hydroxyl group, perform upon this protected compound various<br />
reactions not compatible with the free hydroxyl group but compatible with the silyl ether,<br />
and finaUy remove the silyl protecting group to 'unmask' the alcohol.<br />
8
\ = /<br />
10 R=SiMe3<br />
11 R=H<br />
heat<br />
Major path for<br />
diene 10 *"<br />
Major path for<br />
diene 11 *"<br />
Scheme 2.3<br />
Another class of OrganosiUcon compounds is the silyl enol ethers,<br />
OSiMe^<br />
whose appUcations in organic synthesis are vast and varied. These relatively stable<br />
compounds undergo a large number of useful transformations, and the products most often<br />
no longer incorporate silicon. Therefore, these compounds can serve as another example<br />
of organosUicon compounds as 'ferrymen.' As the research we are reporting herein<br />
involves a transformation of silyl enol ethers, the synthesis and reactions of this class of<br />
compounds will be discussed more thoroughly.<br />
The Synthesis of Silyl Enol Ethers<br />
SUyl enol ethers, of general structure 14, have been known for some time, and<br />
have come to be of considerable utiUty. The most frequently used method<br />
14<br />
of their formation is the trapping of enolate anions, which can be generated under either<br />
kinetic or thermodynamic control (Scheme 2.4).26 Other regioselective methods for the<br />
13
OSiMe. OSiMe.<br />
+<br />
15 16 17<br />
Scheme 2.4<br />
Conditions<br />
Thermodynamic-<br />
Me3SiCl,DMF,<br />
Et3N, heat<br />
Kinetic-LDA,<br />
Me3SiCl, DME<br />
formation of silyl enol ethers are via the rhodium catalyzed hydrosilylation of cx,p<br />
10<br />
16:17<br />
22:78<br />
unsaturated ketones, via silylation of the enolates formed by various conjugate additions to<br />
cyclic a,p-unsaturated ketones, and via the dehydrogenative sUyation of ketones by silyl<br />
hydrides in the presence of a cobalt catalyst (Scheme 2.5).^^<br />
22<br />
O<br />
R^SiH,<br />
Rhodium catalyst<br />
a) Li, NH3, BUHDH^<br />
• ^<br />
b) Me3SiCl '<br />
Me3SiO<br />
a) Me2CuLi<br />
b) Me^SiCl (R=Me) ^<br />
-or-<br />
Et3Al, Me3SiCN<br />
(R=CN)<br />
R3SiH. pvridine ^<br />
C02(CO)8<br />
19<br />
24 25<br />
Scheme 2.5<br />
\ OSiR.<br />
99:1
Reactions of Silvl Enol Ethers<br />
The body of literature concerning the reactions of silyl enol ethers is prodigious,<br />
and has been extensively reviewed.^8 It has been found that suitably substituted silyl enol<br />
ethers can be hydrolytically stable, and many have been purified by distillation. An<br />
important feauire of these compounds is their high electron density relative to simple<br />
aUcenes, a characteristic that often allows chemodifferentiation in multifunctional<br />
compounds. At this point some of the most common reactions wUl be noted, as they serve<br />
to give a sense of the reactivity of these compounds.<br />
An extremely common fate of a silyl enol ether is the cleavage of the siUcon-oxygen<br />
bond via attack on sUicon by a nucleophUe. Nucleophiles commonly used for this purpose<br />
include methyUithium and fluoride ion (from various sources). Subsequently, the free<br />
enolate anion is avaUable to react with an electrophUe to give a product Thus,<br />
regioselective silyl enol ether formation and purification, foUowed by enolate regeneration,<br />
allows for the production of regiodefined enolates, which often react with excellent<br />
selectivity. As an example, regioselectively produced silyl enol ether 26 reacts cleanly with<br />
benzaldehyde, in the presence of a catalytic amount of tetrabutylammonium fluoride, to<br />
give ketone 27 (Scheme 2.6).^^ Reaction of the enolate anion with other types of<br />
electrophUes (alkyl haUdes, for example) is also common.<br />
0SiMe3<br />
26<br />
PHCHO, THF,<br />
5-10% rt-Bu4N'' F"<br />
Scheme 2.6<br />
Me3SiO O<br />
Another common reaction of silyl enol ethers is condensation with Lewis acid<br />
activated electrophUes, the most common Lewis acid used being titanium tetrachloride. For<br />
example, the silyl enol ether 26 was alkylated cleanly with tert-buty\ chloride, a reaction<br />
that would be difficult under other conditions (Scheme 2.7).^^<br />
27<br />
11
0SiMe3<br />
(CH3)3CC1,<br />
TiCL<br />
26 28<br />
Scheme 2.7<br />
Many other reactions of silyl enol ethers are known, although they are used less<br />
commonly. Some examples include hydroboration, oxidation by various oxidants<br />
including mem-chloroperbenzoic acid (MCPBA), ozone, lead(IV) carboxylates, and singlet<br />
oxygen, and cycloaddition reactions including cyclopropanations, [2-F2] reactions with<br />
electron deficient alkenes, and the Diels-Alder reaction of a,p-unsaturated silyl enol ethers.<br />
'Danishefsky's Diene' 29 is an example of a silyl enol ether commonly used in the Diels-<br />
OMe<br />
J<br />
MeoSiO in^V<br />
29<br />
Alder reaction. A recent paper reports several reactions of silyl enol ethers<br />
that involve the addition of an electrophUe, without the loss of the silyl enol ether<br />
functionaUty.31 For example, the triisopropylsilyl (TIPS) enol ether 30 reacts with N-<br />
bromosuccinnimide (NBS) to afford the brominated silyl enol ether 31 in 90% yield<br />
(Scheme 2.8). Although the product in many of these reactions still includes sUicon, this<br />
group is usually removed at a later point, and as such the silicon is usually still acting as a<br />
'ferryman.'<br />
0Si(/-Pr)3<br />
NBS<br />
(90%)<br />
0Si(/-Pr)3<br />
»»»'<br />
Y<br />
30 31<br />
Scheme 2.8<br />
Br<br />
LJ<br />
12
'Silicon FunctionaUzed' Silvl Enol Ethers<br />
AU of the silyl enol ethers discussed thus far have had simple alkyl or aryl groups<br />
as the three non-enoxy Ugands on siUcon. For some time, however, workers in tiiis<br />
laboratory have been interested in the chemistry of sUyl enol ethers with various<br />
functionaUzed Ugands on sUicon, a class of compounds that we caU siUcon functionalized<br />
silyl enol ethers. One would imagine that by placing one or more non-aUcyl Ugands on<br />
sUicon, one could, via electronic or steric effects, modulate the reactivity of silyl enol<br />
ethers, possibly in an extremely useful manner. A manuscript reporting the appUcation of<br />
this concept to silyl ethers, rather than silyl enol ethers, appeared in 1988^2. This paper<br />
reported the synthesis of a number of differentiy substituted aUcoxydialkylchlorosilanes 33,<br />
which were then reacted with dodecanol to afford a variety of dialkoxydiaUcylsUane<br />
protected ethers 34. It was reported that by varying the Ugands of the chlorosilanes 33,<br />
the reactivity of sUyl ethers 34 towards the fluoride ion or towards acidic conditions could<br />
be tailored, in some cases resulting in useful selectivities (Scheme 2.9).<br />
RaSiCIa<br />
32<br />
R'OH,<br />
EtsN,<br />
CH2CI2<br />
OR"<br />
/<br />
RgSi<br />
\<br />
01<br />
CH3(CH2)iiOH,<br />
EtsN, CH2CI2<br />
33<br />
Scheme 2.9<br />
34<br />
OCH2{CH2)ioCH3<br />
Prior to work in this laboratory, few silicon functionaUzed silyl enol ethers had<br />
been reported, and no systematic studies of the synthesis or reactions of these type<br />
compounds had been pubUshed. Thus Walkup's report in 1987 of the synthesis of a<br />
number of pinacolone derived siUcon functionaUzed silyl enol ethers 35 was the first study<br />
of this interesting class of compounds.^^ A subsequent pubUcation from this laboratory<br />
reported the successful synthesis of a number of silicon functionalized sUyl enol etiiers. A<br />
focal point of this study was the resolution of diastereomeric silicon functionalized silyl<br />
enol ethers (36) that were chkal at the siUcon atom. Disappointingly, it was found that the<br />
O XR<br />
XR = alkoxy, enoxy,<br />
allyloxy, or amino<br />
ligand<br />
Xc = a chiral<br />
alkoxy group<br />
13
chirality about the siUcon atom did not result in a stereochemical bias in the MCPBA<br />
oxidation34 or other various reactions^S of these silyl enol ethers. However, expertise<br />
gained in these studies did encourage us to investigate other transformations of silicon<br />
functionaUzed silyl enol ethers.<br />
Conclusions<br />
The field of OrganosiUcon chemistry is rich and varied. The unique properties of<br />
OrganosiUcon compounds insure them an important niche in organic chemistry.<br />
OrganosUicon compounds, although useful as final products, are most often used in<br />
synthetic organic chemistry as 'ferrymen'-that is, they are temporarily utilized to direct<br />
and/or alter the reactivity of an organic substrate, and are eventually removed to afford the<br />
desired organic compound. Silyl enol ethers, a subclass of organosUicon compounds, can<br />
be formed in a number of selective reactions, and have been found to undergo a variety of<br />
interesting and useful transformations. Finally, siUcon functionalized sUyl enol ethers can<br />
be expected to exhibit interesting, and perhaps novel, reactivities.<br />
General<br />
Radical Transformations in Synthetic Organic <strong>Chem</strong>istrv<br />
Literature concerning the study of radical chemistry, a field that dates back to 1900,<br />
documents the painstaking acquisition of a vast amount of insight into the formation,<br />
structure, and reactions of radicals. Fortunately, the groundbreaking work has been<br />
extensively reviewed and summarized.^^ A number of researchers recognized that the<br />
unique properties of radicals could result in useful reactivities, and began to focus on<br />
synthetically useful transformations of these interesting intermediates. The first burst of<br />
activity (other than that concerned with polymer chemistry) began in the 1970's with<br />
investigations of the radical substitution reactions of aromatic compounds. Recent work<br />
has brought rapid development in the use of alkyl radicals in the formation of aliphatic<br />
compounds, especially by the addition of the radicals to various unsaturated compounds.<br />
The utility of radicals in the formation of carbon-carbon bonds has recently been the subject<br />
of several review articles^'^ and a very useful book.^s Articles corroborating computer<br />
models of radical reactions with experimental results have been published recently.39<br />
In this account we will be concerned with the organic chemistry of carbon centered<br />
radicals (henceforth simply called 'radicals'), and thus this introduction will be concerned<br />
with these species, and will not discuss other aspects of radical chemistry. Radicals are<br />
14
most often formed indirectly by the abstraction of some atom or group by another radical.<br />
They are very seldom generated directly. Perhaps the most common metiiod used to<br />
generate carbon centered radicals is the cascade outiined in Scheme 2.10.<br />
Azobisisobutyronitrile (AIBN) undergoes homolytic decomposition readily, under thermal<br />
or photolytic conditions. The resulting isobutyronitrile radical 37 abstracts a hydrogen<br />
atom from tributylstannane (TBTH), a reaction tiiat is favored by the weak tin-hydrogen<br />
bond as well as die polar character of the transition state. The tin radical 38 tiien abstracts<br />
an atom (most often iodine or bromine) or a group 'X' from the functionaUzed organic<br />
compound 40, finaUy forming the desired organic radical 42. A number of other indirect<br />
methods for producing carbon centered radicals are known.^^<br />
AIBN<br />
NC—C- -h n-Bu3SnH<br />
37<br />
n-Bu3Sn- + RX<br />
39 40<br />
heat or light<br />
Scheme 2.10<br />
-^- NC-^C-<br />
37<br />
-f-N,<br />
NC--C—H + n-Bu3Sn-<br />
38 39<br />
n-BugSnX<br />
41<br />
+ R<br />
Radicals are energetic intermediates, with a propensity to combine with themselves<br />
or other radicals, an incUnation not shared by cations or anions. Since radical combination<br />
is usually diffusion controlled in the liquid phase (although the electronic or steric<br />
characteristics of the radical can slow down this process), these combinations occur with<br />
little selectivity. Therefore radical-radical combination reactions are relatively unimportant<br />
as a synthetic method. The most useful reactions of radicals are the result of the<br />
transformation of one radical into another. Reactions of this type include addition<br />
reactions, abstraction reactions, elimination reactions, and rearrangement reactions. These<br />
reactions are well suited for use as propagating steps in a chain reaction, allowing for the<br />
maintenance of low free-radical concentrations and avoiding problems with radical<br />
combination. Of these reactions, addition reactions have taken 'center stage' and found<br />
great utility in preparative organic chemistry.<br />
42<br />
15
The most useful addition reaction of radicals is addition to alkenes or aUcynes.<br />
These reactions result in the formation of a carbon-carbon o bond and the cleavage of a<br />
carbon-carbon n bond, a highly exothermic exchange. Other reactions, such as the<br />
abstraction of a hydroxyl hydrogen or addition to a carbonyl carbon, are endothermic (or,<br />
at least, less exotiiermic), and tiierefore these radical addition reactions are often highly<br />
chemoselective and are tolerant of a large variety of functional groups. Although radicals<br />
are electroneutral (die radical center has no formal charge), substituted radicals are often<br />
considered to be nucleophilic or electrophiUc. Thus radical 43, which is substituted with<br />
electron releasing groups, is considered electron rich and tiierefore nucleophiUc, whereas<br />
radical 44 is electron deficient and electrophiUc. These characteristics of radicals often<br />
CH3^<br />
H-C<br />
CH3O<br />
43<br />
CH3O2C<br />
H-O<br />
CH3O2C<br />
aUow radical additions to act as complements towards otiier types of reactions. For<br />
example, while the cation 45 formed from the bromoglucose 46 will add to electron rich<br />
aUcenes, the corresponding radical 47 will be nucleophUic and wiU tiierefore add efficientiy<br />
to electron poor aUcenes (Scheme 2.11).^<br />
AcO<br />
AcO<br />
CHoOAc<br />
45<br />
O<br />
H<br />
+<br />
^C<br />
AcO \ H<br />
AcO<br />
AcO<br />
CH2OAC<br />
AcO I<br />
Br<br />
46<br />
Scheme 2.11<br />
•H<br />
44<br />
AcO<br />
AcO<br />
CHoOAc<br />
47<br />
O<br />
16<br />
AcO \ H<br />
Radical Cvclizations<br />
Although intermolecular radical additions can be useful,^^ die intramolecularity of<br />
radical cyclizations makes them particularly useful transformations.^^ Probably the most<br />
studied radical cycUzation is tiiat of 5-hexen-l-yl radicals. The kinetics of tiiis cyclization<br />
have been very tiioroughly studied, and it is often used as a 'radical clock' for the indirect<br />
determination of the rates of other radical reactions tiirough competition experiments. In<br />
fact. Wilt has called tiiis cycUzation tiie 'Greenwich Meridian Time' of radical chemistry'."^^
The unsubstituted 5-hexen-l-yl radical 48 cyclizes witii contrathermodynamic<br />
regioselectivity ahnost exclusively to the cyclopentyhnethyl radical 49, with very little of<br />
the more stable cyclohexyl radical 50 produced (Scheme 2.12). This behavior has<br />
6<br />
+<br />
48 49 50<br />
Scheme 2.12<br />
been explained by a greater loss of entropy in the transition state for the 6-endo cycUzation<br />
relative to the 5-exo cycUzation. An alternative explanation is that non-bonded repulsions<br />
cause the transition state for the 6-endo cyclization to be disfavored. However, the most<br />
reasonable explanation seems to be that the geometry necessary for the overlap of the<br />
radical with the n* orbitals of the alkene is less strained in the 5-exo mode of cyclization<br />
(Figure 2.3). Molecular modeling calculations based upon this stereoelectronic approach<br />
have been of good, predictive value in more complex cycUzations, providing evidence for<br />
the vaUdity of this approach.'*^<br />
The Effect of SiUcon Substitution<br />
—cvC*<br />
Figure 2.3<br />
Tra jectory for Radical Add ition<br />
Since stereoelectronic effects are so important in these radical cycUzations, one<br />
would expect that small changes in the geometry of the radical could effect significant<br />
changes in the reaction patiiway. In fact. Wilt reported in 1985 that the replacement of a<br />
C2, C3, or C4 methylene unit in the 5-hexen-l-yl radical with a dimethylsilyl group led to<br />
dramatic changes in the product distribution from the cyclization."^2 These results were not<br />
17
altogether unexpected, as the longer carbon-siUcon bonds, as well as the metiiyl groups<br />
attached to silicon, would be expected to alter the transition state geometry.<br />
In this 1985 publication. Wilt compared rate constants for the reactions indicated in<br />
Scheme 2.13. The behavior of the a-silyl radical 51 as well as the y-silyl radical 52 could<br />
be compared to that of the all-carbon system 53. Compound 54 was also considered, as it<br />
appeared to be a better model for a-sUyl radical 51.<br />
Radical<br />
51<br />
52<br />
53<br />
54<br />
+nBu3Sn»<br />
El<br />
CH2<br />
SiMe2<br />
CH2<br />
CH2<br />
TBTH = «-Bu3SnH Scheme 2.13<br />
E2<br />
SiMe2<br />
CH2<br />
CH2<br />
CMe2<br />
k5(s-i)<br />
6.6x10^<br />
6.5x10"^<br />
2.5x10^<br />
4.7x10^<br />
k6(s-')<br />
1.4x10'<br />
4.3x10^<br />
3.5x10^<br />
unknown<br />
kH(M-^s-^)<br />
1.9x10*^<br />
2.1x10^<br />
2.4x10^<br />
3.6x10^<br />
From the data, one can see that a-sUyl radical 51 cyclizes much more slowly in a<br />
5-exo fashion than does the all carbon system 53. This attenuation in rates was explained<br />
by invoking unfavorable polar contributions in the transition state (Figure 2.4) and strain<br />
.6"<br />
&<br />
^—SiMe2_<br />
Figure 2.4<br />
Polar Contributions in 5-exo<br />
CycUzation of a-SUyl Radical<br />
caused by the long carbon-sUicon bonds. The sUghtly larger kH for tiie a-silyl radical, as<br />
compared to the all-carbon systems 53 or 54 could be explained by invoking favorable<br />
polar contributions to the transition state for this step (Figure 2.5). The slight reduction in<br />
18
Figure 2.5<br />
Polar Contributions in Reduction<br />
of a-SUyl Radical<br />
tiie rate of 5-exo cyclization of y-sUyl radical 52 was attributed to strain caused by the<br />
longer carbon-silicon bonds. Unfortunately, Wilt was unable to dissect rate constants for<br />
the cycUzation of compound 55 as the kH for P-sUyl radicals were not known. All of the<br />
60<br />
56<br />
OtBu<br />
Me, ,CI<br />
^.*°e so^<br />
,s^t^i<br />
a) 5-exo cyclization pi<br />
b) H-atom source<br />
Scheme 2.14<br />
'CH2Br<br />
Et3N, CH2CI2<br />
Scheme 2.15<br />
R^^Y^,<br />
OtBu<br />
R3<br />
O—SiMe.<br />
O—SiMsc<br />
57<br />
OtBu<br />
20
Cyclization of B-Silvl Radicals<br />
The results Wilt obtained with the p-silyl radical 55 are particulary notable. These<br />
radicals showed very Uttie propensity to cyclize, and virtually all cyclization occurred in the<br />
6-endo fashion. These results were explained by invoking this type radical's known<br />
preference to adopt a syn-periplanar conformation, a consequence of the 'P-effect' (see<br />
Figure 2.2). Any cycUzation would require the radical center to twist out of the plane and<br />
lose its homoconjugative stabiUzation. Wilt claimed that a combination of this and the<br />
longer carbon-silicon bonds caused cycUzation, especially in the 5-exo mode, to be<br />
extremely disfavored. A communication pubUshed in 1988 corroborated Wilt's work by<br />
virtue of gas chromatographic (GC) product studies on radicals 55 and 64, without really<br />
reporting anything new (Scheme 2.16)."^'^<br />
r"<br />
egSi—<br />
55 (R=H)<br />
64 (R=CH3)<br />
Conclusions<br />
TBTH ^-<br />
MegSi '<br />
65<br />
(R=H or<br />
CH3, 0%)<br />
+<br />
Scheme 2.16<br />
MOsSi ^<br />
66<br />
(R=H, 7%;<br />
R=CH3, 4%)<br />
+ Me2Si-<br />
H<br />
67<br />
(R=H, 83%;<br />
R=CH3, 61%)<br />
Radical transformations of organic molecules are relatively well understood and are<br />
becoming useful tools for the synthetic organic chemist. Of all the radical reactions of<br />
organic molecules,excluding polymerization, radical cyclizations are the most synthetically<br />
useful. Radical cycUzations of molecules containing siUcon are possible, and these<br />
compounds react differently than the aU carbon systems.<br />
Rationale for this Research<br />
In accord with our ongoing interest in the chemistry of silyl enol ethers, a study of<br />
the free radical chemistry of silicon functionalized silyl enol ethers was enticing. Especially<br />
attractive in this respect was the possibility of an intramolecular free radical reaction of<br />
suitably substituted silyl enol ethers. Although die propensity for simple silicon substituted<br />
5-hexenyl radicals to undergo cycUzation is attenuated relative to tiie all carbon systems, it<br />
21
was not obvious what effects adding an oxygen, especially as part of a silyl enol ether,<br />
would have upon the reactivity of these systems. The success reported for 5-exo<br />
cycUzations of allyl sUyl ethers such as 61 was somewhat suprising given the lack of<br />
cyclization for the simple silicon substituted 5-hexenyl radicals, and was therefore<br />
encouraging. This project was embarked upon with three major goals. First, it was<br />
necessary to discern whether or not a sUyl enol etiier could be suitably functionalized so as<br />
to undergo a free radical cyclization. If this is possible, then a study of how the<br />
incorporation of a functionalized sUyl enol ether affects the outcome of the cycUzation will<br />
add to our knowledge about the properties of siUcon functionalized silyl enol ethers in a<br />
general way. Finally, if such a cyclization is possible, the question can be asked: can it be<br />
developed into a versatUe synthetic method for the use of synthetic organic chemists, who<br />
are more concerned with tiie products of a transformation tiian with the route by which tiie<br />
transformation is carried out.<br />
22
CHAPTER 3<br />
RESULTS AND DISCUSSION<br />
General Considerations<br />
In embarking upon this project the first consideration was the choice of the method<br />
of radical formation. The thermally initiated tri-w-butyltin hydride (TBTH) / azobisiso<br />
butyronitrile (AIBN) method, outiined in Scheme 2.10, was chosen. This is the most<br />
common metiiod of producing syntheticaUy useful carbon centered radicals, and it has been<br />
utUized in previous studies involving the formation of siUcon substituted alkyl<br />
radicals.'^2,45.47 xhe availabiUty of the requisite silicon substituted aUcyl chlorides"^^ causes<br />
this route to be especiaUy appealing. Since die ultimate fate of tiie aUcyl radicals formed in<br />
tills cascade is hydrogen atom abstraction from TBTH, which regenerates tiie tin radical<br />
39, this reaction is a chain reaction. Therefore, only a catalytic amount of the initiator<br />
(AEBN) is necessary.<br />
Chlorosilanes have been demonstrated to be versatUe starting materials for the<br />
synthesis of 'simple' silyl enol ethers,^ as well as 'silicon functionalized' silyl enol<br />
ethers.^^'^'^'^^ Fortunately, chloroalkylsilanes with either 2 or 3 chlorine atoms on silicon<br />
are readily available. As the synthesis of the simplest 'silicon functionalized' silyl enol<br />
ethers requires the addition of two groups, an enol ether and an additional non-alkyl ligand<br />
"L" to the silicon, the commerciaUy avaUable chloroalkyl methyldichlorosUanes 68 were<br />
chosen as starting materials for the initial studies (Scheme 3.1). That only straight chain<br />
chloroalkylsilanes are avaUable was accepted as an initial limitation upon this research.<br />
Addition of an enolate anion and a functionaUzed group "L," although not necessarily in<br />
that order, to the dichlorosilanes 68 would then afford the siUcon functionalized<br />
chloroalkylsilyl enol ethers 69 (Scheme 3.1) desired for the study of their free radical<br />
cyclizations.<br />
Cl\ .^CH3 ^^^ O^ ^(CH2)n-CH2CI<br />
C|/^'^(CH2)n-CH2CI<br />
68<br />
><br />
Scheme 3.1<br />
23
Abstraction of the chlorine atom of 69 by tiie tin radical 39 (generated as shown in<br />
Scheme 2.10) would afford sUyl enol etiier substituted aUcyl radicals of general structure<br />
70 (Scheme 3.2). These radicals could cyclize in an endo fashion to afford radical 71, or<br />
an exo fashion to afford radical 72. Radicals 71 or 72 would finally abstract a hydrogen<br />
atom from TBTH, producing the cyclized products 74 and 75. Alternatively, the radical<br />
70 could abstract a hydrogen atom from TBTH, forming the 'directiy reduced' silyl enol<br />
ether 73. This was by far the predominant reaction of the previously studied 'simple' silyl<br />
substituted 5-hexenyl radicals 55'^2 and 64.^7 Any of these hydrogen atom abstractions<br />
would regenerate the tin radical 39, tiius propagating the chain reaction.<br />
{CH2)n<br />
e<br />
ro<br />
3<br />
CQ n-BusSn*<br />
39<br />
{CH2)n<br />
rt-BusSn- (39)<br />
.SI.<br />
O' •(CH2)n<br />
R' R"<br />
Scheme 3.2<br />
L. .^CH3<br />
R'<br />
R'<br />
^ ^ ^<br />
L^^.^CH3<br />
O-^ "^(CH2)n<br />
/ \<br />
.. H<br />
R<br />
75<br />
O-^ ''^{CH2)n-CH3<br />
24
Initial Studies<br />
Much of the work done in this laboratory involving the study of the synthesis and<br />
reactions of silicon functionalized sUyl enol ethers has utUized tiie 3,3-dimethylbuten-<br />
2-yloxy enol etiier ligand. These silyl enol etiiers have been syntiiesized by reaction of the<br />
enolate of pinacolone (3,3-dimethyl-2-butanone, r-butyl methyl ketone) witii various<br />
chlorosilanes.33.34.35 These compounds have been found to be relatively stable to<br />
hydrolysis, by virtue of the buU
Upon homolytic chlorine atom abstraction from 69 (n=0), tiie a-sUyl carbon centered<br />
radical 70 (n=0) would be formed. What is of interest, then, is the fate of this radical.<br />
Du-ect hydrogen atom atom abstraction by tiiis radical to afford 73 (n=0) would be<br />
relatively uninteresting. Exo cycUzation of radical 69 (n=0), to afford 74 (n=0) (after<br />
hydrogen atom abstraction), would require a 4-exo cyclization and would result in the<br />
formation of a l-oxa-2-sUacyclobutane. Stereoelectronic effects would be expected to<br />
disfavor greatly a 4-exo cyclization of this system. The alternative endo cyclization mode<br />
would require the formation of a l-oxa-2-silacyclopentane via a 5-endo cycUzation.<br />
Although 5-endo cycUzations are rare in free radical chemistry, an intramolecular metal<br />
catalyzed hydrosilylation of silyl ether 78 did produce a product (79) from tiiis type<br />
cyclization (Scheme 3.3).^^ However, the stereoelectronic requirement for the metal<br />
mediated reaction is quite different from that of a radical cyclization.<br />
MSv ^Me Me ^e<br />
O H H2PtCl6 - 6H2O ^ o'^<br />
\ ^^ isopropanol reflux \<br />
78 79<br />
Scheme 3.3<br />
Chloromethyl silyl enol ether 80 was easily synthesized following the 'one pot'<br />
procedure developed by Walkup (75% crude yield, 26% distilled. Scheme 3.4).33 This<br />
compound could not be purified by either silica gel or alumina chromatography, as it was<br />
found to undergo rapid decomposition under these conditions. However, purification by<br />
distillation was successful.<br />
EtO. ^Me<br />
a) 1 eq.LDA, ether, Q CHgCI<br />
-78°. 10 minutes ^<br />
b) 1 eq. 77, -78°- 0°,<br />
1 hour<br />
c) 1 eq. EtsN, 1 eq EtOH,<br />
0°- r.t., 12 hours<br />
Scheme 3.4<br />
Silyl enol ether 80 was submitted to typical free radical cyclization conditions<br />
(Scheme 3.5).'*^ Upon examination of the ^H NMR spectrum of the crude reaction mixture<br />
26
it was obvious that a reaction had occurred, as the signals for the chloromethyl ligand on<br />
the starting material had disappeared. Also, in tiie ^H NMR spectrum, the signal for the<br />
methyl ligand on siUcon, a clear singlet in the starting enol etiier, had disappeared and was<br />
replaced by a number of peaks in the area 5 0.2-0.0 ppm of the spectrum. Unfortunately,<br />
there was no sign in the proton NMR of the crude reaction mixture that the 5-endo<br />
cycUzation had occurred. Analysis of the crude reaction was complicated by the fact tiiat<br />
much of the upfield region (6 1.75-0.75 ppm) of the spectrum was obscured by the alkyl<br />
signals of the tri-/i-butyltin byproducts. However, it was expected that the C5 methine<br />
proton of 5-endo cyclized material 83 would appear as a doublet of doublets, or some<br />
similar pattern, in the relatively clear downfield (~ 6 4.00) region of the ^H NMR<br />
spectrum. Attempted purification of the crude reaction mixture was unsuccessful, with<br />
decomposition of the mixture occurring rapidly. These discouraging results led to an<br />
investigation of the next homologue of the series (69, n=l).<br />
TBT'<br />
80 z: >.<br />
TBT-Cl<br />
EtO. ^Me<br />
O CHg-<br />
Scheme 3.5<br />
a) 4-exo cyclization^^<br />
b)TBTH (-TBT- f<br />
The Svnthesis and Radical CvcUzation of a<br />
2-rhloroethvl Silvl Enol Ether<br />
OEt<br />
• -Me<br />
The free radical chemistry of the next homologue of the chloroalkylsilyl enol ethers<br />
69 (n=l) would involve the intermediacy of the p-silyl radical 70 (n=l). As was noted in<br />
the introduction, p-silyl radicals behave unusually. The unusual behavior of these radicals<br />
27
has been attributed to the homoconjugative stabiUzation that is responsible for the p-effect<br />
(see Figure 2.2). It should be noted tiiat homoconjugation would result in a relatively<br />
electrophiUc radical, which would be an especially good match for the electron rich silyl<br />
enol ether alkene.<br />
P-Silyl radicals 70 (n=l) could undergo a 6-endo cyclization to afford, after<br />
hydrogen atom abstraction, a l-oxa-2-silacyclohexane 74 (n=l). The alternate 5-exo<br />
cyclization would afford a l-oxa-2-sUacyclopentane 75 (n=l). Endo cyclization has been<br />
demonstrated to be the major cyclization path for the simple siUcon substituted 5-hexenyl<br />
radicals 51, 55, and 64. The radical cyclization of the steroidal silyl etiier 84 also<br />
proceeded in a 6-endo fashion.^o This reaction models the radical reaction of 2-<br />
chloroethylsUyl enol ethers more closely, as it includes a silicon-oxygen bond in the chain.<br />
However, the stericaUy hindered environment of the disubstituted intemal end of the double<br />
bond of 84, accentuated by the rigid steroid ring system, may have played a significant role<br />
in directing the course of this cyclization. Recall that the similar sUyl ether 61 cyclized in a<br />
5-exo fashion. This is as one would expect when simply considering the steric bulk<br />
surrounding the double bond. Therefore, in order to study the free radical reactions of 2-<br />
chloroethylsilyl enol ethers, an n=l analogue of 69 (86) was synthesized.<br />
THPO<br />
84<br />
TBTH, AIBN,<br />
benzene reflux<br />
70%<br />
Scheme 3.6<br />
Dichloro(2-chloroetiiyl)metiiylsilane is commercially avaUable. The synthesis of<br />
the 2-chloroethyl silyl enol ether 86, according to the proceedure developed by Walkup,<br />
was uneventful (95% crude yield. Scheme 3.7). However, the purification of 86 was a<br />
problem, and once again distUlation was the purification method of choice, although in<br />
practice the crude reaction mixtures were often utiUzed.<br />
28
a) 1 eq.LDA, ether,<br />
-78°. 10 minutes<br />
b) CI<br />
CI<br />
-78°- 0°,1 hour<br />
c) 1 eq. EtsN, 1 eq EtOH,<br />
0°- r.t., 12 hours<br />
Scheme 3.7<br />
EtO. ^Me<br />
:Si<br />
Silyl enol ether 86 was submitted to the free radical cyclization conditions (Scheme<br />
3.8). Once again, it was clear that abstraction of the chlorine atom had occurred, as the<br />
signals for the methylene group attached to the chlorine atom had completely disappeared.<br />
However, the disappearance of these signals was accompanied by the appearance of four<br />
doublets at -3.45 ppm. This is the pattern expected for the proton on the carbon a to the<br />
r-butyl group of the 6-endo cyclized product 90. That is, the C6 proton of 90 would be a<br />
doublet of doublets as a result of coupUng with the two nonequivalent hydrogens on the<br />
neighboring carbon C5. As E and Z isomers are possible for the ring closed product 90, a<br />
total of two doublets of doublets would be expected. Also noted was the disappearance of<br />
the signals for the enol ether vinyl protons and the appearance of a singlet slightiy upfield<br />
86<br />
TBT-<br />
1<br />
TBT-Cl L.<br />
EtO^ .Me<br />
:Si.<br />
CHo-<br />
Scheme 3.8<br />
a) 5-exo cyclization^<br />
b) TBTH (TBT- f<br />
EtO^ ^Me<br />
O Et<br />
29
from where the enol etiier signals had been. Unfortunately, once again the majority of die<br />
upfield region was obscurred by tiie aUcyl signals of the tin byproducts. Finally, inspection<br />
of tiie region about 0.0 ppm revealed tiiat the signal for tiie metiiyl groups on silicon in the<br />
starting material had disappeared, and tiiree peaks, with approximately equal integrations,<br />
had appeared.<br />
Every attempt to purify this crude reaction mixture by chromatography resulted in<br />
rapid decomposition. In Ught of tiie successes in the purification of the silyl enol etiiers by<br />
distiUation, a large scale radical reaction was performed in order to afford sufficient product<br />
to attempt purification by distiUation.<br />
This distillation did, in fact, result in the isolation of a small amount of the 6-endo<br />
cyclized product 90. The product resulting from hydrogen atom abstraction by the p-silyl<br />
radical 87 prior to cyclization, 88, was also identified. It was found that the singlet that<br />
had appeared slightly upfield from the signals for the vinyUc hydrogens of the starting enol<br />
ether was the vinyl signal for this 'directiy reduced' product. Thus, two of the possible<br />
products for this cyclization (88 and 90) were observed in the crude reaction mixture. No<br />
evidence for product 89, resulting from the alternate mode of cyclization (5-exo), was<br />
noted.<br />
By considering relative ^H NMR integrations in the cmde reaction mixture, it was<br />
calculated that the two diastereomers of the cycUzed material were produced in equal<br />
amounts, and that the directly reduced product (88) and the cycUzed products (90, sum of<br />
both isomers) were produced in approximately equal amounts. Two of the signals in the<br />
region about 0.00 ppm correspond to the cyclized isomers, while the tiiird signal<br />
corresponds to the directiy reduced material. Thus all of the signals in the 0.00 region were<br />
accounted for. This would seem to suggest that the identified products were produced<br />
more or less quantitatively, as any conceivable product would have a metiiyl on silicon that<br />
would appear in this region of the ^H NMR spectrum. Unfortunately, the total isolated<br />
yield of the three identified products was only 53%. This problem of mass balance has<br />
been consistant throughout tiiis project, and has as yet to be explained. Possibly some<br />
volatile and/or insolubile byproducts are formed under tiie reaction conditions.<br />
One probable side reaction would be the loss of ethylene from the starting enol ether<br />
86 to afford the silyl chloride 91 (Scheme 3.9). This kind of fragmentation is known (see<br />
Scheme 2.2). Although this type transformation can be thermaUy initiated,^ a control<br />
experiment in which tiie reaction conditions were duplicated in the absence of AIBN did not<br />
result in the decomposition of 86. However, it should be noted that the radical reaction<br />
(86 => 87) results in a build-up of tri-n-butyltin chloride. This tin chloride could promote<br />
30
the formation of 91, eitiier by acting as a Lewis acid, or by serving as a source of chloride<br />
ion (especially if water is present). Silyl chloride 91 could then catalyze its own formation<br />
in the same manner. In any case, 91 most Ukely would not be easily isolable, because of<br />
problems with volatUity and/or polymerization during workup.<br />
EtO^^.^Me EtO^ ^Me<br />
A, Lewis acid, O CI<br />
or nucleophile •^ I + CHo=CHc<br />
Scheme 3.9<br />
Compounds 88 and 90 were found to decompose upon standing. This was not<br />
suprising, considering that it is well known from previous studies that l-oxa-2-<br />
silacyclohexanes are, as a class, very unstable, polymerizing spontaneously upon contact<br />
with water.51 In any case, it was encouraging that 2-chloroethylsilyl enol ether 86 would<br />
cyclize in an 6-endo fashion. Compound 90 is simply an intramolecularly protected y-silyl<br />
alcohol, and as such this cyclization has resulted in the 'reductive a-alkylation' of the<br />
starting ketone (pinacolone). A general transformation of this type would be a worthwhile<br />
synthetic method, and as such this reaction was studied with this as the ultimate goal.<br />
Optimizing the Recipe for the Radical Cyclization<br />
As a competing direct reduction, at ~50% of the products, would prohibit the use of<br />
this radical cycUzation reaction as a general synthetic method, various attempts to optimize<br />
the ring forming reaction were made. Various reaction conditions were explored, and the<br />
88/90 ratio determined by comparison of the ^H NMR peak areas in the crude reaction<br />
mixtures.<br />
In a preUminary experiment, the radical reaction was performed with<br />
triphenylstannane as the hydrogen atom source. This reagent would not obscure the<br />
upfield region of the ^H NMR spectra of the crude reaction mixtures, thus allowing for<br />
easier analyses of the crude reaction mixtures. However, tiie use of this reagent resulted in<br />
almost complete formation of the directiy reduced product 88.<br />
Acting upon a suggestion that compound 88 could be produced by a competing<br />
'ionic' hydride reduction of the starting p-silyl alkyl chloride, enol ether 86 was treated<br />
with TBTH under the 'standard' free radical conditions, excepti that cyclohexane was used<br />
31
as solvent instead of benzene. The nonpolar solvent would be expected to inhibit ionic<br />
reactions relative to tiie 'polarizable' benzene solvent. However, inspection of tiie ^H<br />
NMR spectrum of the crude mixture from tiiis reaction showed complete conversion of the<br />
enol etiier 86 to the directly reduced material 88, witii no cyclized material evident. Using<br />
THF as tiie solvent gave the same results. Toluene was found to behave similarily to<br />
benzene.<br />
One common method used to lessen competing direct reduction in slow radical<br />
cycUzations is the maintenence of extremely low hydrogen atom donor concentrations. Up<br />
to this point this had been accomplished in this research by adding the tin hydride slowly to<br />
the reaction mixture. Another method is the use of a catalytic amount of the tin reagent,<br />
with a hydride reducing agent utUized to reduce the tin chloride formed during the reaction,<br />
and to therefore regenerate the tin hydride.<br />
An early report of a reaction involving the use of catalytic amounts of the tin reagent<br />
used sodium borohydride (NaBH4) as the reagent to re-reduce the tin chlorides formed<br />
when the tin radicals abstracted chlorine to form the alkyl radicals. To make this reaction<br />
homogeneous, r-butanol was used as solvent. Unfortunately, a model reaction of silyl enol<br />
ether 86, performed in r-butanol under the standard stoichoimetric tin hydride conditions,<br />
resulted in the formation of only 88.<br />
An alternate catalytic recipe used sodium cyanoborohydride (NaCNBH3) as the<br />
reducing agent (Scheme 3.10).^2 These reactions were performed in benzene solvent, with<br />
15-crown-5 added to the reaction mixture in order to solubiUze the borohydride. In fact,<br />
upon UtiUzation of this recipe the ratio of cycUzed:directiy reduced products was enhanced<br />
substantially. The yield of the desired products was not significantly increased, however,<br />
and various other products were formed as evidenced by a proliferation of peaks in the 0.0<br />
ppm region of the ^H NMR spectrum. A control experiment established that this was not a<br />
result of reduction of the alkeke moiety of the silyl enol ether 86 by the NaCNBH3.<br />
However, compound 90 may be relatively sensitive to reducing agents. It is also possible<br />
that the chloroborohydrides formed during the course of the reaction catalyze the<br />
decomposition of the starting materials or products. Another problem was that this catalytic<br />
recipe was found to be somewhat less than reliable; at times no reaction occurred under<br />
these conditions. This may be a consequence of the increased number of reagents, each of<br />
questionable purity, that are used in the catalytic recipe, as compared to the recipe involving<br />
stoichiometric amounts of the tin hydride.<br />
32
EtO. ^Me<br />
,Si 0.10 eq. TBTH,<br />
0.20 eq. AIBN,<br />
1.0eq.NaCNBH3,<br />
Ql O.lOeq. Benzo-15-crown-5,<br />
benzene reflux, 12 hours<br />
Scheme 3.10<br />
EtO. ^Me<br />
O^ ^Et<br />
Oxidative Removal of the SiUcon Atom<br />
+<br />
EtO. ^Me<br />
:Si<br />
Although the formation of l-oxa-2-sUacyclohexane 90 had been demonstrated to be<br />
reproducible, the difficulty of efficiently isolating it was discouraging. Therefore, it was<br />
decided that various derivatization methods should be explored, with the hope of finding a<br />
simple reaction of 90 that would afford easUy isolable products. Also, an efficient method<br />
for forming derivatives was necessary for the development of this reaction as a new<br />
synthetic method, so that sUicon could perform in its role as a 'ferryman.'<br />
A well precedented reaction, the oxidation of substituted silyl ethers by hydrogen<br />
peroxide in the presence of fluoride, was thereby attempted.^3 of the procedures described<br />
in this paper, a 'neutral' recipe worked the best. This reaction resiUted in the production of<br />
the diol 92, which could easily be separated from the nonpolar trialkyl tin byproducts (27%<br />
overall from the silyl enol ether. Scheme 3.11). The isolation of diol 92 also confirms that<br />
EtO. ^Me<br />
.31.<br />
10 eq. aqueous H2O2,<br />
10 eq. KHF2,<br />
DMF, 60°, 48 hours<br />
Scheme 3.11<br />
the l-oxa-2-silacyclohexane was indeed formed. This derivatization was reproducible,<br />
altiiough low yields were always obtained.<br />
In any case, these preliminary explorations have demonstrated that the 6-endo<br />
radical cycUzation of 2-chloroetiiylsilyl enol ethers would proceed, and that the resulting 1<br />
oxa-2-sUacyclohexanes could be derivatized in order to afford isolable materials. To<br />
discern the scope of this reaction an attempt was made to synthesize analogues of 86,<br />
differing only in the nature of the enol ether Ugands. However, this was found to be less<br />
33
than trivial. Although this difficulty was eventually surmounted^^ by modifying a recipe<br />
described by Corey and Gross,^^ it was decided to explore simpler systems during this<br />
preliminary stage of this study.<br />
The Svnthesis and Free-Radical Cvclizations of 2-<br />
Chloroethvldimethvlsilvl Enol Etiiers<br />
In order to simpUfy the synthesis and characterization of the 2-chloroethylsilyl enol<br />
etiiers, we decided to study the dimethyl sUyl substituted analogues. The requisite<br />
dimethylchlorosUane 93 was synthesized by the addition of methyUithium to dichloro(2-<br />
chloroethyl)methylsilane(Scheme 3.12). This product could be distilled to high purity. It<br />
was noted that if the distUlation temperature became elevated (-100° or higher), 93, whose<br />
identity had been demonstrated by ^H analysis of the cmde reaction mixtiu'e, would<br />
decompose to dichlorodimethylsilane 94. The identity of this product was discerned by ^H<br />
NMR and verified by conversion to diethoxydimethylsilane 95. The facile decomposition<br />
of 93, which may have been catalyzed by the presence of lithium chloride (a byproduct of<br />
addition of the alkyllithium to the chlorosilane), accents the inherent high reactivity of the 2-<br />
chloroethylsilanes. ^<br />
M^^c-^" CHsLi ^"^si" A S^2_^ Me^ Me EtOH, Me^ Me<br />
L 94 95<br />
'CI 93 XI<br />
Scheme 3.12<br />
Chlorosilane 93 reacted with a variety of lithium ketone enolates to afford the sUyl<br />
enol etiiers (Scheme 3.13, Table 3.1). This condensation was most efficient when HMPA<br />
was UtUized as a cosolvent. These silyl enol ethers were found to be unstable to silica gel<br />
chromatography. Therefore, the relatively clean 'crude' silyl enol ethers 96 were usually<br />
used 'as is' in the radical reactions.<br />
^^ R^>f"CH2R4<br />
'2 p<br />
Me^ ^Me ^<br />
^Si ^^v. THF:HMPA(2:n.-7,g°<br />
b) LDA,-78°-r.t.<br />
O<br />
Scheme 3.13<br />
34
Table 3.1 tabulates data from the synthesis (% yield, ^H NMR data for hydrogens<br />
on C*) and radical reaction (^H NMR data for C* of tiie directiy reduced compound 97 and<br />
IH NMR data for HA of the cycUzed compound 98) of 2-chloroetiiyldimethylsUyl enol<br />
ethers 99-105.<br />
1.05 eq. TBTH,<br />
0.2 eg. AIBN. ^<br />
benzene reflux, R^<br />
12 hours R,<br />
R3<br />
Scheme 3.14<br />
Me^ ^Me<br />
O^ Et<br />
C*HR4<br />
97<br />
+<br />
^ ^<br />
That silyl enol ethers 99-105 reacted under the radical conditions was<br />
Me. .Me<br />
suggested by the disappearance of the proton NMR signals for the hydrogens on the carbon<br />
originally bound directly to the chlorine atom. However, no products from these reactions<br />
were isolable as pure materials by chromatography. Careful examination of the crude ^H<br />
NMR spectra of the radical reaction mixtures of enol,ethers 99,100, 101, and 103<br />
revealed signals that suggested that these sUyl enol ethers had undergone the previously<br />
observed 6-endo cyclization, as well as direct reduction. These cycUc products would be<br />
trialkyl silanes, which are known to be difficult to oxidize under the conditions developed<br />
by Tamao^3 (and used previously to derivatize the dialkoxy-2-oxa-l-silacyclohexane 90).<br />
Therefore, the crude reaction mixtures were derivatized by reaction with a large excess of<br />
methyUithium in ether at room temperature. NucleophiUc displacement of the alkoxy ligand<br />
opened the l-oxa-2-silacyclohexanes efficientiy, and the expected trimethylsilyl alcohols<br />
were easily isolated by siUca gel chromatography. Thus pinacolone was transformed into<br />
the substituted hexanone 107 in 15% overall yield for the tiiree steps (enol ether formation,<br />
radical cyclization, and methyUithium treatment) without purification of any intermediate<br />
(Scheme 3.15). No other identifiable compounds were isolated. This nucleophUic ring<br />
opening of l-oxa-2-silacyclohexanes is known.55<br />
Me^ ^Me<br />
106<br />
Scheme 3.15<br />
107<br />
R4<br />
98<br />
SIMe^<br />
35
#<br />
99<br />
100<br />
101<br />
102<br />
103<br />
104<br />
105<br />
TABLE 3.1<br />
Synthesis and ^H NMR Data for 2-Chloroetiiyldimetiiyl<br />
SUyl Enol Ethers<br />
ENOXY<br />
LIGAND<br />
^<br />
o'''<br />
a^<br />
6<br />
0'"^<br />
1 °^'^<br />
^ • . . . .<br />
O'"^<br />
YIFID<br />
(%)<br />
91<br />
97<br />
39<br />
33<br />
43<br />
96<br />
53<br />
96^<br />
6 4.10(d)<br />
6 3.90(d)<br />
5 4.03(d)<br />
6 3.95(d)<br />
6 4.85(br t)<br />
6 4.93(d)<br />
6 4.42(d)<br />
6 4.06(d)<br />
6 4.01(d)<br />
6 4.67(br s)<br />
6 4.16(d)<br />
6 3.99(d)<br />
iH NMR^<br />
97c<br />
5 4.08(s)<br />
6 4.00(s)<br />
6 3.96(s)<br />
6 4.92(br t)<br />
6 4.02(s)<br />
6 4.67(br s)<br />
NOTES: a) ^H NMR diagnostic peaks, blanks denote peak not present or not identified<br />
b) hydrogcn(s) on C* c) hydrogen(s) on C* d)HA methine hydrogen<br />
98^^<br />
6 3.46(d of d)<br />
6 3.38(dofd)<br />
6 3.47(d of d)<br />
6 3.42(d of d)<br />
5 3.43(m)<br />
6 3.75(m)<br />
36
When the three step procedure was performed via tiie silyl enol ethers 100 and 103<br />
(Table 3.1) without intermediate purification steps, alcohols 108 and 109 were isolated in<br />
22% and 25% overall yields, respectively, from the ketones. Once again, no other<br />
characterizable products were isolated.<br />
SIMe. SIMer<br />
108 109<br />
SUyl enol ether 101 also underwent radical cyclization and subsequent<br />
nucleophilic ring opening. In this case, however, cis and trans isomers are possible. In<br />
fact, the cis alcohol 110-cis was found to be the predominant isomer of an 8:2 mixture<br />
with trans alcohol 110-trans.^^ This selectivity is a consequence of the steric<br />
environment of the cycUzed radical, as the hydrogen atom source can approach more easily<br />
from opposite the newly formed carbon-carbon bond (Scheme 3.16). This type of<br />
stereoselectivity has been reported for other free-radical cyclizations.^^<br />
110-cis<br />
Scheme 3.16<br />
110-trans<br />
SiMer<br />
A careful examination of the ^H NMR spectrum of the cmde reaction mixture from<br />
the radical reaction of silyl enol ether 104 suggested tiiat no cycUzed product was formed.<br />
Signals were observed tiiat could belong to the directly reduced product. Furthermore, no<br />
identifiable product was isolable after metiiyUitiiium derivatization of this crude reaction<br />
mixture. This absence of cyclization is possibly a result of steric hindrance to die approach<br />
of the alkene by the p-silyl radical. This result was disappointing as silyl enol ether 104, if<br />
it did in fact cyclize, could be expected to show stereoselectivity in the radical addition. In<br />
37
fact, considering die selectivity of tiie hydrogen atom abstraction demonstrated in die<br />
selectivity for the formation of alcohol 110-cis, one would expect die selective formation<br />
of l-oxa-2-silacyclohexane 111. The net result would have been the selective formation of<br />
two new chiral centers.<br />
Me>. ,Me<br />
^81<br />
111<br />
Although the ^H NMR spectrum of the cmde reaction mixture from submission<br />
of 102 to the radical cycUzation conditions showed a complete loss of the signals for the<br />
enol ether protons and the protons on the CH2CI moiety, no signals characteristic of the<br />
cycUzed product nor the directly reduced product were evident. Reaction of the cmde<br />
radical reaction mixture with excess methyUitiiium produced no isolable products.<br />
Apparently this enol ether decomposed under the radical cycUzation conditions. It had been<br />
noted that enol ether 102 was more unstable than the other enol ethers used in this study.<br />
The free radical reaction of silyl enol ether 105 gave similar results. No products,<br />
or even characteristic ^H NMR peaks, were noted in either the cmde radical reaction<br />
mixture or the mixture resulting from reaction of this mixture with methyUithium. This was<br />
disappointing, as this reaction was to produce a novel product and serve as a mechanistic<br />
probe. The free radical cyclization of this compound would produce the intermediate<br />
radical 112, which would rapidly rearrange to a new silyl enol ether 113. Ketone 114,<br />
then, was the product expected after treatment of the cmde radical reaction mixture with<br />
methyUithium (Scheme 3.17).<br />
105<br />
- TBT-Cl<br />
Scheme 3.17<br />
MeLi<br />
114<br />
SiMe.<br />
38
A Stereoselective Tandem CvcUzation<br />
SUyl enol ether 115 was synthesized in order to provide evidence for the radical<br />
nature of this cycUzation protocol. If this reaction does, in fact, proceed tiirough an<br />
intermediate cyclized radical (in the case of 115, radical 116), then a rapid 5-exo<br />
cyclization of 116 would be expected to afford the spirobicyclic compound 117. It was<br />
not at all obvious upon inspection of the ^H NMR of the cmde radical reaction mixture that<br />
117 had been formed. However, treatment of die cmde reaction mixture with<br />
metiiyUithium followed by silica gel chromatography afforded the alcohols 118, in 30%<br />
overaU yield from l-heptene-2-one. It was noted that these diastereomeric alcohols were<br />
formed in a 6:1 ratio (discerned by ^H NMR and HPLC analysis of the crude reaction<br />
mixtures). The major isomer was predicted to be the E isomer, by analogy with similar<br />
selective cyclizations.^^ This assignment was supported by the fact that the isomer<br />
assigned as Z was the major isomer produced upon addition of 3-<br />
trimethylsilylpropylmagnesium bromide to 1-methylcyclopentanone. This exciting<br />
stereoselective transformation, a 'tandem cyclization', enriches the possible synthetic utility<br />
of this radical cyclization methodology.<br />
115<br />
118<br />
TBT-<br />
TBT-Cl<br />
SiMe3 -*-<br />
MeLi IP<br />
Scheme 3.18<br />
117<br />
Dienol ethers 119 and 120 could possibly undergo 'tandem cyclization' similar to<br />
tiiat shown above. Upon treatment of tiiese dienol ethers under the radical reaction<br />
conditions the chlorines were efficientiy abstracted. Both crude reaction mixtures were<br />
treated under the Tamao oxidation conditions (the two oxygen Ugands on the silicon allow<br />
for the success of the oxidation procedure).^^ Diol 92 was isolated (14%) from the<br />
39
mixture that originated with dienol ether 119. This is evidence that this silyl dienol ether<br />
underwent only a single cyclization.<br />
^ \<br />
120<br />
Unsvmmetrical Dialkyl Silvl Enol Ethers<br />
In an attempt to synthesize 2-chloroethylsilyl enol ethers that might exhibit better<br />
stability than the dimethylsUyl enol ethers 96, the unsymmetrical dialkyl-2-chloroethylsilyl<br />
chlorides 121 and 122 were synthesized by adding the appropriate nucleophiles to<br />
dichloro-2-chloroethyl-methylsilane. Unfortunately, the silyl enol ethers 123-126,<br />
derived from these chlorosilanes (and r-butyl-(2-chloroethyl)methyl chlorosilane, which<br />
was synthesized by another worker in this laboratory^^), did not exhibit extraordinary<br />
stabilities nor cyclization propensities. Although it was obvious that the radical cyclization<br />
was sensitive to the nature of the alkyl Ugands on sUicon, no real trend was clear. It was<br />
therefore determined that these chlorosUanes demonstrated no advantage for the possible<br />
development of this synthetic methodology.<br />
Me^ ^CH2CH3<br />
cr<br />
121 C'<br />
Me.. ^CH2CH3<br />
O^<br />
124<br />
CI<br />
Me.<br />
O'<br />
:si ^CH2CH3<br />
123<br />
126<br />
CI<br />
40
Conclusions<br />
A number of differently substituted 2-chloroethylsilyl enol etiiers have been<br />
synthesized. By utilizing the appropriate 2-chloroethylsilyl chloride starting materials,<br />
diaUcyl, alkyl-aUcoxy, and dialkoxy 59 substituted 2-chloroethylsilyl enol ethers were<br />
synthesized. Several of these compounds have been demonstrated to undergo free-radical<br />
cyclizations in a 6-endo fashion, when submitted under 'standard' free radical conditions.<br />
A competing reaction is the 'direct reduction' of die chloroetiiyl group. 6-endo radical<br />
cyclizations are rare, and as such these cyclizations are noteworthy. Apparently, the<br />
inclusion of sUicon in these molecules greatiy alters the stereoelectronic requirements for<br />
cycUzation.<br />
The unstable l-oxa-2-silacyclohexane compounds produced by these cyclizations<br />
are derivatized by oxidation by peroxide or by reaction with methylUthium. These<br />
transformations produce 1,4-diols or y-trimethylsilyl alcohols, respectively. The net<br />
reaction, then, is a 'reductive a-alkylation' of the starting ketones. The overall yields for<br />
the three step transformation - enol ether formation, radical cyclization, and derivatization,<br />
were consistantiy low, limiting this route's usefulness as a general synthetic methodology.<br />
However, the stereoselectivity found for the radical cyclization of the 2-chloroethylsUyl<br />
enol ether of cyclohexanone demonstrates a possible advantage of this intramolecular<br />
cyclization reaction. A tandem cyclization of a suitably substituted 2-chloroethylsilyl enol<br />
ether also proceeded with good stereoselectivity. Finally, it was noted that the cycUzation<br />
behavior of this system depends upon the substituents on silicon, although no trends were<br />
discemed for the dialkyl substituted system. These results have been reported in two<br />
preliminary communications.^^-^^ Improvements in the yields of these transformations<br />
could result in this methodology's becoming tmly useful.<br />
41
CHAPTER 4<br />
EXPERIMENTAL DETAU^S<br />
General Methods<br />
Unless otherwise noted all commerciaUy avaUable starting materials were used as<br />
received. Tetrahydrofuran (THF) and diethyl ether (ether) were distilled under nitrogen<br />
from a dark blue solution containing the sodium ketyl of benzophenone immediately before<br />
use. Dichloromethane and trietiiylamine were distiUed from calcium hydride immediately<br />
before use. EtiiyldUsopropylamine (Hunig's base), dimethylsulfoxide (DMSO), and<br />
hexamethylphosphorictriamide (HMPA) were distiUed over calcium hydride and stored<br />
over oven dried 4A molecular sieves. Etiiyl acetate and hexanes were distiUed before use.<br />
Benzene and toluene used for the free radical reactions were distilled under nitrogen from a<br />
dark blue solution of the sodium ketyl of benzophenone, then deoxygenated either by three<br />
freeze/vacuum purge/thaw cycles under a dry nitrogen atmosphere or by bubbling dry<br />
nitrogen through the solvent for 30 minutes, immediately before use.<br />
NMR spectra were obtained on either an IBM AF-200 (200 MHz for proton, 50<br />
MHz for carbon) or an IBM AF-300 (300 MHz for proton, 75 MHz for carbon)<br />
instmment. Unless otherwise noted, all spectra were obtained in deuterochloroform<br />
(CDCI3) solvent, with either residual chloroform or tetramethylsUane (TMS) as an intemal<br />
reference. Spectra are reported as follows: peak position (6) (multiplicity, coupling<br />
constant[s], number of protons). The peak position (6) is in parts per mUlion (ppm). The<br />
coupling constant (J) is in hertz (Hz).<br />
Infrared (IR) spectra were measured on either a Perkin Ehner 1600 series FT-IR or<br />
a Nicolet MX-S spectrometer. Samples were neat films or concentrated chloroform<br />
solutions between NaCl plates.<br />
High resolution mass spectral (HRMS) analyses were performed by the Midwest<br />
Center for Mass Spectrometry, Lincoln, Nebraska.<br />
Analytical thin layer chromatography (TLC) was performed using Merck silica gel<br />
60 F254 aluminum backed plates. Flash chromatography was performed according to the<br />
method reported by Still,^^ using 230-400 mesh silica gel.<br />
42
(33-Dimetiivlbutenvl-2-oxv)ethoxvmetiivl('chlorometiivl)-<br />
silane (80)<br />
^OCH2CH3<br />
.81.<br />
CioH2iC102Si f.w. = 236.82<br />
rt-Butyllithium (1.7 ml of a 2.4 M solution in hexane, 4.0 mmol) was added<br />
dropwise over a 5 minute period to 0.57 ml (4.0 mmol) dUsopropylamine and 10 ml ether<br />
stirring at -78° under nitrogen. After stirring for 10 minutes, 0.50 ml (4.0 mmol)<br />
pinacolone was added, and the mixture was stirred at -78° for an additional 30 minutes.<br />
Dichloro(chloromethyl)methylsUane 77 (0.51 ml, 4.0 mmol)was carefully added, and the<br />
resulting mixture allowed to warm to -0° over one hour. Triethylamine (0.56ml, 4.0<br />
mmol), then dry ethanol (0.23 ml, 4.0 mmol) were added, and the milky suspension<br />
aUowed to warm to room temperature overnight The reaction mixture was added to a<br />
seperatory funnel containing 25 ml ether and 50 ml saturated aqueous NaHCOs, the layers<br />
seperated, and the aqueous layer extracted with three 25 ml portions of ether. The organic<br />
extracts were then combined, washed with 25 ml brine, dried over MgS04, and<br />
concentrated under vacuum to afford 0.75 g (79%) of a clear liquid. Although ^H and ^^c<br />
NMR analysis suggested that the desired product was formed, attempts at purification by<br />
flash column chromatography on silica gel or neutral alumina were unsuccessful.<br />
Purification by distillation was successful, although only 0.25 g (26%) of the product was<br />
isolated.<br />
iR (200 MHZ) 6 4.17 (d, J=1.7, IH), 4.14 (d, J=1.7, IH), 3.86 (q, J=7.0, 2H),<br />
2.86 (s, 2H), 1.26 (t, J=7.0, 3H), 1.07(s, 9H), 0.35 (s, 3H).<br />
13C (50 MHZ) 6 166.05, 87.49, 59.20, 36.59, 27.99, 26.74, 18.16, -6.21.<br />
BP 58° at 1.0 mm Hg.<br />
43
(33-Dimethvlbutene-2-oxv)ethoxv-2-chloroethvl-<br />
metiivlsilane (86^<br />
O'<br />
^OCH2CH3<br />
Si.<br />
CI<br />
Ci iH23C102Si f.w. = 250.84<br />
n-ButyUithium (1.7 ml of a 2.4 M solution in hexane, 4.0 mmol) was added<br />
dropwise over a 5 minute period to 0.57 ml (4.0 mmol) dUsopropylamine and 10 ml ether<br />
stirring at -78° under nitrogen. After stirring for 10 minutes, 0.50 ml (4.0 mmol)<br />
pinacolone was added, and the mixture was stirred at -78° for an additional 30 minutes.<br />
Dichloro(2-chloroethyl)methylsilane (0.71 ml, 4.0 mmol)was carefully added, and the<br />
resulting mixture aUowed to warm to 0° over one hour (at —15° [bath temperature] a white<br />
precipitate separated from the reaction mixture). Triethylamine (0.56ml, 4.0 mmol), then<br />
dry ethanol (0.23 ml, 4.0 mmol) were added, and the milky suspension allowed to warm to<br />
room temperature overnight. The reaction mixture was added to a separatory funnel<br />
containing 25 ml ether and 50 ml saturated aqueous NaHCOs, the layers separated, and the<br />
aqueous layer extracted with three 25 ml portions of ether. The organic extracts were then<br />
combined, washed with 25 ml brine, dried over MgS04, and concentrated under vacuum to<br />
afford 0.95 g (3.8 mmol, 95%) of a clear liquid. ^H and 13c NMR suggested tiiat the<br />
desired product was formed cleanly. Although this product could not be purified by flash<br />
column chromatography, distUlation under high vacuum was successful, allowing the<br />
isolation of a good yield of pure product.<br />
IH (300 MHZ) 6 4.20 (d, J=1.6, IH), 4.15 (d, J=1.6, IH), 3.86 (q, J=7.0, 2H),<br />
3.77 (d of d , 1=9.7,7.8, 2H), 1.43 (d of d, 1=10.6,7.8, 2H), 1.27 (t, J=7.0, 3H),<br />
1.04 (s, 9H), 0.22 (s, 3H).<br />
13C (75 MHZ) 6 166.00, 87.06, 58.64, 41.75, 36.40, 27.97, 20.82, 18.18, -4.12.<br />
IR 2980, 1680, 1660, 1560, 1520, 1385, 1322.<br />
B P 66-69° at 0.5 mm Hg.<br />
44
3-(tert-Butvl)-l-ethoxv-l-methvl-l-sila-2-oxacvclohexane<br />
{901<br />
EtO^ ^Me<br />
CiiH2402Si f.w-= 216.40<br />
A 500 ml three necked flask, fitted with a stopper, a water jacketed reflux condensor, and a<br />
pressure equaUzing 100 ml addition funnel, was carefully flushed with nitrogen, then<br />
charged with 2.5 g (10 mmol) freshly distiUed sUyl enol etiier 86 and 450 ml dry,<br />
deoxygenated benzene. The addition funnel was charged with 2.7 ml (10 mmol) tributyltin<br />
hydride, 0.025 g (0.15 mmol) AIBN, and 25 ml benzene. The flask was then immersed in<br />
an oil bath and brought to reflux, then the contents of the addition funnel were added in<br />
eight portions over a four hour period. The mixture was allowed to reflux for an additional<br />
15 hours, then cooled and concentrated under vacuum to afford a clear oil, consisting of a<br />
1:1 mixture of the 1:1 mixture of diastereomers 90 and the sUyl enol ether 88, as well as a<br />
large amount of tin containing byproducts (^H NMR). Distillation under a nitrogen<br />
atmosphere afforded 1.14 g (5.3 mmol, 53%) of various mixtures of 90 and 88 distilling<br />
over the range 40-47°, with the lowest boding fraction almost pure 'directiy reduced<br />
product' 88, and with the final fraction almost pure l-oxa-2-silacyclohexane 90. The final<br />
fraction was separated by HPLC to afford the pure diastereomers of 90. This product<br />
decomposed rapidly upon chromatography, and slowly (ti/2 =-7 days) upon storage.<br />
Less polar diastereomer:<br />
iR (200 MHZ, relative to CHCI3 at 6 7.24) 6 3.78 (q, J=7.0, 2H), 3.39 (d of d,<br />
1=11.0,2.1, IH), 2.12-1.98 (m, IH), 1.69-1.60 (m, IH), 1.56-1.33 (m, IH),<br />
1.24-1.11 (m, IH), 1.20 (t, J=7.0, 3H), 0.846 (s, 9H), 0.60-0.51 (m, 2H), 0.10<br />
(s, 3H).<br />
13C (75 MHZ) 6 83.68, 58.11, 29.33, 26.35, 25.71, 21.99, 18.48, 11.18, -4.19.<br />
B P 47°.<br />
MS (GC, retention time=283 seconds, m/e) 216 (M+), 201 (M+-CH3), 159 (M+-<br />
C(CH3)3, base peak).<br />
45
More polar diastereomer:<br />
iR (300 MHZ, relative to CHCI3 at 6 7.24) 6 3.75 (q, J=7.0, 2H), 3.47 (d of d,<br />
1=11.1,1.8, IH). 2.12-1.98 (m, IH), 1.69-1.60 (m, IH), 1.56-1.33 (m, IH),<br />
1.24-1.11 (m, IH), 1.23 (t, J=7.0, 3H), 0.834 (s, 9H), 0.83-0.73 (m, IH), 0.55-<br />
0.30 (m, IH), 0.08 (s, 3H).<br />
13C (75 MHZ) 6 82.59, 57.82, 29.27, 26.35, 25.66, 21.92, 18.42, 11.48, -3.05.<br />
B P 47°.<br />
MS (GC, retention time=293 seconds, m/e) 216 (M+), 201 (M+-CH3), 159 (M+-<br />
C(CH3)3, base peak).<br />
Directly reduced silyl enol ether (88):<br />
IR (300 MHZ, relative to CHCI3 at 6 7.24) 6 4.08 (d, J=1.3, 2H), 3.78 (q, 7.0, 2H),<br />
B P 40°.<br />
1.21 (t, J=7.0, 3H), 1.05 (s, 9H), 0.97 (t, 1=8.9, 3H), 0.70-0.55 (m, 2H), 0.15<br />
(s, 3H).<br />
MS (GC, retention time=238 seconds, m/e) 216 (M+), 201 (M+-CH3), 159 (M+-<br />
C(CH3)3, base peak), 117 (M+-C6H11O).<br />
5.5-Dimetiivl-1.4-hexanediol (92)<br />
C8H18O2 f.w. = 146.23<br />
Aqueous H2O2 (5.88 ml of a 30% solution, 58 mmol), then KHF2 (0.75 g, 9.6<br />
mmol) were added to 0.13 g (0.60 mmol) of a 2:1 mixture of l-oxa-2-silacyclohexane 90<br />
and silyl enol ether 88 stirring at room temperature in 10 ml DMF. The resulting mixture<br />
was warmed to 60° (oil bath temperature) for 48 hours, then cooled and washed with 50 ml<br />
distilled water. The aqueous layer was extracted with 3x25 ml ether, and the organic<br />
extracts combined, washed with 25 ml brine, dried over MgS04, and concentrated under<br />
vacuum. Flash column chromatography (20 g sigel, 8:2 hexanes:etiiyl acetate eluent)<br />
aforded 0.024 g (0.16 mmol, 27%) of the desired product as a clear oU.<br />
46
IR (300 MHZ, rel to CHCI3 at 6 7.26) 6 3.63 (m, 2H), 3.17 (d of d, 1=10.5,1.3,<br />
IH), 2.53 (br s, 2H[variable, -OH]), 1.66 (m, 2H), 1.25 (m, 2H), 0.84 (s, 9H).<br />
13C (75 MHZ) 6 80.17, 63.07, 34.98, 30.44, 28.43, 25.67.<br />
IR 3331, 2955, 2868.<br />
TLC Rf = 0.50 (1:1 hexanes:ediyl acetate eluent).<br />
GC (GC, retention time = 5.04 min, m/e) 89 (M+ - C(CH3)3, base peak).<br />
Chloro(2-chloroethvl)dimethvlsilane(93)<br />
:8i<br />
CK<br />
^Cl<br />
C4HioCl2Si f.w. = 157.12<br />
MethyUithium (60 ml of a 1.4 M solution in hexanes, 84 mmol) was added to 15 ml<br />
(84 mmol) dichloro(2-chloroethyl)methylsUane and 25 ml ether stirring in a 100 ml round<br />
bottomed flask under a nitrogen atmosphere at -78°(bath temperature). After one hour, the<br />
mixture was allowed to warm to room temperature, and the resulting milky suspension was<br />
allowed to stir for an additional 12 hours. The suspension was then filtered through a 5 cm<br />
pad of oven dried ceUte and concentrated under vacuum, taking care to minimize exposure<br />
to the atmosphere. Distillation under aspirator vacuum (-35 mm Hg) afforded 6.8 g (43<br />
mmol, 51%) of the pure product as the fraction distilling between 70-75°. The lower<br />
boiUng fractions were mixtures of the desired product and dichlorodimethylsilane. NOTE:<br />
care must be taken to avoid heating this material too >100°, as its decomposition with the<br />
loss of ethene is facile.<br />
iR (300 MHZ, relative to CHCI3 at 6 7.26) 6 3.76-3.71 (m, 2H), 1.53-1.48 (m, 2H),<br />
0.48 (s, 6H).<br />
13C (50 MHZ) 6 41.70, 24.39, 1.94.<br />
IR 2960 cm-1.<br />
B P 70-75° at 35 mm Hg.<br />
Density (three measurements) 1.06 g/ml.<br />
47
Diethoxvdimetiivlsilane (95)<br />
CH3\ ^OCH2CH3<br />
CH3^ '^OCH2CH3<br />
C6Hi602Si f.w. = 148.28<br />
Ethanol (0.64 ml, 11 mmol) was added to 1.5 ml (11 mmol) triethylamine, 10 ml<br />
dichlorometiiane, and 0.64 g of a compound proposed to be dichlorodimetiiylsUane 94<br />
(5.0 mmol if so), stirring in a 10 ml pear shaped flask under a nitrogen atmosphere at room<br />
temperature. After one hour, die milky suspension was added to a separatory funnel<br />
containing 15 ml distilled water, die layers separated, and die organic layer washed with 10<br />
ml brine. The organic extract was dried over MgS04 and concentrated under vacuum to<br />
afford 0.52 g (3.5 mmol, 70%) of the product 95, >90% pure by iR NMR. The crude ^H<br />
NMR data was deemed sufficient to prove the nature of the starting material, and as such<br />
this volatile product was not purified or characterized further.<br />
IR (200 MHZ, relative to CHCI3 at 6 7.26) 6 3.76 (q, 1=6.3, 4H), 1.21 (t, J=6.3,<br />
6H), 0.13 (s, 6H).<br />
(3.3-Dimethylbutene-2-oxy)(2-chloroethyl)dimethylsUane<br />
199)<br />
CioH2iC10Si f.w. = 220.82<br />
Lithium dusopropylamide (0.37 ml of a 1.5 M solution in hexane, 0.55 mmol) was<br />
added dropwise to 0.063 ml (0.50 mmol) pinacolone, 0.5 ml HMPA, and 1 ml THF<br />
stirring in a 5 ml pear shaped flask at -78° (bath temperature) under a nitrogen atmosphere.<br />
This mixture was stirred for 10 minutes, and then 0.078 ml (1.1 mmol)<br />
48
chloro(2-chloroetiiyl)dimetiiylsilane, 93, was added. The resulting solution was allowed<br />
to warm to room temperature over two hours and stirred at room temperature for an<br />
additional 10 hours, tiien quenched by addition to 15 ml distilled water and 10 ml ether in a<br />
separatory funnel. The layers were separated, the aqueous phase extracted with 3x10 ml<br />
ether, and die combined organic extracts washed with 10 ml brine. Removal of the<br />
solvents under vacuum after drying over MgS04 afforded 0.10 g (0.46 mmol, 91% ) yield<br />
of the desired product, contaminated with a small amount of a siUcon containing impurity<br />
(according to ^H NMR).<br />
iR (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.10 (d, 1=1.7, IH), 3.90 (d, J=1.7,<br />
IH), 3.78-3.70 (m, 2H), 1.44-1.35 (m, 2H), 1.04 (s, 9H), 0.26 (s, 6H).<br />
13C (50 MHZ) 5 166.75, 86.05, 42.24, 36.39, 28.00, 23.15, -1.24.<br />
IR 3122, 2962 cm-l.<br />
(l-CVclohexvlethenvloxv)(2-chloroethvl)dimethvlsilane<br />
£1001<br />
Ci2H23C10Si f.w. = 246.89<br />
Lithium diisopropylamide (0.37 ml of a 1.5 M solution in hexane, 0.55 mmol),<br />
followed immediately by 0.5 ml HMPA, were added dropwise to 0.069 ml (0.50 mmol)<br />
cyclohexyl methyl ketone, 0.082 ml (0.55 mmol) chloro(2-chloroethyl)dimethylsilane,<br />
93,and 2 ml THF stirring in a 5 ml pear shaped flask at -78° (bath temperature) under a<br />
nitrogen atmosphere. The resulting solution was allowed to warm to room temperature<br />
over two hours, stirred at room temperature for an additional 2 hours, then quenched by<br />
addition to 5 ml distiUed water and 5 ml ether in a separatory funnel. The layers were<br />
separated, the aqueous phase extracted with 3x10 ml ether, and die combined organic<br />
extracts washed with 10 ml brine. Removal of the solvents under vacuum after drying over<br />
49
50<br />
MgS04 afforded 0.12 g (0.49 mmol, 97%) of the desired product as a clear oil, >95% pure<br />
(according to iR NMR).<br />
iR (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.03 (d, 1=1.1, IH), 3.95 (d, 1=1.1,<br />
IH), 3.77-3.68 (m, 2H), 1.81-1.73 (m, 5H), 1.43-1.34 (m, 2H), 1.24-1.11 (m,<br />
6H), 0.25 (s, 6H).<br />
13C (50 MHZ) 6 163.71, 87.62, 44.24, 42.23, 30.81, 26.17, 24.80, 23.07, -1.16.<br />
(2-Chloroethvl)( 1 -cvclohexenvloxv)dimethvlsilane (101)<br />
CioHi9C10Si f.w. = 218.80<br />
Lithium diisopropylamide (0.37 ml of a 1.5 M solution in hexane, 0.55 mmol),<br />
followed immediately by 0.5 ml HMPA, were added dropwise to 0.052 ml (0.50 mmol)<br />
cyclohexanol, 0.082 ml (0.55 mmol) chloro(2-chloroethyl)dimethylsilane, 93,and 2 ml<br />
THF stirring in a 10 ml pear shaped flask at -78° (bath temperature) under a nitrogen<br />
atmosphere. The resulting solution was allowed to warm to room temperature over two<br />
hours, stirred at room temperature for an additional 2 hours, then quenched by addition to<br />
15 ml distilled water and 10 ml ether in a separatory funnel. The layers were separated, the<br />
aqueous phase extracted with 3x10 nd ether, and the combined organic extracts washed<br />
with 10 ml brine. Removal of the solvents under vacuum after drying over MgS04<br />
afforded 0.066 g (0.19 mmol, 39% adjusted for impurities) of the desired product and<br />
silicon containing impurities as a -1:1 mixture (according to ^H NMR).<br />
iR (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.85 (br t, 1=3.8, IH), 3.76-3.62 (m,<br />
2H), 2.00-1.98 (m, 4H), 1.69-1.63 (m, 2H), 1.56-1.48 (m, 2H), 1.36-1.27 (m,<br />
2H), 0.22 (s, 6H).<br />
13C (50 MHZ, partial) 6 104.55, 42.38, 29.78, 24.33, 23.74, 23.08, 22.22, -0.94.
(l-Phenvlethenvloxv)(2-chloroetiivl)dimethvlsUane(102)<br />
x^<br />
CnHnClOSi f.w. = 240.83<br />
Lithium diisopropylamide (0.73 ml of a 1.5 M solution in hexane, 1.1 mmol) was<br />
added dropwise to 0.12 ml ( 1.0 mmol) acetophenone, 1 ml HMPA, and 2 ml THF stirring<br />
in a 10 ml pear shaped flask at -78° (bath temperattire) under a nitrogen atmosphere. This<br />
mixture was stirred for 10 minutes,and then 0.24 g (1.5 mmol) chloro(2-chloroethyl)-<br />
dimethylsilane, 93, was added. The resulting solution was aUowed to warm to room<br />
temperature over two hours and stirred at room temperature for an additional 5 hours, then<br />
quenched by addition to 15 ml distiUed water and 10 ml ether in a separatory funnel. The<br />
layers were separated, the aqueous phase extracted with 3x10 ml ether, and the combined<br />
organic extracts washed with 10 ml brine. Removal of the solvents under vacuum after<br />
drying over MgS04 left a 4:1 mixture of the desired product and unwanted sUicon<br />
containing byproducts (according to ^H NMR). Flash column chromatography (20 g silica<br />
gel, hexanes eluent) afforded 0.12 g of a 1:1 mixture of the desired product and the siUcon<br />
byproducts, corresponding to -0.33 mmol (33%) of the product, corrected for its purity.<br />
^n(desired product) (200 MHZ, relative to CHCI3 at 6 7.26) 6 7.59-7.54 (m,<br />
2H), 7.37-7.31 (m, 3H), 4.93 (d, 1=1.9, IH), 4.42 (d, 1=1.9, IH), 3.70-3.62<br />
(m, 2H), 1.28-1.20 (m, 2H), 0.32 (s, 6H).<br />
^H(impurity) (200 MHZ, relative to CHCI3 at 6 7.26) 6 3.79-3.70 (m, 2H),<br />
1.50-1.42 (m, 2H), 0.13 (s, 6H).<br />
51
(2-Chloroethvl)(l-heptenvl-2-oxv)dimethvlsUane(103)<br />
CiiH230ClSi f.w. = 234.84<br />
Lithium dusopropylamide (0.73 ml of a 1.5 M solution in hexane, 1.1 mmol) was<br />
added dropwise to 0.14 ml ( 1.0 mmol) 2-heptanone, 1 ml HMPA, and 2 ml THF stirring<br />
in a 10 ml pear shaped flask at -78° (bath temperature) under a nitrogen atmosphere. This<br />
mbcture was stirred for 10 minutes,and then 0.24 g (1.5 mmol) chloro(2-chloroethyl)-<br />
dimethylsilane, 93, was added. The resulting solution was aUowed to warm to room<br />
temperature over two hours and stirred at room temperature for an additional 5 hours, then<br />
quenched by addition to 15 ml distiUed water and 10 ml ether in a separatory funnel. The<br />
layers were separated, the aqueous phase extracted with 3x10 ml ether, and the combined<br />
organic extracts washed with 10 ml brine. Removal of the solvents under vacuum after<br />
drying over MgS04, foUowed by flash column chromatography (20 g siUca gel, hexanes<br />
eluent) afforded 0.10 g (0.43 mmol, 43%) of the desired product, contaminated with
(3'S.6'R)-(2-Chloroethvl)(3'-(l"-methvlethvl)-5'-methvl-<br />
r-cvclohexenvl-2'-oxv)dimethvlsilane(104)<br />
Ci4H27C10Si f.w. = 274.95<br />
Lithium diisopropylamide (0.68 ml of a 1.5 M solution in hexane, 1.1 mmol),<br />
followed immediately by 1 ml HMPA, were added dropwise to 0.17 ml (1.0 mmol)<br />
menthone, 0.15 g (1.0 mmol) chloro(2-chloroethyl)dimethylsilane, 93, and 4 ml THF<br />
stirring in a 10 ml pear shaped flask at -78° (bath temperature) under a nitrogen atmosphere.<br />
The resulting solution was allowed to warm to room temperature over two hours and<br />
stirred at room temperature for an additional 3 hours, then quenched by addition to 15 ml<br />
distUled water and 10 ml hexanes in a separatory funnel. The layers were seperated and the<br />
aqueous layer wasextracted with 3x10 ml hexanes. The combined organic extracts were<br />
dried over MgS04 and filtered through a 2 cm pad of ceUte. Removal of the solvents under<br />
vacuum afforded 0.26 g (0.96 mmol, 96% yield) of the desu-ed product in -90% purity<br />
(according to^H NMR).<br />
1R (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.67 (br s, IH), 3.77-3.68 (m, 2H),<br />
2.25-1.97 (m, 2H), 1.41-1.32 (m, 2H), 1.31-1.20 (m, 5H), 0.94-0.83 (m, 6H),<br />
0.74 (d, 1=6.7, 3H), 0.23 (s, 6H).<br />
53
l-Cvclopropvlethenvloxv(2-chloroethvl)dimethvlsilane<br />
005)<br />
CgHnClOSi f.w. = 204.77<br />
Lithium diisopropylamide (0.67 ml of a 1.5 M solution in hexane, 1.0 mmol),<br />
followed immediately by 1 ml HMPA, were added dropwise to 0.099 ml (1.0 mmol)<br />
cyclopropyl metiiyUcetone, 0.15 g (1.0 mmol) chloro(2-chloroethyl)dimethylsilane, 93,<br />
and 4 ml THF stirring in a 10 ml pear shaped flask at -78° (badi temperature) under a<br />
nitrogen atmosphere. The resulting solution was allowed to warm to room temperature<br />
over two hours and stirred at room temperature for an additional three hours, then<br />
quenched by addition to 15 ml distilled water and 10 ml hexanes in a separatory funnel.<br />
The layers were seperated and the aqueous layer was extracted with 3x10 ml hexanes. The<br />
combined organic extracts were dried over MgS04 and filtered through a 2 cm pad of<br />
celite. Removal of the solvents under vacuum afforded 0.11 g (0.53 mmol, 53% yield) of<br />
the desired product of -90% purity (according to ^H NMR).<br />
1R (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.16 (d, 1=1.3, IH), 3.99 (d, 1=1.3,<br />
IH), 3.74-3.66 (m, 2H), 1.41-1.33 (m, 2H), 0.59-0.55 (m, 4H), 0.88 (br t,<br />
1=0.9, IH), 0.25 (s, 6H).<br />
2.2-Dimethvl-6-trimethylsilyl-3-hexanol(107)<br />
SIMe.<br />
CiiH260Si f.w. = 202.41<br />
MethyUithium (2.0 ml of a 1.4 M solution in hexanes, 1.8 mmol) was slowly added<br />
to 5 ml ether and die cmde reaction mixture (a -1; 1 mixture of tiie cycUzed [106] and<br />
54
directiy reduced products by iR NMR) from free radical reaction of 0.225 mmol silyl enol<br />
ether 99, stirring in a 10 ml round bottomed flask under a nitrogen atmosphere at room<br />
temperature. After two hours, the mbcture was quenched by careful addition to a<br />
separatory funnel containing 10 ml etiier and 10 ml saturated aqueous ammonium chloride,<br />
die layers separated, and die aqueous layer extracted witii 3x5 ml etiier. The combined<br />
organic extracts were then washed with brine, dried over MgS04, and concentrated under<br />
vacuum to afford the desired product as a minor component of a yellow gum. Flash<br />
column chromatography (10 g siUca gel, 8:2 hexanes:ethyl acetate eluent) afforded 0.0069<br />
g (0.034 mmol, 15% from sUyl enol ether, 30% for tiiis reaction if the purity of the starting<br />
material is taken into account) of the desired product as fine white crystals.<br />
IR (300 MHZ, relative to CHCI3 at 6 7.26) 5 3.19 (br s, IH), 1.56 (br s, lH[OH,<br />
variable]), 1.60-1.20 (m, 4H), 0.89 (s, 9H), 0.50 (d of t, 1=11.2,4.8, 2H), -0.02<br />
(s, 9H).<br />
13C (50 MHZ) 6 79.73, 35.46, 34.86, 25.69, 21.42, 16.77, -1.64.<br />
1 -Cvclohexvl-4-trimethylsilyl-1 -butanol (108)<br />
8i(CH3).<br />
Ci3H280Si f.w. = 228.50<br />
MethyUithium (1.5 ml of a 1.4 M solution in hexanes, 2.0 mmol) was slowly added<br />
to 5 ml ether and the cmde reaction mixture (a 1:1:1 mixture of starting material, the<br />
cycUzed material, and the directiy reduced material) from free radical reaction, using<br />
stoichiometric tin hydride, of 0.50 mmol silyl enol ether 100, stirring in a 10 ml round<br />
bottomed flask under a nitrogen atmosphere at room temperature. After two hours, the<br />
mixture was quenched by careful addition to a separatory funnel containing 10 ml etiier and<br />
10 ml saturated aqueous ammonium chloride, the layers separated, and the aqueous layer<br />
extracted with 3x5 ml ether. The combined organic extracts were then washed widi brine,<br />
dried over MgS04, and concentrated under vacuum to afford die desired product as a minor<br />
component of a yellow oil (^H NMR). Flash column chromatography (10 g sUica gel, 8:2<br />
55
56<br />
hexanes:etiiyl acetate eluent) afforded 0.025 g (0.034 mmol, 22% from cyclohexyl methyl<br />
ketone) of the desked product as a clear oil.<br />
IR (300 MHZ, relative to CHCI3 at 6 7.26) 6 3.25-3.21 (m, IH), 1.72 (br s, lH[OH,<br />
variable], 1.56-1.44 (m, 13H), 1.24-1.18 (m, 2H), 0.96-0.72 (m, 2H), -0.02 (s,<br />
9H).<br />
13C (50 MHZ) 6 75.99, 43.65, 38.05, 29.19, 27.72, 26.54, 20.39, 16.75, -1.66.<br />
MS (high resolution, m/e) 137.1331 (M+-H2O, Si(CH3)3, 0.6 ppm deviation).<br />
l-TrimethvlsUvl-4-nonanol (109)<br />
.8iMe3<br />
Ci2H280Si f.w. =216.49<br />
MethyUithium (1.5 ml of a 1.4 M solution in hexanes, 2.0 mmol) was slowly added<br />
to 5 ml ether and the cmde reaction mixture (an -1:1 mixture of starting material, the<br />
cycUzed material, and the directiy reduced material) from free radical reaction, using<br />
stoichiometric tin hydride, of 0.50 mmol silyl enol ether 103, stirring in a 10 ml round<br />
bottomed flask under a nitrogen atmosphere at room temperature. After two hours, the<br />
mixture was quenched by careful addition to a separatory funnel containing 10 ml ether and<br />
10 ml saturated aqueous ammonium chloride, the layers separated, and the aqueous layer<br />
extracted with 3x5 ml ether. The combined organic extracts were then washed with brine,<br />
dried over MgS04, and concentrated under vacuum to afford the desired product as a minor<br />
component of a yellow oU (^H NMR). Flash column chromatography (10 g sUica gel, 8:2<br />
hexanes:ethyl acetate eluent) afforded 0.028 g (0.13 mmol, 25% from cyclohexyl metiiyl<br />
ketone) of the desired product as a clear oil.<br />
iR (300 MHZ, relative to CHCI3 at 6 7.26) 6 3.66-3.52 (m, IH), 1.49-1.13 (m,<br />
lOH), 0.91-0.85 (m, 6H), 0.54-0.44 (m, 2H), -0.02 (s, 9H).<br />
13C (50 MHZ) 6 71.79, 41.48, 37.51, 31.91, 25.34, 22.66, 20.06, 16.75, 14.06,<br />
-1.66.<br />
MS (high resolution, m/e) 129.0735 (M+-C5Hii,CH3, 0.6 ppm deviation).
2-(2'-TrimethylsUvlethvl)cvclohexanol (110)<br />
8i(CH3),<br />
CiiH240Si f.w. = 200.44<br />
MethyUitiiium (1.5 ml of a 1.4 M solution in hexanes, 2.0 mmol) was slowly added<br />
to 5 ml ether and the cmde reaction mixttire (an -1:1 mixture of cyclized and directiy<br />
reduced products) from free radical reaction, using stoichiometric tin hydride, of 0.50<br />
mmol silyl enol ether 101, stirring in a 10 ml round bottomed flask under a nitrogen<br />
atmosphere at room temperature. After two hours, the mixture was quenched by careful<br />
addition to a separatory funnel containing 10 ml ether and 10 ml saturated aqueous<br />
ammonium chloride, the layers separated, and the aqueous layer extracted with 3x5 ml<br />
ether. The combined organic extracts were then washed with brine, dried over MgS04,<br />
and concentrated under vacuum to afford the desired product as a component of a yellow<br />
oU. HPLC analysis of this mixture suggests a 4:1 mixture of diastereomers. Flash column<br />
chromatography (10 g sUica gel, 8:2 hexanes:ethyl acetate eluent) afforded 0.019 g (0.097<br />
mmol, 19% from cyclohexanone) of the desired product as a clear oil. The diastereomers<br />
were well separated by careful flash chromatography of this oil (10 g sUica gel, hexanes -<br />
8:2 hexanes:ethyl acetate eluent gradient).<br />
Major Diastereomer (110-cis, as shown):<br />
IR (300 MHZ, relative to CHCI3 at 5 7.26) 6 3.29-3.17 (br s, IH,), 1.96-1.62 (m,<br />
4H), 1.43 (br s, lH[OH, variable]), 1.29-1.02 (m, 6H), 0.88 (m, IH), 0.60 (d of<br />
t, 1=14.5,4.2, IH), 0.32 (d of t, 1=4.2,14.5, IH), -0.02 (s, 9H).<br />
13C (50 MHZ) 6 74.43, 47.65, 35.73, 29.39, 25.84, 25.57, 24.95, 12.68, -1.77.<br />
MS (high resolution, mixture of diastereomers, m/e) 200.1595 (M+, 0.5 ppm<br />
deviation), 185.1354 (M+-CH3, 4.1 ppm deviation), 170.1129 (M+-C2H6, 1.2<br />
ppm deviation), 167.1257 (M+-CH3,H20, 0.6 ppm deviation).<br />
RPLC tR=870 seconds, 9:1 hexanes:ethyl acetate eluent at 1.0 ml/min on a 25 cm<br />
(4.6 mm ID) Dupont Zorbax siUca gel column.<br />
57
Minor Diastereomer (110-trans, not shown):<br />
IR (300 MHZ, relative to CHCI3 at 6 7.26) 6 3.94 (br s, IH), 1.82-1.16 (m, 12H),<br />
0.47 (m, 2H), -0.01 (s, 9H).<br />
RPLC tR=678 seconds, 9:1 hexanes:ethyl acetate eluent at 1.0 ml/min on a 25 cm<br />
(4.6 mm ID) Zorbax silica gel column.<br />
(2-Chloroethvl)ri -6-heptadien-2-vloxv)dimethvlsUane (115)<br />
CiiH2iC10Si f.w. = 232.83<br />
Lithium dusopropylamide (0.68 ml of a 1.5 M solution in hexane, 1.1 mmol),<br />
followed immediately by 1 ml HMPA, were added dropwise to 0.13 g (1.1 mmol) 1-<br />
heptene-6-one,62 0.15 g (1.0 mmol) chloro(2-chloroethyl)dimethylsUane, 93, and 4 ml<br />
THF stirring in a 10 ml pear shaped flask at -78° (bath temperature) under a nitrogen<br />
atmosphere. The resulting solution was allowed to warm to room temperattu-e over two<br />
hours and stirred at room temperature for an additional 3 hours, then quenched by addition<br />
to 15 ml distilled water and 10 ml ether in a separatory funnel. The phases were separated<br />
and the organic layer was dried over MgS04 and filtered tiirough a 2 cm pad of celite.<br />
Removal of the solvents under vacuum afforded 0.28 g (1.2 mmol, >100% yield) of the<br />
desired product, contaminated with a silicon containing impurity (^H NMR). Flash column<br />
chromatography (20 g silica gel, hexanes eluent) afforded 0.16 g (0.68 mmol, 68%) of the<br />
desired product<br />
iR (300 MHZ, relative to CHCI3 at 5 7.26) 6 5.87-5.74 (m, IH), 5.01 (d, J=19.2,<br />
IH), 4.97 (d, 1=7.9, IH), 4.07 (d, J=l.l, IH), 4.03 (d, J=l.l, IH), 3.75-3.69<br />
(m, 2H), 2.10-1.99 (m, 4H), 1.59-1.49 (m, 2H), 1.35-1.28 (m, 2H), 0.25 (s,<br />
6H).<br />
13C (75 MHZ) 5 158.84, 138.48, 114.73, 90.32, 42.13, 35.71, 30.03, 25.98, 22.94,<br />
-1.19.<br />
58
2-Methyl-l-(3-trimethvkilylDropvl)cvcloDentanol (118)<br />
8i(CH3)3<br />
Ci2H260Si f.w. = 214.43<br />
MethyUithium (1.5 ml of a 1.4 M solution in hexanes, 2.0 mmol) was slowly added<br />
to 5 ml ether and the cmde reaction mixture from free radical reaction, using stoichiometric<br />
tin hydride, of 0.50 mmol silyl enol ether 115, stirring in a 10 ml round bottomed flask<br />
under a nitrogen atmosphere at room temperature. After two hours, the mixture was<br />
quenched by careful addition to a separatory funnel containing 10 ml ether and 10 ml<br />
saturated aqueous ammonium chloride, the layers separated, and the aqueous layer<br />
extracted with 3x5 ml ether. The combined organic extracts were then washed with brine,<br />
dried over MgS04, and concentrated under vacuum to afford the desired product as a<br />
component of a yellow oU. HPLC analysis of this mixture suggests a 6:1 E:Z ratio. Flash<br />
column chromatography (10 g siUca gel, 8:2 hexanes:ethyl acetate eluent) afforded 0.032 g<br />
(0.015 mmol, 30% from 6-heptene-2-one) of the desired product as a clear oil. The<br />
diastereomers were separated by careful flash chromatography of this oil (10 g silica gel,<br />
hexanes - 8:2 hexanes:ethyl acetate eluent gradient).<br />
Major Isomer (E, as shown):<br />
iR (300 MHZ, relative to CHCI3 at 67.26) 62.05 (m, IH), 1.85 (m, IH), 1.73-1.12<br />
(m, 9H), 1.13 (s, lH[OH, variable]), 0.84 (d, J=7.1, 3H), 0.50 (m, 2H), -0.06<br />
(s, 9H).<br />
13C (75 MHZ) 684.05, 44.36, 40.42, 36.85, 31.77, 20.64, 18.27, 17.53, 16.63,<br />
-1.60.<br />
IR 3383, 2953, 1248, 862, 837 cm-L<br />
RPLC tR=582 seconds, 9:1 hexanes:ethyl acetate eluent at 1.0 ml/min on a 25 cm<br />
(4.6 mm ID) Zorbax silica gel column.<br />
MS (mixture of diastereomers, high resolution, m/e) 214.1759 (M+, 1.0 ppm<br />
deviation), 199.1516 (M+-CH3, 1.0 ppm deviation), 196.1651 (M+-H2O, 2.1<br />
ppm deviation, 181.1414 (M+-CH3,H20, 0.8 ppm deviation).<br />
59
Minor Isomer (Z, not shown):<br />
IR (300 MHZ, relative to CHCI3 at 6 7.26) 6 1.83-1.25 (m, 1 IH), 0.97 (s, lH[OH,<br />
variable]), 0.92 (d, 1=6.7, 3H), 0.49 (t, 1=8.3, 2H), -0.02 (s, 9H).<br />
13C (75 MHZ) 6 82.17, 43.72, 42.90, 38.14, 31.93, 20.92, 18.99, 17.50, 12.50,<br />
-1.60.<br />
IR 3481, 2953, 2930, 2870, 1248, 862, 837 cm-l.<br />
RPLC tR=426 seconds, 9:1 hexanes:ethyl acetate eluent at 1.0 ml/min on a 25 cm<br />
(4.6 mm ID) Zorbax silica gel column.<br />
Bis-(33-dimethvlbutene-2-oxy)(2-chloroethyl)methvlsilane<br />
am<br />
Ci5H29C102Si f.w. = 304.94<br />
n-ButyUithium (1.8 ml of a 2.2 M solution in hexane, 4.0 mmol) was added<br />
dropwise over a 5 minute period to 0.57 ml (4.0 mmol) dUsopropylamine and 10 ml ether<br />
stirring at -78° under nitrogen. After stirring for 10 minutes, 0.50 ml (4.0 mmol)<br />
pinacolone was added, and the mixture was stirred at -78° for an additional 30 minutes.<br />
Dichlorochloroethyl-methylsUane (0.28 ml, 2.0 mmol) was carefully added, and die<br />
resulting mbcture allowed to warm to room temperature. After stirring at rcx)m temperature<br />
overnight (-18 hours) the reaction mixture was added to a seperatory funnel containing 25<br />
ml ether and 50 ml saturated aqueous NaHC03, the layers seperated, and die aqueous layer<br />
extracted witii tiiree 25 ml portions of etiier. The organic extracts were tiien combined,<br />
washed witii 25 ml brine, dried over MgS04, and concentrated under vacuum to afford<br />
0.74 g (>100%) of a yellow liquid. Flash column chromatography (20g Sigel, hexanes<br />
eluent) afforded 0.33 g (54%) of relatively pure product. Attempted free-radical cyclization<br />
60
61<br />
of the crude product resulted in the formation of a number of products (TLC and ^H NMR)<br />
with no evidence of products resulting from cyclization (iR NMR).<br />
IR (200 MHZ, rel to CHCI3 at 6 7.24) 6 4.15 (d, 1=1.6, 2H), 4.12 (d, 1=1.6, 2H),<br />
3.74 (d of d, 1=8.6,8.6, 2H), 1.45 (d of d, 1=8.6,8.6, 2H), 1.05 (s, 18H), 0.29<br />
(s, 3H).<br />
IR 2955, 1910, 1868, 1631, 1472, 1293, 1171, 1036, 1015 cm-l.<br />
Rf 0.65 (hexanes).<br />
Bis-(cvclohexen-l-vloxv)(2-chloroethvl)methvlsilane(12Q)<br />
Ci5H25C102Si f.w. = 300.90<br />
In an attempt to synthesize (cyclohexen-l-yloxy)ethoxy(2-chloroethyl)methylsilane,<br />
n-butyUithium (1.7 ml of a 2.4 M solution in hexane, 4.0 mmol) was added dropwise over<br />
a 5 minute period to 0.57 ml (4.0 mmol) dUsopropylamine and 10 ml ether stirring at -78°<br />
under nitrogen. After stirring for 10 minutes, 0.42 ml (4.0 mmol) cyclohexanone was<br />
added, and the mixture was stirred at -78° for an additional 30 minutes. Dichloro-(2-<br />
chloroethyl)methylsilane (0.56 ml, 4.0 mmol)was carefully added, and the resulting<br />
mixture allowed to warm to -0° over one hour. Triethylamine (0.56ml, 4.0 mmol), then<br />
dry ethanol (0.23 ml, 4.0 mmol) were added, and die miUcy suspension allowed to warm to<br />
room temperature overnight. The reaction mixture was added to a seperatory funnel<br />
containing 25 ml etiier and 50 ml saturated aqueous NaHC03, the layers seperated, and the<br />
aqueous layer extracted with three 25 ml portions of ether. The organic extracts were then<br />
combined, washed witii 25 ml brine, dried over MgS04, and concentrated under vacuum to<br />
afford 1.1 g (4.4 mmol, >100% if mono-enol ether; 3.7 mmol, 91% if bis-enol ether) of a
clear Uquid. Altiiough iR and 13c NMR analysis suggested that die desUed product was<br />
formed (albeit in low yield) flash column chromatography (20 g sigel, 9:1 hexanes;etiiyl<br />
acetate eluent) successfuUy purified only the bis-enol etiier (0.128 g, 0.43 mmol, 11 %).<br />
IR (200 MHZ, rel to CHCI3 at 6 7.24) 5 4.98 (br s, 2H), 3.74-3.65 (m, 2H), 2.03-<br />
1.96 (m, 8H), 1.68-1.62 (m, 4R), 1.54-1.43 (m, 4H), 11.38-1.33 (m, 2H), 0.25<br />
(s, 3H).<br />
13C (50 MHZ) 6 149.26, 105.25, 41.62, 29.43, 23.68, 22.98, 22.11, 20.94,<br />
-3.32.<br />
Chloro(2-chloroetiivl)ethvlmethvlsilane(121)<br />
cr ^<br />
XI<br />
C5Hi2Cl2Si f.w. = 171.16<br />
Ethylmagnesium chloride (19 ml of a 2.0 M solution in THF, 38 mmol) was added<br />
dropwise over a 10 minute period to 5 ml (36 mmol) dichloro(2-chloroethyl)methylsUane<br />
and 20 ml ether stirring in a 100 ml round bottomed flask under a nitrogen atmosphere at<br />
-78°(bath temperature). After one hour, the mixture was aUowed to warm to room<br />
temperature, and the resulting milky suspension was allowed to stir for an additional 12<br />
hours. The suspension was then filtered through a 5 cm pad of oven dried celite and<br />
concentrated under vacuum, taking care to minimize exposure to the atmosphere.<br />
DistiUation under high vacuum (-1 mm Hg) afforded 2.4 g (14 mmol, 39%) of the pure<br />
product.<br />
iR (300 MHZ,, relative to CHCI3 at 6 7.26) 6 3.75 (d of d of d, 1=7.5,3.2,0.9, IH),<br />
3.72 (d of d of d, 1=7.5,3.0,0.8, IH), 1.51 (d of d, 1=7.5,3.2, IH), 1.49 (d of d,<br />
1=7.5,3.0, IH), 1.04 (t oft, 1=8.2,1.4, 3H), 0.91-0.81 (m, 2H), 0.42 (s, 3H).<br />
13C (75 MHZ) 6 41.28, 22.82, 9.75, 6.38, -0.31.<br />
Density (three measurements) 1.04 g/ml.<br />
62
Chloro(2-chloroetiivDmethvl( 1 -metiivlpropvl) silane (122)<br />
CI' Si<br />
CI<br />
CvHieChSi f.w. = 199.20<br />
5-ButyUithium (62 ml of a 1.3 M solution in hexanes, 80 mmol) was added to 11<br />
ml (80 mmol) dichloro(2-chloroethyl)methylsilane and 25 ml ether stirring in a 100 ml<br />
round bottomed flask under a nitrogen atmosphere at -78°(bath temperature). After one<br />
hour, the mixture was allowed to warm to room temperature, and die resulting milky<br />
suspension was allowed to stir for an additional 2 hours. The siispension was then filtered<br />
through a 5 cm pad of oven dried ceUte and concentrated under vacuum, taking care to<br />
minimize exposure to the atmosphere. High vacuum distiUation (1 mm Hg) afforded 3.0 g<br />
(15 mmol, 19%) of die pure product as the fraction distUUng between 73-77°. The product<br />
appears to be a 5:4 mixture of d,l and meso products.<br />
iR (300 MHZ, relative to CHCI3 at 6 7.26) 6 3.77-3.69 (m, 2H), 1.55-1.40 (m, 2H),<br />
1.29-1.18 (m, 2H), 1.05 (d, 1=2.8, 1.3H), 1.02 (d, J=2.8, 1.7H), 0.98 (t, 1=7.3,<br />
3H), 0.93-0.83 (m, IH), 0.415 (s, 1.7H), 0.411 (s, 1.3H).<br />
13C (75 MHZ) 6 41.49, 23.60, 23.12, 21.89, 13.07, 12.62, -1.46.<br />
B P 73-77° at 1 mm Hg.<br />
Ethvl(2-chloroethvl)( 1 -cvclohexvletiienyloxv)methylsilane<br />
11231<br />
\ ^CH2CH3<br />
Ci3H25C10Si f.w. = 260.92<br />
63
Lithium diisopropylamide (0.67 ml of a 1.5 M solution in hexane, 1.0 mmol),<br />
followed immediately by 0.75 ml HMPA, were added dropwise to 0.14 ml (1.0 mmol)<br />
cyclohexyl methyUcetone, 0.17 ml (1.0 mmol) chloro-(2-chloroediyl)-etiiylmethylsilane,<br />
and 2 ml THF stirring in a 5 ml pear shaped flask at -78° (badi temperature) under a<br />
nitrogen atmosphere. The resulting solution was allowed to warm to room temperature<br />
over two hours, stirred at room temperature for an additional 2 hours, tiien quenched by<br />
addition to 5 ml distUled water and 5 ml hexanes in a separatory funnel. The layers were<br />
separated, die aqueous phase extracted with 3x5 ml hexanes, and die combined organic<br />
extracts washed with 5 ml brine. Drying over MgS04 and filtration tiirough a 2 cm pad of<br />
celite, followed by removal of solvents afforded 0.24 g (0.92 mmol, 92%) of the desired<br />
product in -90% purity ( according to iR NMR).<br />
1R (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.02 (d, 1=0.9, IH), 3.94 (d, 1=0.9,<br />
IH), 3.75 (d, 1=9.9, IH), 3.71 (d, 1=10.1, IH), 1.92-1.59 (m, 8H), 1.40 (t,<br />
1=8.0, IH), 1.38 (t, 1=9.5, IH), 1.30-1.18 (m, 3H), 0.99 (t, 1=7.5, 3H), 0.73 (t,<br />
1=7.5, 2H), 0.22 (s, 3H).<br />
(33-Dimethvlbutenvl-2-oxv)ethvl(2-chloroethvl)-<br />
methvlsUane (124)<br />
>^ ^CHoCHo<br />
O^ ^<br />
CI<br />
CiiH23C10Si f.w. = 234.88<br />
Lithium diisopropylamide (0.67 ml of a 1.5 M solution in hexane, 1.0 mmol),<br />
followed immediately by 0.75 ml HMPA, were added dropwise to 0.13 ml (1.0 mmol)<br />
pinacolone, 0.17 ml (1.0 mmol) chloro-(2-chloroethyl)-ethylmethylsilane, and 2 ml THF<br />
stirring in a 5 ml pear shaped flask at -78° (bath temperature) under a nitrogen atmosphere.<br />
The resulting solution was allowed to warm to room temperature over two hours, stirred at<br />
room temperature for an additional 2 hours, then quenched by addition to 5 ml distilled<br />
water and 5 ml hexanes in a separatory funnel. The layers were separated, the aqueous<br />
64
phase extracted witii 3x5 ml hexanes, and the combined organic extracts washed witii 5 ml<br />
brine. Drying over MgS04 and filtration through a 2 cm pad of ceUte, foUowed by removal<br />
of solvents under vacuum afforded 0.16 g (0.66 mmol, 66%) of die desired product in<br />
-90% purity (according to iR NMR).<br />
iR (200 MHZ, relative to CHCI3 at 6 7.26) 6 4.09 (d, 1=1.7, IH), 3.89 (d, 1=1.7,<br />
IH), 3.76 (d, 1=9.9, IH), 3.72 (d, 1=10.0, IH), 1.40 (t, 1=8.0, IH), 1.37 (t,<br />
1=9.6, IH), 1.05 (s, 9H), 1.04-0.96 (m, 2H), 0.78-0.67 (m, 3H), 0.23 (s, 3H).<br />
r-Butvl(2-chloroethvl)methvl( 1 -phen vlethenvloxv)silane<br />
(125)<br />
Ci5H23C10Si f.w. = 282.89<br />
Lithium dusopropylamide (0.73 ml of a 1.5 M solution in hexane, 1.1 mmol) was<br />
added dropwise to 0.12 ml (1.0 mmol) acetophenone, 1 ml HMPA, and 2 ml THF stirring<br />
in a 10 ml pear shaped flask at -78° (bath temperature) under a nitrogen atmosphere. This<br />
mixture was stirred for 10 minutes,and then 0.22 g (1.1 mmol) r-butylchloro(2-<br />
chloroethyl)methylsilane,3^ was added. The resulting solution was allowed to warm to<br />
room temperature over two hours and stirred at room temperature for an additional 10<br />
hours, then quenched by addition to 15 ml distilled water and 10 ml ether in a separatory<br />
funnel. The layers were separated, the aqueous phase extracted with 3x10 ml ether, and<br />
the combined organic extracts washed widi 10 ml brine. Removal of the solvents under<br />
vacuum after drying over MgS04, followed by flash column chromatography (20 g sigel,<br />
hexanes eluent) afforded 0.27 g (0.88 mmol, 88%) of the desired product.<br />
IR (200 MHZ, relative to CRCI3 at 6 7.26) 6 7.61-7.56 (m, 2R), 7.37-7.27 (m, 3H),<br />
65
4.91 (d, 1=1.9. IH), 4.43 (d, 1=1.9, IH), 3.75 (d of d, 1=6.7,1.7, IH), 3.69 (d<br />
of d, 1=6.7,1.4, IR), 1.56-1.40 (m, 2H), 1.01 (s, 9H), 0.27 (s, 3H).<br />
13C (50 MHZ) 6 128.42, 128.18, 126.23, 125.15, 167.51, 194.37, 91.43, 42.59,<br />
25.80, 18.78, -5.68.<br />
r-Butvl(2-chloroethvl)(l -heptenvl-2-oxv)methvlsilane (126)<br />
Ci4H29C10Si f.w. = 276.93<br />
Lithium dusopropylamide (0.73 ml of a 1.5 M solution in hexane, 1.1 mmol) was<br />
added dropwise to 0.14 ml (1.0 mmol) 2-heptanone, 1 ml HMPA, and 2 ml THF stirring<br />
in a 10 ml pear shaped flask at -78° (bath temperature) under a nitrogen atmosphere. This<br />
mixture was stirred for 10 minutes, and then 0.22 g (1.1 mmol) tert-butylchloro(2-<br />
chloroethyl)-methylsilane,^^ was added. The resulting solution was aUowed to warm to<br />
room temperature over two hours and stirred at room temperature for an additional 10<br />
hours, then quenched by addition to 15 ml distilled water and 10 ml ether in a separatory<br />
funnel. The layers were separated, the aqueous phase extracted with 3x10 ml ether, and<br />
the combined organic extracts washed with 10 ml brine. Removal of the solvents under<br />
vacuum after drying over MgS04, foUowed by flash column chromatography (20 g silica<br />
gel, 9:1 hexanes:ethyl acetate eluent) afforded 0.23 g (0.83 mmol, 83%) of the desired<br />
product, contaminated with
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18.<br />
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35. Obeyesekere, N.O. PhD. Dissertation, Texas Tech University, 1988.<br />
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70
PARTE<br />
A SYNTHESIS OF THE CARBON-9 TO CARBON-21<br />
SUBUNIT OF THE APLYSL\TOXINS AND<br />
OSCILLATOXINS
CHAPTER 5<br />
BACKGROUND<br />
Discovery of the Aplysiatoxins and OsciUatoxins<br />
The aplysiatoxins and oscillatoxins are natural products isolated from marine<br />
sources. The first toxins isolated, aplysiatoxin (1) and debromoaplysiatoxin (2), were<br />
found in the digestive gland of the moUusk Stylocheilus longicauda which were coUected<br />
off the coast of Hawaii. ^ Later, Moore reported the isolation of these compounds from the<br />
bluegreen alga Lyngbya majuscula.'^ Outbreaks of severe contact dermatitus affecting<br />
swimmers off the coast of Hawau have been attributed to these toxins.^ It has been shown<br />
that the toxins found in the moUusks were a result of these animals feecUng upon toxin<br />
producing algae. Several other algal sotu^ces of these classes of toxins have been<br />
identified. Debromoaplysiatoxin (2) and 19-bromoaplysiatoxin (3), as well as the 31-nor<br />
compounds oscillatoxin A (4), 17-bromoosciUatoxin (5), and 17,19-dibromooscillatoxin A<br />
(6), were isolated from a mixture of the bluegreen algae Schizothrix calcicola and<br />
Oscillatoria nigroviridis?-<br />
1: Aplysiatoxin -<br />
R=CH3, Xi=Br, X2=H<br />
2: Debromoaplysiatoxin -<br />
R=CH3, Xi=X2=H<br />
3: 19-Bromoaplysiatoxin -<br />
R=CH3, Xi=X2=Br<br />
4: Oscillatoxin A -<br />
R=H, Xi=X2=H<br />
5: 17-Bromooscillatoxin A -<br />
R=H, Xi=Br, X2=H<br />
6: 17,19-DibromooscUlatoxin A<br />
R=H,Xi=Br, X2=Br<br />
CH3O Xi CH3O Xi<br />
72<br />
7: Anhydrodebromoaplysiatoxin -<br />
R=CH3, Xi=X2=H<br />
8: Anhydro-19-bromoaplysiatoxin<br />
R=CH3, Xi=X2=Br<br />
9: Anhydrooscillatoxin A -<br />
R=H, Xi=X2=H<br />
10: Anhydro-17-bromooscillatoxin A -<br />
R=H, Xi=Br, X2=H<br />
11: Anhydro-19-dibromooscillatoxin A<br />
R=H, Xi=Br, X2=Br
The anhydrotoxins 7-11 were also isolated, and it was claimed that these<br />
compounds always accompany the otiier toxins and are not artifacts of the isolation<br />
conditions.4 Finally, the toxins 12-16 were also isolated from this mixture of algae. The<br />
oscillatoxins 12, 13, and 16 have also been isolated, along with 2 and 7, from an<br />
OscUlatoriaceae alga collected near the MarshaU Islands.^<br />
I<br />
OH OH<br />
12: OsciUatoxin B1 -<br />
Ri=CH3, R2=CH3, R3=OH<br />
13: Oscillatoxin B2-<br />
Ri=CH3, R2=OR, R3=CH3<br />
14: 31-NorosciUatoxin B -<br />
Ri=CH3, R2,R3=CH3,OH<br />
(mixture of epimers)<br />
Stmcture Determination<br />
15: OsciUatoxin D<br />
R=H<br />
16: 30-MethyloscUlatoxin D<br />
R=CH3<br />
Using information gleaned from chemical ionization mass spectroscopy (CI-MS),<br />
ultraviolet spectroscopy (UV), infrared spectroscopy (IR), and proton nuclear magnetic<br />
resonance spectroscopy (^H NMR) on the natural compounds and on various degradation<br />
products, Kato and Scheuer were able to discern the gross stmctures of aplysiatoxins 1 and<br />
2.1 Careful consideration of ^H NMR data and of the chemical behavior of these<br />
molecules (as well as the anhydro compound 7) under certain reaction conditions allowed<br />
these researchers to propose relative stereochemistry for the stereo-centers from C4 to<br />
Cii.^ Subsequent ^H NMR studies by Moore et al. supported these assignments."^<br />
Difference Nuclear Overhauser Effect (NOE) experiments allowed Moore's group to extend<br />
the assignment of relative stereochemistries to include C12 and C29. The absolute<br />
stereochemistry at C15 was determined by comparing the circular dichroism (CD) spectra of<br />
compounds 2 and 4 with those of compounds 17-22. The authors proposed that the a-<br />
band (the band arising from a 7r-7t* transition in an aromatic chromophore situated in a<br />
chiral environment) in these compounds' CD spectra should be comparable, as they are all<br />
73
substituted benzyl alcohols. In fact, the CD spectra of compounds 2, 4, and 17-22 are<br />
remarkably similar, with each exhibiting positive Cotton effects at -280 nm. Therefore<br />
Moore assigned to the natural product die corresponding stereochemistry (15S). However,<br />
Johnson has questioned the appUcability of these comparisons.'^ Since CD spectra result<br />
OH<br />
17<br />
OH<br />
20<br />
OH<br />
'V^<br />
OH<br />
18<br />
OCH.<br />
^ ^<br />
OCH.<br />
OCH.<br />
from the net chiral environment surrounding the chromophore, significant<br />
21<br />
changes in the conformation of the model compounds and the natural products could cause<br />
a perturbation of the spectra. EspeciaUy worrisome is the possibiUty of through-space<br />
effects upon the chromophore by the large chiral ring system of the natural product.<br />
Johnson also noted that the achh-al substitution pattern on tiie aromatic ring can greatly<br />
affect the magnitude and even the sign of CD spectra of similar compounds, an important<br />
point that suggests that comparisons of die CD spectra of the aplysiatoxins with those of<br />
the model compounds 17,18, and 20-22 are especially risky. Although Johnson claims<br />
to have addressed this last point by making CD comparisons of 2 with compound 23<br />
(synthesized as part of his Ph.D. research), it appears that this comparison attracts his very<br />
own criticisms. In any case, his results supported Moore's assignment of the absolute<br />
configuration of C\s in compounds 1-11 as S.<br />
H3C,<br />
OCH3<br />
23<br />
OCH.<br />
OH<br />
74
Moore was able to complete the assignment of the absolute stereochemistry of<br />
compounds 1-6 (and thus 7-11 by analogy) and 12-16 by utilizing various other CD and<br />
molecular rotation comparisons,^ and even though several of die analogies that he drew<br />
seem to be questionable,'^ a total synthesis of 2 has demonstrated that these assignments are<br />
correct.^<br />
At this point the relationship between oscUlatoxin D (15) and oscillatoxin A (4),<br />
and analogously that between 30-metiiylosciUatoxin D (16) and debromoaplysiatoxin (2),<br />
should be noted (Scheme 5.1). One could imagine that compound 24, the triketo tautomer<br />
^^T]) ?^<br />
0=<br />
HO><br />
^^-^^^^^^4^0<br />
OCH,<br />
Scheme 5.1<br />
-H2O<br />
H2O ^ 15<br />
of OsciUatoxin A (4), could undergo an intramolecular aldol condensation with concomitant<br />
loss of water to afford cyclohexenone 25. The combination of a Michael addition of the<br />
hydroxyl group to the doubly activated alkene and a dehydrative translactonization would<br />
then complete the transformation to 15.<br />
75
Bioloeical Activity of the Aplvsiatoxins and Oscillatoxins<br />
As was previously noted, aplysiatoxin (1) and debromoaplysiatoxin (2) can cause<br />
severe contact dermatitus. The aplysiatoxins have been demonstrated to be potent<br />
cocarcinogens. Cocarcinogens are compounds tiiat enhance tumor development in tissues<br />
diat have been exposed to a carcinogen, but tiiat wiU not act as a carcinogen in then- own<br />
right. Much of the interest that these compounds have garnered is a result of this activity.^<br />
As dieir names suggest, the aplysiatoxins (1-3) and A osciUatoxins (4-6) are highly toxic,<br />
although the corresponding anhydrotoxins (7-11) and die B and D oscillatoxins (12-16)<br />
do not appear to have significant toxicity. Debromoaplysiatoxin (2) and oscillatoxin A (4)<br />
have demonstrated interesting antileukemic activity against the P-388 ceU line.<br />
Unfortunately the optimum antUeidcemic activity occurred at toxic levels. Interestingly,<br />
Professor R.E. Moore observed that the D oscUlatoxins are active against the L1210<br />
leukemia cell line, but that insufficient material was avaUable to thoroughly analyze this<br />
activity.^o Since these " toxins" have been shown to be relatively non-toxic, this<br />
antileukemic activity should be further explored. Also, as it has been shown that bromine<br />
substitution of the aromatic ring of the aplysiatoxins significantiy enhances their biological<br />
activity, it seems possible that the hitiiero unisolated brominated D oscillatoxins could also<br />
have enhanced biological activities.<br />
Progress Toward the Total Svnthesis of<br />
OsciUatoxin D<br />
The total synthesis of oscillatoxin D (15) is an ongoing project in our laboratories.<br />
The aforementioned biological activity of this compound is a major reason for our interest<br />
in pursuing this project. A total synthesis could allow sufficient quantities of 15 (as well<br />
as various analogues including bromine containing analogues) to be produced to allow a<br />
diorough evaluation of their biological activities. The inherent chaUenge in synthesizing<br />
such a complex molecule is also a factor in our interest. This spirobicyclic allyl ether is a<br />
foreboding challenge, especially considering die requisite stereochemistry.<br />
76
Model stucUes performed in our laboratory have recentiy culminated with the ring<br />
closure of 26 to form spirobicycle 27 (Scheme 5.2).ii Although this compound was<br />
formed unexpectedly and has undesired functionaUty at C9, its formation has provided us<br />
with confidence that the proposed route to the desired model 30 (as indicated in Scheme<br />
5.2), and eventually to the natural product, will be successful. Especially encouraging was<br />
R3SiO<br />
28<br />
26<br />
0<br />
(TBDMS = tBuMe2Si,<br />
Tf=CF3S02,<br />
TMSE = Me3SiCH2CH2)<br />
• ^ TM8E0<br />
iPrNEt2,<br />
TBDMSOTf.<br />
CH2CI2<br />
29<br />
Scheme 5.2<br />
TBDM80<br />
27<br />
(major diastereomer<br />
of a 12:2:2:1 mixture,<br />
72% combined yield)<br />
30<br />
77
78<br />
the high degree of stereoselectivity found for die formation of the spirobicyclic ring of 27<br />
from 26.<br />
A route to compound 26 is outUned in Scheme 5.3. Aldol condensation between<br />
thepara-methoxyphenylmethyl (MPM)-protected 6-hydroxy ketone 31 and the aldehyde<br />
32, foUowed by oxidation to the P-diketone and removal of the 11-hydroxyl protecting<br />
group affords alcohol 33 in good yield. 12 Acid catalyzed ketalization of 33 with<br />
concomitant dehydration, followed by removal of the MPM protecting group and oxidation<br />
produces aldehyde 34, which is then condensed widi trimetiiylsilylethyl (TMSE) acetate<br />
enolate 35 followed by oxidation to afford p-ketoester 26. It can be easily recognized that<br />
if one utilizes aldehyde 36 instead of aldehyde 32, then an analogue of 26 containing all of<br />
the functionality required to complete the total synthesis of Oscillatoxin D (15) will be<br />
produced. Therefore, as part of a convergent total synthesis of 15, an efficient means of<br />
producing the C9-C21 aldehyde 36 was necessary, and such was the goal of the project<br />
described herein.<br />
26<br />
TMSEO<br />
35<br />
Scheme 5.3<br />
O OR OCH.<br />
36<br />
-^ MPMO' O<br />
O<br />
OR'<br />
.y^^^L<br />
34<br />
33<br />
11<br />
{/
Other Synthetic Studies on the Aplysiatoxins<br />
The synthesis of a suitably protected C9-C21 aldehyde 36 is especially attractive, as<br />
this compound is a potential precursor for the total synthesis of all of the compounds 1-14.<br />
Several research groups have focused tiieir attention upon the total synthesis of aplysiatoxin<br />
1 and debromoaplysiatoxin 2, because of the interesting stmctures and biological activities<br />
of these compounds. Kishi's group at Harvard University has reported the only total<br />
synthesis of any of these compounds to date.^ They synthesized compound 37, a C8-C21<br />
synthon simUar to our proposed C9-C21 aldehyde intermediate 36, en route to the total<br />
synthesis of 2. An outUne of their synthesis of this fragment is given in Scheme 5.4.<br />
(Bn = C6H5CH2)<br />
BnO<br />
OH<br />
A<br />
S02Ph<br />
3 steps,<br />
54%<br />
+<br />
OBOM<br />
13 steps,<br />
31% A<br />
11<br />
OBn<br />
/ A \ 8 steps,<br />
68%<br />
(-)-diethyl D-tartrate Scheme 5.4<br />
OBOM<br />
3 steps,<br />
43%<br />
79
Kishi obtained the correct stereochemistry at C9-C12 by a performing series of<br />
stereoselective reactions upon the protected triol 39, which was synthesized according to<br />
Uterature precedent in eight steps of 68% overaU yield. Thirteen steps, in 31 % overall<br />
yield, transformed 39 into sulfone 38, which is a precursor of the anionic nucleophile that<br />
Kishi required. The stereochemistry at C15 was obtained by Kishi by stcu-ting with tiie<br />
enantiomerically pure amidoalcohol 41, which was converted to the benzyloxymethyl<br />
(BOM) protected epoxide 40 in three steps and 43% overall yield. Coupling of 38 and<br />
40, followed by reductive desulfurization and ether formation, afforded 37 in three steps<br />
and 54% yield. Therefore, the longest linear series of reactions involved twentyfour<br />
transformations and was performed in 11% overaU yield.<br />
A 1988 pubUcation by Ireland et al^^ reported an aborted attempt at the total<br />
synthesis of aplysiatoxin. Although they did not synthesize a subunit comparable to ours,<br />
the method that they used to establish the absolute stereochemistry at C15 should be noted.<br />
Thus, ketone 42 was stereoselectively reduced using the Uthium aluminum hydride (LAH)<br />
/ (S)-2,2'-binapthol complex developed by Noyori (Scheme 5.5).^^ This reaction yielded<br />
71% of the alcohol 43 of greater than 95% ee on a large scale (>3 grams 43 isolated).<br />
OCH.<br />
(S)-2,2'-binapthol,<br />
EtOH. LAH. THF^<br />
-100°for4hrs<br />
-78°forl6hrs<br />
Scheme 5.5<br />
OH<br />
OCH.<br />
The total synthesis of the unnatural aplysiatoxin, 3-deoxydebromoaplysiatoxin 44,<br />
was the subject of two recent papers by .^^ Compound 44 has been shown to have<br />
biological activity comparable to diat of debromoaplysiatoxin (2). The removal of die labile<br />
hemUcetal functionality should increase the stabiUty of compound 44 relative to the natural<br />
products, and as such this compound deserves attention. (This is a good example of a<br />
totally synthetic analogue that may have characteristics superior to the natural product.)<br />
This synthesis of 44 utilized the protected C8-C21 tetraol 46 (TBDMS = tert-<br />
butyldimethyl-silyl), which is comparable to Kishi's intermediate 37 and to our proposed<br />
aldehyde 36. Ketone 45 was synthesized in 13 steps and 19% overall yield from<br />
diacetone glucose. This ketone was then stereoselectively reduced by Brown's<br />
dUsocampheylchloroborane (Ipc2BCl) reagent,^^ i^ 74% yield and -100% ee, and finally<br />
80
methylated to afford the ether 46 (Schema '^ A^ TU AH , • , •<br />
iiici HO (,5cneme 5.6). Thus 46 was obtamed in 15 steps and<br />
14% overall yield from commercially available material.<br />
44 R=H<br />
2 R=OH<br />
OBn<br />
1) IPC2BCI,<br />
THF, -25°<br />
2)NaH, THF,<br />
Mel<br />
Scheme 5.6 OBn<br />
Conclusions<br />
The aplysiatoxins and osciUatoxins are interesting new marine natural products.<br />
Oscillatoxin D is a minor constituent of several algae, and although it is of interest because<br />
of reported antileukemic activity, thorough biological testing has not been done as<br />
insufficient quantities of it have been isolated. Model studies performed by another student<br />
in this laboratory have demonstrated a possible synthetic route to the spirocyclic ring<br />
system, therefore a C9-C21 aldehyde 36 is necessary to complete the actual synthesis of the<br />
natural product. Synthetic work on the aplysatoxins has resulted in the development of<br />
several lengthy pathways to compounds similar to aldehyde 36, a compound which could<br />
be used in the synthesis of all of the aplysiatoxins.<br />
81
CHAPTER 6<br />
RESULTS AND DISCUSSION<br />
Retrosynthetic Analysis of Target Aldehyde (36)<br />
In approaching the synthesis of aldehyde 36, it was recognized that it would be<br />
advantageous to introduce the aldehyde functionaUty at the very end of the synthesis. This<br />
would reduce problems stemming from the high reactivity of this functional group. An<br />
aldehyde functionaUty would also facilitate epimerization of the Cio chiral center.<br />
Therefore compound 47, in which an MPM (para-metiioxyphenyhnethyl) protected C9<br />
hydroxy group serves as a 'masked' aldehyde, was chosen as a penultimate target (Scheme<br />
6.1).<br />
OCH.<br />
V<br />
MPMO OR OCH3<br />
MPM = ^ara-methoxyphenylmethyl OR'<br />
MPMO OR X<br />
V OCH.<br />
+<br />
Scheme 6.1<br />
82<br />
OR'
The MPM group was chosen because it could be removed under mild oxidizing<br />
conditions (2,3-dichloro-5,6-dicyanobenzoquinone, DDQ) that most other hydroxyl<br />
protecting groups wiU widistand.17 Selective removal of the MPM protecting group,<br />
foUowed by oxidation, would afford aldehyde 36.<br />
To simplify matters further a convergent route, where some fragment containing<br />
C9-C13 would be somehow joined with a fragment containing C15-C21, was considered.<br />
In fact, the previous syntheses of C8-C21 containing intermediates 37 and 46 have utilized<br />
this approach.8.15 Consequentiy, compounds 48 and 49, in which X and Y are functional<br />
groups that will aUow coupling, were chosen as targets. The coupUng combinations that<br />
were considered were a C13 nucleophile displacing a C14 leaving group, and a C14<br />
nucleophile displacing a C13 leaving group. Kishi's synthesis of debromoaplysiatoxin 2<br />
used the former combination to form the C13-C14 bond (Scheme 5.4),8 while Yamamura's<br />
synthesis of deoxydebromoaplysiatoxin 44 relied upon a Wittig coupling, followed by<br />
hydrogenation of the resulting C13-C14 double bond.^^<br />
Studies on the Stereoselective Synthesis of a<br />
Ci4zCi5.Epo2cide<br />
The oxirane functional group is very useful in synthetic organic chemistry.^^ One<br />
common transformation of this functional group is attack by a nucleophile at the least<br />
substituted carbon. In this manner oxiranes function as 'masked' a-hydroxy electrophUes.<br />
Since this is exactly what was desired for the C14-C21 synthon, the fu-st goal of this project<br />
was an efficient stereoselective synthesis of a C14-C15 epoxide. Kishi utilized this strategy<br />
in his total synthesis of debromoaplysiatoxin 2, where he opened a C14-C15 epoxide with<br />
an a-sulfonyl carbanion.8 It was perceived that Kishi's route to the epoxide could possibly<br />
be improved, and that a C13 nucleophile (probably an a-sulfonyl carbanion similar to<br />
Kishi's but one relevant to die synthesis of oscUlatoxin D) could be fashioned and would<br />
open the epoxide in an analogous fashion. Also noted was that it would be desirable to<br />
protect the phenolic hydroxy group with a hardy protecting group, as this hydroxy group<br />
should remain protected until the last stages of the total synthesis. Therefore efforts to<br />
synthesize epoxide 50, where R is an appropriate protecting group, were begun.<br />
83
The first attempt at synthesizing the chiral epoxide 50 reUed upon an asymmetric<br />
reduction of a-ketoester 51 to the a-hydroxy ester 52 by an actively fermenting culture of<br />
bakers yeast (Scheme 6.2). Compound 51 was synthesized from meta-hydoxy-<br />
acetophenone by protection as a benzyl etiier, permanganate oxidation to the a-keto acid,<br />
and esterification. Yeast cultures have been demonstrated to reduce simUar aromatic a-<br />
ketoesters (lacking, however, tiie meta substituents) in high yield and witii exceUent<br />
stereoselectivity. 19 Although the desired a-hydroxy ester 52 was produced,20 the low<br />
overaU yield (-30%) and especially the disappointing 80% enantiomeric excess (e.e.) of the<br />
product necessitated a search for a more efficacious route.<br />
l)BnBr<br />
2) KMn04<br />
3) MeOH, H^<br />
(Bn^CgHsCHz)<br />
^-<br />
MeOoC<br />
21<br />
51 BnO<br />
Scheme 6.2<br />
actively fermenting<br />
bakers yeast<br />
MeOsC<br />
OH<br />
21<br />
52 BnO<br />
One of the most powerful methods for producing chiral alcohols is the Sharpless<br />
epoxidation.2i This reaction relies upon the directing affect of an allyUc alcohol, coupled<br />
with a chiral titanium - tartrate complex, to achieve phenomenal sucess in an<br />
enantioselective epoxidation by r-butylhydroperoxide. The mechanism has been studied<br />
and a complex mechanistic picture proposed,22 although Corey has recently proposed a<br />
simpler mechanistic explanation for the selectivity of this reaction.23 This reaction can also<br />
be used to resolve a racemic mixture of aUyUc alcohols (Scheme 6.3).24 For example,<br />
when a racemic mixture of allylic alcohol 53 was treated with -0.6 molar equivalents of<br />
rerr-butyl hydroperoxide under Sharpless' epoxidation conditions, one enantiomer of the<br />
alcohol (S) was rapidly epoxidized, while the other enantiomer reacted very slowly (for this<br />
allylic alcohol kfast/ksiow=83). Separation of the epoxide 55 from the unreacted 'resolved'<br />
alcohol 54 afforded products with high e.e.'s.<br />
84
OH<br />
53<br />
(racemic<br />
mixture)<br />
CH2{CH2)4CH3<br />
1.0 eq. Ti(0-i-Pr)4<br />
1.2 eq. (+)-diisopropyl<br />
, tartrate<br />
0.6 eq. t-BuOOH<br />
CH2Cl2,-20°C,12days<br />
Scheme 6.3<br />
OH<br />
54<br />
96% e.e.<br />
OH<br />
55<br />
92% e.e.<br />
CH2{CH2)4CH3<br />
CH2(CH2)4CH3<br />
To investigate this appUcation of Sharpless' resolution procedure to our problem,<br />
aUyUc alcohol 57, synthesized by addition of vinylmagnesium bromide to meta-<br />
benzyloxybenzaldehyde 56, was submitted to the kinetic resolution conditions (Scheme<br />
6.4). The resolution and subsequent separation of the enantiomer 59 of this alcohol posed<br />
TsO,<br />
56<br />
NaH,<br />
THF<br />
0„<br />
BnO<br />
OH<br />
a)CH2CHMgBr<br />
b)H20<br />
BnO<br />
61<br />
L>-v^-^<br />
62 BnO<br />
57<br />
BnO<br />
TsCl, EtsN<br />
-^-<br />
(Ts=paratoluenesulfonyl)<br />
1.0eq.Ti(O-i-gr)4<br />
1.2 eq. (+)-diisopropyl<br />
tartrate<br />
0.6 eq. t-BuOOH<br />
CH2CI2. -20°C<br />
OH<br />
Scheme 6.4<br />
OH<br />
59<br />
BnO<br />
85
no special problems, altiiough it should be noted tiiat as this is a resolution the maximum<br />
yield is 50%. The subsequent reductive ozonolysis of die resolved allylic alcohol 59 was<br />
especially troublesome, and low yields (-40%) of 60 were isolated.20 Jt was demonstrated<br />
that tills diol could be converted to its monotosylate 61 (33%), and finally to the desired<br />
epoxide 62 (61%)(Scheme 6.4).<br />
Faced with the low yields and lengthy procedure of the route outlined in Scheme<br />
6.4, another route to a chiral epoxide 49 was explored. It has been demonstrated that<br />
enolates derived from chUal binapthol monoesters of phenylacetic acids undergo alkylation<br />
stereoselectively.25 It was envisioned that the oxidation of an enolate 64 could proceed<br />
witii some similar directing effect by the molecular chiraUty of the binapthol system. A<br />
compound that has been demonstrated to oxidize ester enolates efficiently to a-hydroxy<br />
esters is the MoOPH26 reagent (MoOspyridineHMPA [hexamethylphosphorictriamide])<br />
developed by Vedejs.2'7 Although the synthesis of the binapthol monoester 63 was<br />
relatively straightforward, attempts at using the MoOPH reagent to oxidize the enolate 64<br />
to an a-hydroxy ester were unsuccessful (Scheme 6.5). At this point, the lack of success<br />
in synthesizing a chiral C14-C21 epoxide in high yield and with high stereoselectivity made<br />
the alternate 48 + 49 coupUng route attractive.<br />
63<br />
MeO<br />
2.2 eq.LDA<br />
(Lithium<br />
diisopropylamide)<br />
Scheme 6.5<br />
iy4.^H^l c^fndies on thp. Alkvlation of a Cu Anion<br />
(M0O5: (PA =<br />
pyridine: hexamethyl-<br />
HMPA) Phosphorict<br />
triamide)<br />
N.R.<br />
The lack of success in developing an improved route to a C14-C15 epoxide led to<br />
the exploration of the reaction of a C14 carbanionic nucleophile with a C9-C13 synthon<br />
fitted with a leaving group on C13. Since it had been established for similar systems that a<br />
86
Ct5 ketone can be stereoselectively reduced to give the desired 15S configuration n.is it<br />
was proposed that the simplest Ct4 nucleophile would be the enolate (66) of a<br />
trimetiiylsilylethoxymethyl (SEM) protected m.r.-hydroxy acetophenone 65 (Scheme 6 6)<br />
This protecting group,28 which was fitted to m-hydroxyacetophenone in 80% yield, was<br />
chosen because this phenolic hydroxyl group required a robust protecting group so as to<br />
survive until the very end of the total synthesis. Model studies were performed in an<br />
attempt to react enolate 66 with a model mesylate (67) or iodide (68). However, very little<br />
alkylation of enolate 66 was observed under various conditions, including an attempt to<br />
increase the reactivity of the enolate by the addition of HMPA to the reaction mixture.<br />
^<br />
HO<br />
O<br />
SEMC^<br />
R3N<br />
CH2CI2<br />
(SEM = trimethylsilylethoxymethyl<br />
TBDMS = tert-buiyldimethylsilyl)<br />
Scheme 6.6<br />
67 or<br />
67, HMPA ^^<br />
68 or ^^^^^^^"<br />
68, HMPA''<br />
TBDMSO OSOoMe TBDMSO<br />
In an attempt to find a 'ketone enolate equivalent' that would<br />
67 68<br />
undergo the desired alkylation, it was noted that imine anions are known to undergo<br />
efficient alkylation.29 The synthesis of the imine derivative (69) of the ketone 65 was<br />
straightforward (Scheme 6.7), and involved simply refluxing the ketone and<br />
cyclohexylamine in benzene with powdered 4A molecular sieves (cmde yield = 91%).<br />
Imine 69 was readily distilled to high purity, and it was found to be suprisingly stable<br />
towards hydrolysis.<br />
Gratifyingly, it was found that reaction of the imine anion 70 with model iodide 68<br />
was efficient, and that pH 4 buffer rapidly hydrolyzed the imine to the desired alkylated<br />
ketone 71 in 72% overall yield (Scheme 6.8). This alkylation was deemed sufficiently<br />
effective that the model study was halted and the synthesis of a C9-C13 synthon 47, where<br />
'X' is iodine, was explored.<br />
87
LDA<br />
69 ^<br />
8EM0<br />
65<br />
N-LI-<br />
15<br />
^<br />
8EM0<br />
70<br />
o NH,<br />
powdered 4 A *^^<br />
molecular selves<br />
benzene reflux SEMO<br />
Scheme 6.7<br />
a) 68<br />
b) pH 4 buffer<br />
Scheme 6.8<br />
69<br />
TBDMSO<br />
Stereoselective Aldol Route to the CQ-CI^ Segment of the<br />
OsciUatoxins and Aplvsiatoxins<br />
The aldol condensation is a transformation which has long been a powerful tool for<br />
the construction of carbon-carbon bonds. 18 Thus, the recent development of chiral<br />
enolates which participate in the aldol condensation with a high degree of stereocontrol has<br />
provided the organic chemist witii a powerful new tool for the stereocontrolled synthesis of<br />
multifunctional acyclic compounds. Especially useful are the boron enolates of chiral<br />
oxazolidinones (for example 72 [Scheme 6.9]) reported by Evans, who has found that<br />
these enolates undergo aldol condensations witii a high degree of stereoselectivity.^o The<br />
relative stereochemistry of the aldol product is always syn, while the absolute<br />
stereochemistry is determined by the configuration of the ring substituents of the chiral<br />
auxiUary. Removal of the chiral auxiliaries under reducing, transesterifying, or<br />
saponifying conditions allows the isolation of p-hydroxy acids (74), p-hydroxy esters<br />
88
89<br />
75 , or M.01S 76) as well as the recovery of the oxazolidinone chiral auxtliaty (Scheme<br />
6.9). Compour,ds 74-76 are typically isolated in high yields (70-90%), and in >90%<br />
dtastereomeric purity . The utUity of these reactions is enhanced by Evans' observation that<br />
die enolate chiraUty strongly overrides die resident chirality of die aldehyde. For example,<br />
the aldol condensation of chiral aldehyde 77 with the dibutylboron enolate 72 gave 78<br />
(86% yield, >96% diastereomeric excess [d.e.J, Scheme 6.10), while reaction of the same<br />
aldehyde with the enolate 79 gave 80 (>99% d.e.).3i<br />
Given the obvious similarity between 78 and target compound 48, die possible<br />
utiUty of this asymmetric aldol reaction for the stereoselective synthesis of a C9-C13 iodide<br />
81, where R is an appropriate protecting group, is appealing (Scheme 6.11). The syn<br />
relationship and absolute configuration of chiral centers Cn and C12 suggest that the C12-<br />
Ci3 portion of 81 could be derived from the boron enolate 72. The C9-C11 portion would<br />
then have its start as the aldehyde 82, with the absolute configuration of Cio already fixed.<br />
An enantiomerically pure MPM protected p-hydroxy aldehyde 82 was required to<br />
utUize Evans' aldol methodology in the synthesis of 81. Fortunately, an eariy worker in<br />
this laboratory, during the course of studies aimed at the total syntiiesis of oscillatoxin A<br />
(4), had synthesized this very compound.32 This was accomplished in three steps and in<br />
55% overall yield from the commercially available (S)-methyl 3-hydroxy-2-<br />
methylpropionate (83). Recent improvements of tiiis synthesis have resulted in the
n-Bu^BO O a)<br />
A<br />
H<br />
N "O<br />
Ph ^) ^2^2<br />
72<br />
(Ph = C6H5)<br />
n-BupBO O<br />
79<br />
a)<br />
O<br />
YY^"<br />
(77)<br />
b) H2O2<br />
Scheme 6.10<br />
MPMO MPMO<br />
CHO<br />
11 + 72<br />
Scheme 6.11<br />
— ^<br />
><br />
x'<br />
HO O O<br />
production of 82 in two steps (Scheme 6.12). Thus, p-hydroxy ester 83 was protected as<br />
its MPM ether (67%) by reaction with para-methoxyphenylmethyl trichloroacetimidate (85)<br />
under acid catalysis, as per Yonemitsu.l^ Direct reduction of the protected hydroxy ester<br />
84 to the aldehyde 82 was accomplished efficientiy (-100%) using a modification of<br />
Corey's procedure.^^ This protocol allowed the production of multigram quantities of<br />
aldehyde 82 in good overall yield (67-73%).<br />
N-Propionyl oxazolidinone 87 was prepared by reaction of propionyl chloride<br />
with the Uthium amide of oxazolidinone 86 (80%, Scheme 6.13).30 \i j^g Q^^^^ of this<br />
project the commercially available 86 was used. However, it was found that its synthesis<br />
90
MeO"^-^ 85<br />
CO2CH3 85<br />
NH<br />
A<br />
ecu<br />
20 % PPTS<br />
(pyridinium paratoluenesulphonate)<br />
Scheme 6.12<br />
CO2CH3 l.Oeq.DIBAL-H<br />
eg- ^<br />
(diisobutyl<br />
aluminum hydride)<br />
CH2CI2, -78°<br />
is simple and economical.^^ Using the procedure developed in Evans' laboratories at<br />
Harvard University,3^ enolization of oxazolidinone 87 by treatment with ethyldUsopropyl<br />
amine and di-n-butylboron triflate, foUowed by condensation with aldehyde 82, produced<br />
the desired aldol product 88 as a crystalline mass. Upon recrystalUzation from<br />
ethenhexanes, pure 88 was obtained in moderate yields (43-57%) (Scheme 6.13). A smaU<br />
O<br />
1<br />
O o<br />
HN' . ^O P aU,05ea:J^Mli_J^^"^^^'l ? a) 1.1 eq n-Bu^BOTf 72<br />
\ / b) L1 eq. CH3CH2COCI \ / b) EtN(i-Pr)2<br />
86<br />
n-Bu2B0 O<br />
^ Ph<br />
72<br />
1)82<br />
2) H2O2 -^"<br />
Scheme 6.13<br />
(Tf=CF3S02)<br />
88 ' Ph<br />
43-57% recrystalUzed<br />
amount «10%) of another isomer of the product was evident from the 'H NMR spectrum<br />
of the crude product. The stereochemistry of this byproduct was not ascertained.<br />
Subsequently, it was noted that the yields from this type condensation were irreproducible<br />
over time when commercially available solutions of di-«-butyIboron triflate were used, and<br />
that the use of freshly prepared diethylboron triflate alleviated this problem." Therefore,<br />
an aldol condensation between 82 and the diethylboron enolate of 87 (prepared follown,g<br />
82<br />
91
92<br />
Oppoltzer's protocol for the in situ formation of diethylboron triflate), was performed.36<br />
Altiiough the cmde product from this procedure appeared to be much cleaner, the<br />
recrystalUzed yield from tiiis reaction was stUl low (53%).<br />
The stereochemistry of the aldol product was assumed to be as desired by analogy<br />
witii previous work, and was further verified by iH NMR experiments. A decoupling<br />
experiment in which the methyl group attached to C12 of compound 88 was irradiated<br />
revealed a 3.4 hertz (Hz) coupling between the hydrogens attached to Cn and C12. This is<br />
in the range expected for a syn aldol product,^^ suggesting tiiat the relative stereochemistry<br />
at Cu and C12 is syn as shown in 88. DDQ oxidation of 88 produced the acetal 89 in<br />
60% yield (Scheme 6.14). This compound was found to have an 11 Hz coupUng between<br />
HA and HB, which is consistent with the trans-diaxial disposition expected for these<br />
hydrogens. Therefore the stereochemistry of compound 88, both relative and absolute, is<br />
as shown. With the C9-C13 carbon skeleton in place with the desired stereochemistry,<br />
protection of the Ci 1 hydroxyl and manipulation of the C13 amide to a primary iodide will<br />
afford the C9-C13 electrophiUc synthon 81. Therefore these functional group<br />
manipulations were the next goal of this project.<br />
88 CH2CI2/PH 7<br />
buffer (5:1)<br />
p-MeOC6H4<br />
Scheme 6.14<br />
F1nhor;irion of the, Aldol Product (88) to a<br />
C9iCi3jQdide_(901<br />
It was expected that aldol product 88 could be transformed into a suitably protected<br />
iodide 81 in a straightforward fashion using common reactions. It was detemiined that the<br />
Cn hydroxyl group would be protected as a rm-butyldimetiiylsilyl (TBDMS) etiier. This<br />
protecting group is stable to a large number of syntiietic transfomiations.3S Especially<br />
important, the TBDMS ether is stable to the conditions used to remove the MPM protecting<br />
group. Indeed, this protecting group was used in eariy model studies concerning the
93<br />
fomiation of the spirobicycle 27.n Iodide 90, then, was the actual C-C,3 intermediate<br />
desired.<br />
MPMO OTBDMS<br />
Besides choosing a protecting group for the Cn hydroxyl group, it was also<br />
necessary to consider by what means die oxazolidinone chiral auxiliary would be removed<br />
and Ci3 converted into a primary iodide group. It was noted that the reductive removal of<br />
the chiral auxiliary (c./. 73-76, Scheme 6.9) results in tiie conversion of Cn to a primary<br />
alcohol, a functionality that can be readily converted to a primary iodide. Therefore a<br />
reductive removal of the chiral auxiliary was considered especiaUy attractive, and was the<br />
first route to 90 that was pursued.<br />
Although lithium borohydride has been shown to reduce efficiently aldol products<br />
such as 88 to diols,^! it was deemed desirable to avoid problems in the differentiation the<br />
two hydroxyl groups. Protection of the Cn hydroxyl group of 88 as its TBDMS etiier<br />
was straightforward (Scheme 6.15), and the silyl ether 91 was isolated in good yield<br />
(82%). It was expected that this compound would be reduced by Uthium borohydride to<br />
the Ci3 alcohol 92. In practice, however, amino alcohol 93 was the major product<br />
88<br />
MPMO<br />
L25 eg .TBDMSOTf,<br />
1.5 eq. 2,6-lutidine<br />
CH2CI2<br />
OTBDMS<br />
MPMO<br />
H OH<br />
TBDMSO<br />
Scheme 6.15<br />
MPMO OTBDMS
94<br />
isolated upon reduction of 91 under a variety of conditions (in 40-60% purified yields).<br />
Apparentiy either the free Cn hydroxyl is necessary to direct the reducing agent to die Cn<br />
carbonyl, or the buUcy TBDMS etiier in 91 is blocking tiie approach of the reducing agents<br />
to the Ci3 carbonyl. In any case, this route to 92 was abandoned, and a less direct<br />
approach to 90 was explored.<br />
Since the Uthium borohydride reduction of aldol products such as 88 to diols such<br />
as 93 was so well precedented, this strategy was the next one pursued (Scheme 6.16).<br />
This route had earUer been shunned so as to avoid problems with the differentiation of the<br />
primary and secondary hydroxyl groups of the diol. However, it was assumed that the<br />
hydroxyl groups could most lUcely be easily differentiated by reaction of 94 with one<br />
equivalent of methanesulfonyl chloride in the presence of an amine base. A similar reaction<br />
was found to be useful in the synthesis of epoxide 62 from diol 60 via the monotosylate<br />
61 (Scheme 6.4). The reduction of 88 to diol 94 and the chiral auxiliary oxazolidinone<br />
86 proceeded cleanly. However, the chromatographic separation of these two products<br />
was difficult. This problematic separation, which would be exacerbated by the large scale<br />
(multigrams) desirable in the early stages of the lengthy total synthesis, led to the<br />
exploration of another route to transfom 88 into the Cg-Cn iodide 90.<br />
88 J^^—•<br />
MPMO<br />
Scheme 6.16<br />
+<br />
O<br />
A,<br />
HN' ^O<br />
94 86<br />
It was envisioned that esterification of the aldol product 88 to the methyl ester 95<br />
would be a simple method to remove the chiral auxiUary 86. The esterification of the<br />
products from aldol reactions using the oxazolidinone chiral auxiliary is well established.^o<br />
It was hoped that the ester 95 would behave more predictably than the carbamate 88, and<br />
as such would finally allow the synthesis of 90. Therefore, reaction of the aldol product<br />
88 with a slight excess of sodium methylate afforded ester 95 and the oxazolidinone 86.<br />
These compounds were easily separated, with 95 isolated in excellent yield (77-83%,<br />
Scheme 6.17). Direct reduction of p-hydroxy ester 95 witii DIBAL-H was uneventful,<br />
with tiie crystaUine diol 94 isolated in 75-89% yield (72% after rectystallization). In order<br />
to avoid the intennediacy of diol 94 and the aforementioned differentiation problems, ester<br />
Ph
95 was protected as its Cn TBDMS ether Qfi (Q9io/ \ o«^ o u<br />
95<br />
^ ^ ^ ^ ^^"^^ ^" ^^^^o) and subsequentiy reduced to afford<br />
the alcohol 92 in 71% yield.<br />
J.J. LI eg.MeO Na""^<br />
MeOH, CH2CI2<br />
0'', 15 min<br />
OTBDMS<br />
COoMe 2.5 eg. DffiAL-H,<br />
CH2CI2, 0°<br />
Scheme 6.17<br />
OMe<br />
4 eq. DIBAL-H<br />
CH2Cl2,0° r.t.<br />
MPMO OTBDMS<br />
An attempt to convert alcohol 92 to iodide 90 via the intermediate mesylate was<br />
made next. Thus, upon reaction with excess methanesulfonyl chloride in the presence of<br />
trietiiylamine, compound 92 was cleanly converted to the mesylate 97 (92%<br />
chromatographed yield. Scheme 6.18). However, attempted displacement of the mesylate<br />
by iodide resulted in the quantitative (according to ^H NMR) formation of the pyran 98<br />
plus MPM-iodide! At first this was thought to be a result of an acid catalyzed loss of the<br />
o- 1.25 eq. MeS02Cl<br />
L5 eq. EtsN, *"<br />
CH2CI2, 0°<br />
MPMO OTBDMS<br />
97<br />
OSOoMe<br />
Scheme 6.18<br />
Nal (sat'd soln),<br />
acetone retlux, ^<br />
6 hrs<br />
92<br />
0<br />
•9 13<br />
94<br />
11 ^^ "', V<br />
OTBDMS<br />
MPM protecting group, followed by an intramolecular displacement of a leaving group (X<br />
= MeS03 or I), as indicated in Scheme 6.19. However, a recipe for die desired<br />
displacement which had been shown to be useful for acid sensitive substrates, the addition<br />
of 1% EtN(i-Pr)2 to the acetone solvent as an acid scavenger,39 had no affect upon the<br />
results of this reaction. As a final attempt, a recipe that involves the in situ formation and<br />
displacement of a triflate by iodide was followed.^o This reaction occurs rapidly at<br />
98
moderate temperatures, and it was hoped that side reactions would be minimized.<br />
Unfortunately, pyran 98 was the major product under these conditions, also.<br />
97<br />
r<br />
p-CH30C6H4^''^ 4:0<br />
a) 2 eq. n-Bu4N'' I",<br />
2.2 eq. pyridine,<br />
CH9CI9, -78°<br />
b) 2 eq. (CF3S02)0,<br />
-78° to 0°, 1 hour.<br />
• ^ -<br />
Scheme 6.19<br />
O<br />
9 13<br />
11 ^'''/i V<br />
OTBDMS<br />
Acting on the assumption that the bulky silyl protecting group was somehow<br />
causing this unexpected side reaction, we synthesized the unprotected hydroxy mesylate 99<br />
by careful monomesylation of the diol 94, a reaction which proceeded in approximately<br />
quantitative yield (Scheme 6.20). Displacement of the mesylate under the conditions used<br />
for acid sensitive substrates followed by column chromatography afforded 60% of the<br />
desired iodide 100, which could then be silylated under standard conditions in high yield<br />
94 1 eq. MeS02Cl<br />
L2eq. EtsN, *"<br />
CH2CI2, 0°<br />
MPMO<br />
OSOoMe<br />
Scheme 6.20<br />
98<br />
5eqNaI,<br />
refluxing acetone,<br />
1% EtN(i-Pr)2<br />
100<br />
96
97<br />
(99%) to afford the C9-C13 iodide 90. Thus the intermediate 90 was synthesized in eight<br />
steps and 17% overall yield from die commercially avaUable p-hydroxy ester 83.<br />
Altiiough tiie yields for the aldol reaction and the iodide displacement could<br />
possibly be optimized to higher levels, this route to the C9-C13 fragment 90 allowed the<br />
production of significant quantities of tiiis iodide, and it was decided at this point to test die<br />
viabiUty of die remainder of the proposed synthesis of the entire C9-C21 fragment,<br />
beginning with the coupUng of the C14-C21 imine (69) and the C9-C13 iodide (90).<br />
CoupUng of CQ-CI^ Iodide (90^ and C^A-Co^ Imine (69)<br />
Witii concise routes to both the iodide 90 and the imine 69 available, the next step<br />
was die coupling of these two fragments. This coupling to form the ketone 101 would put<br />
in place the entire C9-C21 backbone of oscillatoxin D (15), and of the rest of the<br />
oscUlatoxins and aplysiatoxins as well. Since the stereochemistry of the iodide 90 had<br />
been estabUshed, the C10-C12 stereochemistry of tiiis coupled C9-C21 backbone would be<br />
in place in 101.<br />
MPMO OTBDMS<br />
Model studies had demonstrated that the lithium anion of the imine<br />
69 would react with a primary iodide (68) having branching at the a-position and a<br />
101<br />
SEMO<br />
TBDMS ether at the P-position. Although this model iodide closely resembles iodide 90,<br />
compound 90 is more hindered as it has additional branching at the P-position. In any<br />
case, it was found that deprotonation of 1.5 equivalents of the imine 69 with lithium<br />
dusopropylamide (LDA), followed by die addition of tiie iodide 90, resulted in alkylation<br />
of the imine anion. An excess of the imine anion was used, as the imine was the less<br />
'precious' reagent, and it was desirable to consume all of the hard-won iodide. Subsequent<br />
hydrolysis of the alkylated imine afforded the desired ketone 101 (Scheme 6.21).
69<br />
SEMO<br />
a) 1 eq. LDA, THF, 0°, 30 min<br />
b) 0.67 eq. 90,0°-r.t.. 12 hrs<br />
c) pH 4 buffer, 1 hr<br />
57%<br />
a) 1 eq. LDA, LO eq. HMPA<br />
THF, 0°, 30 min<br />
b) 1.07 eq. 90, 0°- r.t. 10 hrs<br />
c) pH 4 buffer, 1 hr<br />
78%<br />
Scheme 6.21<br />
Unfortunately, the yield for tiiis coupling, with 90 as the limiting reagent, was<br />
lower than that obtained with die model system (57% vs. 72% for the model alkylation). A<br />
further shortcoming of tiiis condensation was that the separation of tiie aUcylated ketone<br />
101 from the excess of starting ketone 65 (generated upon hydrolysis of the leftover imine<br />
starting material 69) was difficult.<br />
Because of die problems associated with die previous aUcylation protocol (low<br />
yield, difficult separation), when it became necessary to repeat this coupling a recipe for<br />
imine aUcylation that included HMPA (to increase die nucleophiUcity of tiie imine anion)<br />
was foUowed.'^i The iodide 90 was used in 7% excess, which was expected to be<br />
advantageous for the purification of the product (as it had been noted that die separation of<br />
the iodide 90 from the alkylated ketone 101 was easier than the separation of the starting<br />
ketone 65 from 101). In fact, this procedure was a substantial improvement over the one<br />
used previously, and a simple purification afforded a 78% purified yield (from the imine<br />
69) of the ketone 101 (Scheme 6.21). •<br />
Asymmetric Reduction of Ci^ Ketone (101)<br />
With ketone 101 in hand, an asymmetric reduction and a subsequent methylation of<br />
the Ci5 carbonyl would afford the 'masked' aldehyde 102 (36, where R=TBDMS and<br />
R'=SEM). Therefore the next step examined was the asymmetric reduction of the Cn<br />
carbonyl of the ketone 101.<br />
Ireland, in die course of an attempted syntiiesis of aplysiatoxin,i3 found good<br />
success in the reduction of the Cn ketone 42 using Noyori's binapthol modified LiAUl4<br />
reagent (Scheme 5.5). 13 The active reducing agent is a trialkoxy Utiiium aluminum hydride<br />
103. The (S)-reagent, which is prepared from (S)-2,2'-binapthol and which reduces<br />
101<br />
98
MPMO OTBDMS OCH.<br />
102<br />
OSEM<br />
prochiral aryl ketones to the (S)-alcohols, is shown (103). (R)-2,2'-Binapthol is used to<br />
make the (R)-reducing reagent, which reduces prochiral aryl ketones to die (R)-alcohols.<br />
The reducing agent is prepared by the addition of ethanol, followed by 2,2'-binapthol, to a<br />
solution of LiAlH4 in THF. This preparation requires extreme care with the stoichiometry<br />
of die reagents. An excess of either ethanol or binaptiiol results in the formation of a<br />
nonreducing tetraalkoxy aluminate. On the other hand, an excess of the LiAlH4 results in<br />
the existence of various hydride species with less than three alkoxy Ugands. These species<br />
would reduce ketones with little or no stereoselectivity, and at higher rates than 103.<br />
Although good control of the stoichiometry is easy to obtain on a large scale (Ireland's<br />
reduction was a 65 mmol scale, with -19 g binapthol used), the small amounts of material<br />
avaUable for preUminary explorations precluded such large scale reactions. It was not<br />
suprising, then, that an attempt to reduce ketone 101 on a 1 mmol scale failed (the ketone<br />
was recovered). At this point it was decided to try another method for tiiis reduction, one<br />
which seemed simpler to perform.<br />
103<br />
An alternative to Noyori's binapthol/LAH reagent 103 was suggested by<br />
Yamamura's utilization of Brown's diisocampheylchloroborane (Ipc2BCl) reagent (104)^6<br />
as an important step in the total synthesis of 3-deoxydebromoaplysiatoxin 44 (Scheme<br />
5.6).i5 Yamamura reported good success at stereoselectively reducing the Cn ketone 45<br />
with this reagent. Although the synthesis of this reagent is reported to be straightforward<br />
OEt<br />
Li"<br />
99
100<br />
(Scheme 6.22), in this laboratory efforts to produce 104 failed. Although this problem<br />
could surely be sumiounted, work with this compound was stopped when the advantages<br />
of a new chiral catalyst developed by Corey became evident.<br />
(+)-a-pinene<br />
92% e.e<br />
BH3-SMe2,<br />
THF,0'<br />
99% e.e.<br />
Scheme 6.22<br />
.o^V2 )2BH<br />
HCl(anhv), ^<br />
Et20, -78°- 0°<br />
104<br />
99% e.e.<br />
In 1987 Corey and coworkers reported the development of a catalytic<br />
enantioselective method for the reduction of prochiral ketones to chiral secondary<br />
,»»vV2 )2BC1<br />
alcohols.'^^ The catalyst for this reaction was oxaboroUdine 105, which is prepared by the<br />
reaction of the chiral amino alcohol 106 with borane (Scheme 6.23). As both enantiomers<br />
of the amino alcohol are available, either isomer of the catalyst 105 is readily synthesized.<br />
o-^<br />
Ph<br />
Ph<br />
\ /<br />
V NH MW OH<br />
106<br />
BH3 - SMe2<br />
THF, A<br />
Scheme 6.23<br />
MeB(0H)2, ^^<br />
4A mol sieves,<br />
benzene reflux<br />
Ph<br />
^—N. .0 ^ 05<br />
^B^<br />
I<br />
H<br />
^ ipitiiiiiir^<br />
Compound 105 efficiently catalyzed the enantioselective reduction of a variety of ketones<br />
I<br />
CH,<br />
via the borane complex 107. This complex has several qualities which cause it to be a<br />
useful chiral catalyst. Complexation of BH3 with the tertiary amine activates it for hydnde<br />
donation, which causes the reduction in the presence of 105 to be much faster than in the<br />
absence of 105. The boron bound between the oxygen and the quaternary (cation.c)<br />
nitrogen is especially Lewis acidic, and as such will coordinate strongly to carbonyl groups<br />
and thereby activate them. In practice, catalyst 105 is quite effective. For example.<br />
Ph
a<br />
^Ph<br />
H,B-<br />
I<br />
R<br />
Ph<br />
iHiiniK^<br />
107 R=H<br />
109 R=CHc<br />
benzophenone was reduced to (S)-phenylethanol in quantitative yield and in 97% e.e. by<br />
BH3 in the presence of 10 mol % of 105.^2<br />
Although catalyst 105 was quite useful, its air and water sensitivity caused it to be<br />
difficult to work with. This led to the development of the catalyst 108,^^3 which is<br />
synthesized by reaction of amino alcohol 106 with methylboronic acid."^ Compound 108<br />
reacts with borane to form complex 109, which reduces ketones as rapidly and as<br />
stereoselectively as 107. As oxazaborolidine 108 is much less air and water sensitive than<br />
105, it is more easUy handled and thus is more practical than the reactive borane 105. As<br />
such, 108 is the reagent of choice in this procedure for asymmetric reductions.<br />
Oxazaborolidine 108, prepared according to the Uterature and used as a cmde<br />
reaction mixture, catalyzed the reduction of ketone 101 by BH3 - THF (Scheme 6.24).<br />
The product, alcohol 110, was isolated in 74% yield. Comparison of the 300 MHz ^^c<br />
NMR spectrum of this product with the spectmm of a 1:1 mixture of diastereomers<br />
epimeric at Cn (produced by a NaBH4 reduction of 101) suggested that the ratio of<br />
diastereomers produced in this reaction was >12:1. The assignment of the absolute<br />
stereochemistry at this newly formed chu-al center was made by analogy with examples of<br />
this reduction in the Uterature. Support for this assignment was provided by the CD<br />
spectrum of 111 (Scheme 6.26, below) which demonstrated positive Cotton effects at 268<br />
and 271 nm, simUar to those observed for debromoaplysiatoxin 2 and various analogues<br />
(17-22). Thus die carbon backbone and absolute stereochemistry of the C9-C21 fragment<br />
110 of the aplysiatoxins and oscUlatoxins have been established.<br />
MPMO OTBDMS MPMO OTBDMS OH<br />
101<br />
L0eq.BH3,<br />
0.20 eq. lof<br />
^ THF,0°.<br />
SEMO 110 SEMO<br />
Scheme 6.24<br />
101
Elaboration of Alcohol (110) tn Aldehyde (119)<br />
With the C9-C21 backbone and absolute stereochemistry in place, metiiylation of the<br />
alcohol 110 using Ireland's recipe proceeded in high yield (75%, Scheme 6.25).<br />
Interestingly, tiie minor Cn epimer of the ether 102 was not evident in the purified<br />
product, as determined by a comparison of the i^c NMR specn-um of this product with that<br />
of a 1:1 mixture of diastereomers epimeric at Cn- Possibly there was a kinetic resolution<br />
in the methylation. That is, it is possible that the 'natural' Cn(S) diastereomer was<br />
methylated more rapidly than the 'unnatural' Cn(R) epimer, and as such the 'unnatural'<br />
isomer was left unreacted and upon chromatography separated from the 'natural' metiiyl<br />
ether. Another explanation is that upon chromatography the isomers were separated, and<br />
that this went unnoticed. In any case, the 'masked aldehyde' 102 was synthesized in<br />
eleven steps and in 7.4% overall yield from the commercially available p-hydroxyester 83.<br />
110<br />
a) 2.5 eq. KH,<br />
THF. 0°- r.t. 1 hr.^<br />
b) 2.5 eq. Mel,<br />
0°, 2 hrs.<br />
MPMO OTBDMS OMe<br />
Scheme 6.25<br />
102<br />
SEMO<br />
Removal of the MPM protecting group from the 9-hydroxyl oxygen was<br />
straightforward. Treatment of the methyl ether 102 with an excess of DDQ in a<br />
CH2CI2/PH 7 buffer solvent mixture produced the alcohol Ul which was isolated in 86%<br />
yield (Scheme 6.26). The mixture of epimers at Cn behaved simUarily. The CD spectmm<br />
of compound 111 had a positive Cotton curve with peaks at 268 and 271 nm, supporting<br />
102<br />
1.3 eq. DDQ. r.t,<br />
CH2Cl2:pH 7 buffer (5:1),<br />
30 min 86%<br />
Scheme 6.26<br />
OTBDMS OMe<br />
111<br />
SEMO<br />
102
103<br />
the stere«:hemical assignt^ent a. €.. TT,e CD spectrum was obtained on this compound<br />
tnstead of an earher tntermediate because of possible interference by the absorption of the<br />
MPM chromophore.<br />
Further progress on this synthesis was perfomied on the mixture of epimers at Cn<br />
so as to preserve the precious diastereomerically pure material. Alcohol lll,asa 1 I<br />
mixture of Cn epimers, was cleanly oxidized to the aldehyde 112 by the conditions<br />
developed by Swern (Scheme 6.27).45 The crude material, which was >95% pure by ^H<br />
NMR, was obtained in quantitative yield. Thus, this aldehyde was syntiiesized in 13 steps<br />
and 8.4% overall yield from 83. This compares weU with Kishi's synthesis of the C8-C21<br />
intertnediate 37 (24 steps from (-)-diethyl D-tartrate, 12% overall yield), and Yamamura's<br />
synthesis of the C8-C21 intermediate 45 (15 steps, 14% overall yield. It is assumed that<br />
the diastereomerically pure alcohol 111 will undergo oxidation in the same manner as the<br />
mixture of epimers to yield the target aldehyde.<br />
111<br />
a) DMSO, (C0C1)2,<br />
CH2CI2, -78°, 20 min<br />
-^~<br />
b) EtsN, -78° - r.t.,<br />
quantitative<br />
Scheme 6.27<br />
O OTBDMS OMe<br />
111<br />
Recent Progress - Synthesis of a C^-C9i Subunit of the<br />
Aplysiatoxins and Oscillatoxins<br />
SEMO<br />
With the aldehyde 112 as a 1:1 mixture of Cn epimers in hand, the C8-C9 bond<br />
forming aldol reaction was attempted. The lithium enolate of ketone 31, kindly provided<br />
by Doug Boatman of our laboratory, was found to undergo an efficient aldol reaction with<br />
112, resulting in the formation of alcohols 113 (Scheme 6.28). The analysis of the ^^C<br />
NMR data for this product is complicated by the fact that the starting aldehyde is a mixture<br />
of diastereomers. However, the ^H NMR spectrum is relatively uncomplicated, suggesting
MPMO o^*\<br />
31<br />
MPMO<br />
a) 1 eq. LDA, THF,<br />
-78°<br />
b) 0.67 eq. 112 '^<br />
Scheme 6.28<br />
113<br />
SEMO<br />
that die aldol reaction may have possibly proceeded with some amount of stereochemical<br />
control. In the investigation of a model system, odier workers in this laboratory have<br />
observed the formation of a 4:1 mixture of diastereomers in die aldol reaction between the<br />
enolate of 31 and aldehyde 32 (R=TBDMS or Bn, Scheme 6.29).ll'l2 This stereocontrol<br />
has been attributed to a chelation-controUed addition of tiie enolate to the protected p-<br />
hydroxy aldehyde 32. If this is the case, tiien the aldol product would favor a 9S<br />
configuration. This is an important point, as the natural products 1-14 require this very<br />
stereochemistry at C9. Further work needs to be done to elucidate die actual C9<br />
stereochemistry of the aldol product 113.<br />
31<br />
a) LDA<br />
32<br />
Scheme 6.29<br />
•'t<br />
3<br />
^ /<br />
MPMO 0^<br />
114<br />
7<br />
HO*^<br />
9 i<br />
OR<br />
1 ^<br />
1<br />
Finally, the p-hydroxy ketone 113 was oxidized, once again utilizing the Swern<br />
protocol, to the P-diketone 115 in moderate yield (53% after purification, Scheme 6.30).<br />
This compound, a protected version of the model compound 33, is the most advanced<br />
intermediate of oscillatoxin D (15) yet synthesized in this laboratory. Because results from<br />
the model study tiiat resulted in the fortiiation of the spirocyclic ether 27 appear to suggest<br />
that sUght modifications in the syntiietic plan may be advantageous (for example, a different<br />
Cn hydroxyl protecting group may be desirable), further work on the total synthesis of<br />
oscUlatoxin D (15) awaits die completion of those model studies.<br />
104
4 eq. DMSO,<br />
2 eq. (C0C1)2,<br />
CH2CI2. -78°<br />
MPMO<br />
a) 113, -78°<br />
b) EtsN, -78°-r.t.<br />
Scheme 6.30<br />
0
CHAPTER 7<br />
EXPERIMENTAL DETAILS<br />
General Method
(R)-2'-Hvdroxy-2'-(T'-hPn.yi^vy>)phenvlPthylA-<br />
(methvnhenyenesulfonntp (^1)<br />
TsO<br />
9H<br />
BnO<br />
C22H22O5S f.w. = 380.48<br />
To 0.024g (0.10 mmol) of (R)-l-(3'-benzyloxy)phenyl-l,2-ethanediol,48 9.7 [Jd<br />
pyridine, and 0.5 ml CH2CI2 stirring at room temperature in a 5 ml round bottomed flask<br />
was added 0.019 g (0.12 mmol) 4-methylbenzenesulfonyl chloride. This mixuire was<br />
allowed to stir for 12 hours, then added to a separatory funnel containing 10 ml ether and<br />
10 ml water. The phases were separated and the aqueous layer was washed with 3x5 ml<br />
ether. The organic layers were combined, washed with 10 ml brine, and dried over<br />
MgS04. Concentration under vacuum followed by flash column chromatography (10 g<br />
silica gel, 1:1 hexanes:ethyl acetate) afforded 0.011 g (0.033 mmol, 33%) of the epoxide.<br />
iH (200 MHz, relative to CHCI3 at 7.26 ppm) 8 7.79-7.73 (m, 2H), 7.45-7.18 (m,<br />
8H), 6.95-6.85 (m, 3H), 5.03 (s, 2H), 5.00-4.91 (m, IH), 4.18-3.97 (m, 2H),<br />
2.68-2.50 (m, 1H[-0H, variable]), 2.44 (s, 3H).<br />
13C (50 MHz) 5 159.02, 145.07, 139.83, 136.67, 132.55, 129.92, 129.74, 128.58,<br />
128.00, 127.94, 127.49, 118.66, 114.83, 112.62, 74.29, 71.80, 69.97, 21.64.<br />
(RV1-(3'-rBenzyloxv1phenyl)ethane-1.2-epoxide(62)<br />
BnO<br />
CnHi402 f-w. = 226.29<br />
107
To 0.0077 g (0.020 mmol) tosylate 61 and 4 ml THF stirring under a nitrogen<br />
atmosphere at room temperature in a 10 ml round bottomed flask was added 0.0023 g NaH<br />
(-0.056 mmol, 60% suspension in mineral oil). After 1.5 hours the reaction mixture was<br />
diluted witii 10 ml hexanes and filtered tiirough a medium fritted funnel. Concentration<br />
under vacuum afforded the desired product and mineral oil (^H NMR). Flash column<br />
chromatography (10 g sUica gel, solvent gradient form 98:2 to 8:2 hexanes:etiiyl acetate)<br />
afforded 0.0028 g (0.012 mmol, 61%) of the desired product as a clear oU, which was<br />
only characterized by ^H-NMR spectrometry.<br />
IH (200 MHz, relative to CHCI3 at 7.26 ppm) 5 7.45-7.22 (m, 6H), 6.93-6.89 (m,<br />
3H), 5.06 (s, 2H), 3.84 (d of d, 1=4.2,2.5, IH), 3.13 (d of d, 1=5.6,4.2, IH),<br />
2.76 (d of d, 1=5.6,2.5, IH).<br />
(RVl"-(2"'-NaDthon-2"-napthyl 2-(3'-methoxv)phenvlethanoate<br />
(63)<br />
MeO<br />
C29H22O4 f.w. =434.51<br />
To 1 66 g (10 mmol) 2-(3'-metiioxy)phenylethanoic acid and 15 ml<br />
CH2Cl2,stirring in a 25 ml round bottomed flask at room temperature under a nitrogen<br />
with a reflux condenser and brought to reflux for 16 hours. The tlasK<br />
a distillation apparatus and the solvent and residual thionyl chloride - - ~ ^ ;^<br />
distiUation under a nitrogen atmosphere. The distillation apparatus -^ *-;> ^^<br />
high vacuum (0.4 mm Hg) and the residue distilled to afford 0.77 g (42%) of 2-(3<br />
niethoxy)phenylethanoic acid chloride.<br />
108
109<br />
IH (200 MHz, relative to CHCI3 at 7.26 ppm) 5 7.29 (t, J=7.8, IH), 6.91-6.79 (m,<br />
3H), 4.11 (s, 2H), 3.81 (s, 3H).<br />
BP 97° at 0.4 mm Hg.<br />
To 0.063 g(0.2 mmol) (R)-2,2'-binaptiiol and 3 ml ether stirring at 0° (ice bath)<br />
under a nitrogen atmosphere in a 5 ml round bottomed flask was added 0.096 ml n-BuLi<br />
(2.3 M solution in hexanes, 0.22 mmol), followed by 0.037 g (0.2 mmol) of the<br />
aforementioned acid chloride in 1 ml ether. The resulting suspension was allowed to warm<br />
to room temperature over 1 hour, and then was stirred at room temperature for an additional<br />
3 hours. The milky suspension was then dissolved in 25 ml CH2CI2, filtered through a<br />
fritted glass funnel (medium frit), and concentrated under vacuum to afford 0.84 g (97%)<br />
of a white solid that is >95% pure (according to ^H NMR). This product was characterized<br />
only by ^H NMR spectrometry.<br />
IH (300 MHz, relative to TMS at 0.00) d 7.99-7.88 (m, 4H), 7.44-7.33 (m, 6H),<br />
7.15 (t, 1=6.0, 2H), 6.98 (t, 1=7.9, IH), 6.69 (m, IH), 6.49 (d, 1.7, IH), 6.42<br />
(d, J=7.5, IH), 3.68 (s, 3H), 3.36 (s, 2H).<br />
:^-(2'-TrimetiiylsUvlethoxy)methoxvacetophenone(65)<br />
'SiMeg<br />
Ci4H2203Si f-w. = 266.45<br />
To a 25 mL round bottomed flask containing 0.641 g (4.71 mmol) of 3-hydroxy-<br />
acetophenone, 1.22 mL (7.00 mmol) ethyldUsopropylamine, and 10 mL dichloromethane<br />
stimng at 0° under nitrogen was added 1.00 mL (5.65 mmol) 2-trimetiiylsilylethoxymethyl<br />
chloride dropwise over die course of 5 minutes. The reaction mixture was then dUuted<br />
widi 25 mL ether and washed successiyely witii 25 mL water, 25 mL saturated NaHC03,<br />
and 25 mL brine, then dried over MgS04 and concentrated under vacuum. Flash
110<br />
chromatography (20 g siUca gel, 9:1 hexanes:etiiyl acetate) afforded 0.985 g (3 70 mmol<br />
78.6%) of the desired compound as a clear liquid.<br />
'" ?'? ^^' -lative to CHC13 at 7.26 ppm) 6 7.59 (m, 2H), 7.38 (t, 1=7.8, IH),<br />
7.24 (m, IH), 5.26 (s, 2H), 3.76 (d of d, 1=8.3,8.3, 2H), 2.59 (s, 3H), 0 96<br />
(d of d, 1=8.3,8.3, 2H), -0.01 (s, 9H).<br />
13C (50 MHz) 5 197.76, 157.57, 138.48, 129.56, 121.72, 121.02, 115 64 92 84<br />
66.39, 26.70, 17.98, -1.46. ' " '<br />
IR 3072, 2954, 2898, 1686, 1584, 1485 cm-l.<br />
TLC Rf = 0.53 (8:2 hexanes:ethyl acetate).<br />
ANALYSIS Calcd: C, 63.12; H, 8.33<br />
Found: C, 63.09; H, 8.43.<br />
(S)-l-(r-Butyldimedivnsilyloxv-2-methvlpropyl<br />
methanesulfonate. (67)<br />
TBDMSO OSOoMe<br />
CiiH2604SSi f.w. = 282.52<br />
To 0.82 g (4 mmol) (R)-3-(r-butyldimethyl)silyloxy-2-methylpropanol,48 0.78 ml<br />
(0.56 mmol) triethylamine and 20 ml CH2CI2 stirring at 0° (ice/water bath) under nitrogen<br />
in a 50 ml round bottomed flask was added 0.37 ml (4.8 mmol) methanesulfonyl chloride.<br />
After 1 hour the reaction mixture was diluted with 50 ml ether and washed with 25 ml<br />
water, 25 ml saturated aqueous NaHC03, and 25 ml brine. The mixture was then dried<br />
over MgS04 and concentrated under vacuum to afford 1.09 g (96%) of the desired product<br />
as a yellow oil.<br />
*H (300 MHz, relative to CHCI3 at 7.26 ppm) 5 4.23 (d of d, 1=9.5,5.9, IH), 4.15 (d<br />
of d, 1=9.5,5.7, IH), 3.61 (d of d, 1=10.1,4.8, IH), 3.49 (d of d, 1=10.1,6.5,<br />
IH), 3.00 (s, 3H), 2.06 (m, IH), 0.98 (d, 1=6.9, 3H), 0.89 (s, 9H), 0.05 (s,<br />
6H).<br />
*^C (50 MHz) 5 71.69, 63.62, 36.98 35.69, 25.85, 18.24, 13.20, -5.51.
IR 2955, 1472, 1356 cm-i.<br />
TLC RpO.33 (9:1 hexanes:etiiyl acetate).<br />
(S)-l-Iodo-2-metiiyl-^-?-hutyldimp.fhy1silvloxvpentnne (f^R)<br />
TBDMSO<br />
C10H23IO f.w. = 286.23<br />
To a solution of the crude mesylate 67 (1.09 g, 3.9 mmol) and 20 ml acetone in a<br />
50 ml round bottomed flask was added Nal until tiie solution was saturated. The flask was<br />
then fitted with a water cooled condenser and heated to reflux for 4 hours. The reaction<br />
mixture was cooled, diluted witii 100 ml ether, and washed with 4x50 ml water and 50 ml<br />
brine. The ether solution was then dried over MgS04 and concentrated under vacuum to<br />
afford an orange oil. Flash column chromatography (20 g sUica gel, hexanes eluent)<br />
afforded 0.98 g (3.4 mmol, 86% overall for two steps) of die desired product as a clear oil.<br />
IH (200 MHz, relative to TMS at 0.00 ppm) 5 3.53 (d of d, 1=9.9,5.0, IH), 3.39 (d<br />
of d, 1=9.9,6.8, IH), 3.28 (m, 2H), 1.63 (m, IH), 0.95 (d, 1=6.7, 3H), 0.90<br />
(s,9H), 0.06 (s, 6H).<br />
13C (50 MHz) 5 66.64, 37.32, 25.88, 18.23, 17.22, 13.80, -5.38.<br />
TLC Rf=0.51 (hexanes).<br />
Ill
N-l-(3'-[(2"-Trimethylsilylethoxy)mPfbr.vy]ph.n:'n<br />
etiivlidinecyrlohexvlaminp (^Q)<br />
C20H33NO2Si<br />
SiMe.<br />
f.w. = 347.63<br />
A 25 mL round bottomed flask fitted with a magnetic stir bar, a Dean-Stark trap<br />
(filled with benzene), and a condenser under nitrogen was charged witii 2.00 g (7.51<br />
mmol) of m-(2-trimetiiylsilylethoxy)methoxyacetophenone, 3.43 mL (30.0 mmol)<br />
cyclohexylamine (distilled from CaH2 and stored under nitrogen until use), 5 g activated<br />
4A molecular sieves, and 15 mL benzene. The mixture was then brought to reflux (bath<br />
temperature 95°) for 12 hours, filtered, and the molecidar sieves washed with 10 mL<br />
benzene. Concentration of the organic extracts afforded 2.36 g (6.79 mmol, 90.5% cmde)<br />
of a sUghtiy yeUow oU that is >95% pure by ^H NMR. In practice this imine was used as<br />
the cmde material. It could, however, be distUled to high purity witii minimal loss of<br />
material.<br />
*H (300 MHz, relative to CHCI3 at 7.26 ppm) 5 7.41 (m, IH), 7.37 (d of d of d,<br />
1=7.7,1.6,1.1, IH), 7.26 (d of d, 1=7.7,8.1, IH), 7.04 (d of d of d,<br />
1=8.1,2.5,1.1, IH), 5.24 (s, 2H), 3.76 (d of d, 1=9.6,7.1, 2H), 3.47 (m, IH),<br />
2.22 (s, 3H), 1.83 (m, 2H), 1.70 (m, 3H), 1.60 (m, 2H), 1.36 (m, 3H), 0.96 (d<br />
of d, 1=9.6,7.1, 2H), 0.00 (s, 9H).<br />
13C (50 MHz) 5 162.10, 157.33, 143.50, 129.09, 120.14, 116.71, 114.77, 92.90,<br />
66.15, 59.85, 33.51, 25.78, 24.85, 18.01, 15.36, -1.42.<br />
BP 185° at 2 mm Hg.<br />
IR 2928, 2854, 1634, 1580, 1249, 1088, 1015, 859, 836, 692 cm"!.<br />
TLC Rf = 0.42 (8:2 hexanes:ethyl acetate).<br />
112
ANALYSIS Calcd: C, 69.11; H, 9.57<br />
Found: C, 69.48; H, 9.66.<br />
(R)-l-(3'-[(2"-Trimethylsilylethoxv^methoxy1php.ny1)-4methyl-5-(f-butvldimp.thynsUvloxV-1<br />
-pentanone (11)<br />
TBDMSO<br />
SEMO<br />
C24H4404Si2 f.w.= 452.86<br />
To 0.092 g (0.26 mmol) of die imine 69 and 1 ml THF stming at 0° under a<br />
nitrogen atmosphere in a 5 ml pear shaped flask was added 0.26 ml (0.29 mmol) LDA<br />
dropwise. The resulting bright yellow solution was aUowed to stir for 30 minutes, then<br />
0.089 g (0.40 mmol) of iodide 68 in 1 ml THF was added dropwise. The solution was<br />
aUowed to warm to room temperature over 1 hour, then stirred at room temperature for an<br />
additional 12 hours. The mixtiu-e was then added to a 50 ml round bottomed flask<br />
containing 20 ml THF and 20 ml pH 4 buffer, and this mixmre was allowed to stir at room<br />
temperamre for 4 hours. The layers were tiien separated, the aqueous layer extracted widi<br />
3x10 ml ether, and die organic layers combined and washed witii 25 ml brine. The organic<br />
solution was dried over MgS04 and concentrated under vacuum to afford a yellow oil.<br />
Hash column chromatography (20 g silica gel, 9:1 hexanes:ethyl acetate) afforded 0.085 g<br />
(0.19 mmol, 72%) of the desired ketone as a clear oil.<br />
*H (200 MHz, relative to CHCI3 at 5 7.26 ppm) d 7.63-7.58 (m, 2H), 7.39 (t, J=7.7,<br />
IH), 7.28-7.23 (m, IH), 5.28 (s, 2H), 3.78 (d of d, 1=2.9,2.9, 2H), 3.49 (d of d,<br />
1=4.8,1.0, 2H), 3.00 (t, 1=8.2, 2H), 1.95-1.50 (m, 6H), 1.02-0.89 (m, 5H),<br />
0.91 (s, 9H), 0.06 (s, 6H), 0.02 (s, 6H).<br />
IR 2928, 2856, 1689, 1631, 1581 cm-i.<br />
TLC Rf = 0.68 (8:2 hexanes:ethyl acetate).
(S)-3-(4'-MethoxvphenYl)mPthoxv-2-methylprnp.n.l(QO)<br />
C12H16O3 f.w. = 208.28<br />
Dnsobutylaluminum hydride (10.49 ml of a 1.5 M solution in toluene, 15.73<br />
mmol) was added over the course of two hours to a 250 ml round bottomed flask<br />
containing 3.00 g (12.58 mmol) of the ester and 125 ml CH2CI2 stirring under nitrogen at<br />
-78°. After stirring one additional hour at -78°, tiiis mixture was quenched by the dropwise<br />
addition of 3 ml anhydrous methanol. This solution was warmed to room temperature and<br />
dUuted with 100 ml ether, then 2 ml water were carefuUy added, and the mixture was<br />
aUowed to stir for 15 minutes. The milky suspension was then filtered through a 3 cm pad<br />
of ceUte, and the filtrate concentrated under vacuum to afford 2.69 g (102.7%) of a clear<br />
liquid. Proton NMR indicated that the aldehyde was ~95% pure, with a small amount of<br />
the alcohol (resulting from over reduction) as the other major product. No attempt was<br />
made to obtain an elemental analysis of this compound.<br />
IR (200 MHz, relative to TMS at 0.00 ppm) 5 9.71 (d, J=1.5, IH), 7.24 (d, J=8.6,<br />
2H), 6.88 (d, 1=8.6, 2H), 4.45 (s, 2H), 3.80 (s, 3H), 3.63 (m, 2H), 2.65 (m,<br />
IH), 1.08 (d, 1=6.1, 3H).<br />
13c (50 MHz) 5 204.0, 159.2, 130.0, 129.2, 113.8, 73.0, 69.7, 55.2, 46.8, 10.7.<br />
IR 2962, 2935, 2858, 1723, 1612, 1513, 1458, 1302, 1248, 1034, 819cm-l.<br />
TLC Rf = 0.49 (7:3 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D= +29.42° (c=0.0906 g/ml, CH2CI2).
(S)-Methyl3-(4'-mp,rhnvyphqnvnmethnyY-9methvlpropionate<br />
(84)<br />
MPMO<br />
XOaCHg<br />
C13H18O4 f.w. = 238.31<br />
To a 250 ml round bottomed flask containing 17.03 g (123.29 mmol) p-methoxy-<br />
benzyl alcohol and 150 ml etiier stming under nitrogen was added 0.46 g sodium hydride<br />
(60% mineral oil dispersion, 11.63 mmol), and the resulting suspension allowed to stir<br />
untU the solid had dissolved and gas evolution ceased (one hour). The mixture was then<br />
cooled to 0° and 12.36 ml (123.30 mmol) of trichloroacetonitrile was added over 14<br />
minutes. After stirring for an additional five minutes at 0° the mixture was warmed to room<br />
temperature, stured for an additional 20 minutes, then washed with 50 ml saturated<br />
aqueous NaHCOs foUowed by 50 ml brine, dried over MgS04 and concentrated under<br />
vacuum. The resulting yellow oU was dissolved in 150 ml CH2CI2 and 9.07 ml (82.20<br />
mmol) of (S)-methyl 3-hydroxy-2-methylpropionate, foUowed by 0.90 g (3.59 mmol) of<br />
pyridinium p-toluenesulfonate, were added, and the mixture allowed to stk at room<br />
temperature for 22 hours (a white crystalUne precipitate fornis). The reaction mixture was<br />
then washed with 75 ml saturated NaHCOs foUowed by 75 ml brine, dried over MgS04,<br />
and concentrated under vacuum. The trichloroacetamide was precipitated by the addition of<br />
a 1:1 mixture of hexanes:CH2Cl2, the solid filtered off, the liquid phase concentrated under<br />
vacuum, and the remaining dissolved trichloroacetamide removed by a bulb to bulb<br />
distillation at 0.07 mm Hg. The residue was distilled through a 10 cm vigreux column,<br />
collecting the distiUate over die range 98° - 110° (0.07 mm Hg), for a yield of 13.10 g<br />
(54.97 mmol, 66.9%)) of die desUed product.<br />
iH (200 MHz, relative to TMS at 0.00 ppm) 5 7.24 (d, J=8.7, 2H), 6.87 (d, 1=8.7,<br />
2H), 4.45 (s, 2H), 3.80 (s, 3H), 3.69 (s, 3H), 3.63 (d of d, J=9.2, 7.4, IH),<br />
3.45 (d of d, 1=9.2, 5.9, IH), 2.77 (m, IH), 1.17 (d, J=7.1, 3H).<br />
13C (50 MHz) 6 159.2, 129.4, 129.2, 113.8, 72.8, 71.7, 71.5, 55.3, 51.7, 40.2,<br />
14.0.<br />
IR 2951, 2861, 1739, 1612, 1513, 1248, 820cm-l.
BP 98-110° at 0.07 mm Hg. ^16<br />
TLC Rf = 0.51 (7:3 hexanes:ethyl acetate)<br />
OPTICAL ROTATION [a]o= +9.67° (c=0.0838 g/ml CHCh)<br />
ANALYSIS Calcd: C, 65.52; H, 7.61 '<br />
Found: C, 65.34; H, 7.45.<br />
phenyl]methoxy-r,4^dim^h^^^^i^^^<br />
Phenvl-4-mPfhy1-9-oxa7.nliHmnn.(g^)<br />
MPMO<br />
C25H31NO6 f.w. =441.57<br />
To a 100 mL round bottomed flask containing 13.72 ml (1.0 M in hexanes, 13.72<br />
mmol) triethylborane stining at 0° under nitrogen was added dropwise 1.11 ml (12.58<br />
mmol) of triflic acid. The flask was tiien immersed in a 40° oU batii and allowed to stir for<br />
30 minutes (gas evolution ceased at that time), tiien cooled in an ice/water bath and 2.69 g<br />
(11.44 mmol) of the N-propionyloxazoUdinone^o in 56 ml CH2CI2 was added, followed<br />
by 2.19 ml (12.58 mmol) of ethyl dUsopropylamine. After 30 minutes, the reaction<br />
mixture was cooled to -78° and 2.62 g (12.58 mmol) of the freshly prepared aldehyde was<br />
slowly added. The reaction was maintained at -78° for one hour, then warmed to room<br />
temperature. Four hours later the reaction was quenched by the addition of 20 ml pH 7<br />
buffer, the layers separated, and the buffer was back extracted with 2x25 ml CH2CI2. The<br />
combined organic extracts were concentrated, and the resulting yellow oil was dissolved in<br />
35 ml methanol, cooled in an ice/water bath, and 12 ml 30% aqueous H2O2 were added.<br />
The solution was warmed to room temperature over the course of one hour, then water (60<br />
ml) was added, the methanol was removed under vacuum, and the aqueous suspension<br />
was extracted with 3x75 ml CH2CI2. The combined organic extracts were then washed<br />
with 100 ml brine, dried over MgS04, and concentrated to afford a yellow oil which<br />
slowly crystaUized. The product was recrystalUzed from ether/hexanes (two crops; 2.46 g,<br />
5.57 mmol, 48.7% combined) and die mother liquor chromatographed (60 g flash silica
gel, 8:2 hexanesxthyl acetate), then recrystallized (0.24g, 0.54 mmol, 4.8%) to afford a^ '^<br />
total of 2.70 g (6.11 mmol, 53.1%) of the diastereomerieaUy pure aldol product as white<br />
needles.<br />
iH (300 MHz, relative to TMS at 0.00 ppm) 5 7.40 (m, 5H), 7.28 (d, 1=8 7 2H)<br />
6.87 (d, 1=8.7, 2H), 5.62 (d, 1=7.1, IH), 4.74 (p, 1=6.7, IH), 4.45 (s, 2H), '<br />
3.90 (m, 2H), 3.80 (s, lH[OH, variable]), 3.79 (s, 3H), 3.57 (m, 2H), 1.97 (m,<br />
IH), 1.22 (d, 1=6.8, 3H), 0.96 (d, J=7.0, 3H), 0.90 (d, 1=6.6, 3H). '<br />
13C (50 MHz) 6 175.93, 159.27, 152.82, 133.24, 129.82, 129.40, 128.71(two<br />
carbons), 125.62, 113.81, 78.95, 75.58, 74.75, 73.19, 55.26, 55.21, 40.85,<br />
35.90, 14.31, 13.55, 9.51.<br />
IR 3478, 3056, 2966, 2935, 1770, 1698, 1613, 1586, 1514, 1455 cm-l.<br />
MP 113-115°<br />
TLC Rf = 0.55 (1:1 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D= +23.74° (c=0.1100 g/ml, CH2CI2).<br />
ANALYSIS Calcd: C, 68.00; H, 7.08<br />
Found: C, 69.13; H, 7.20.<br />
(l"S.2'R.3"S.4'R.5S.6"S)-3-(2'-r6"-Methvl-2".4"-dioxa-<br />
3"-(4"methoxyphenyl)cyclohex-l"-yl1-propanoyl)-5-phenyl-<br />
4-methyl-2-oxazolidinone (89)<br />
p-MeOC6H4<br />
C25H29NO6 f.w. = 439.55<br />
To 0.042 g (0.1 mmol) of the aldol (88), 4 ml CH2CI2, and 1 ml pH 7 buffer<br />
stining at room temperature in a 10 ml round bottomed flask was added 0.030 g (0.13<br />
mmol) 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The solution immediately turned<br />
brownish green. After 30 minutes the mixture was added to 50 ml hexanes, and this<br />
mixture was extracted witii 10x25 ml samrated aqueous NaHC03. The ether layer was
118<br />
then dried over MgS04 and concentrated under vacuum. Hash column chromatography<br />
(10 g silica gel, 8:2 hexanes:etiiyl acetate) afforded 0.027 g (0.06 mmol) of die acetal as a<br />
colorless oU. This compound was only characterized by iH NMR spectrometry.<br />
iH (300 MHz, relative to TMS at 0.00 ppm) 5 7.45-7.28 (m, 7H), 6.88-6 92 (m<br />
2H), 5.65 (d, 1=7.1, IH), 5.49 (s, IH), 4.72 (p, 1=6.7, IH), 4.17-4.09 (m 2H) 4 01-<br />
3.38 (m, 2H), 3.80 (s, 3H), 3.58 (d of d, 1=11.0,11.0, IH), 2.11-2.01 (m, lH),'l.68-<br />
1.55 (m, 2H), 1.48-1.15 (m, 6H), 1.22 (d, J=6.8, 3H), 0.97 (d, J=7.0, 3H), 0.88 (d<br />
J=6.6, 3H).<br />
(2S,3S,4S)-l-Iodo-5-(4'-methoxvphenvnmethoxv-2.4dimethyl-3-r-hutvldimetiiylsiloxvpentane(Qn)<br />
MPMO OTBDMS<br />
C2iH3703SiI f.w. = 492.57<br />
To the iodo alcohol (0.50 g, 1.33 mmol), 2,6-lutidine (0.23 ml, 2.00 mmol) and 2<br />
ml CH2CI2 stirring in a 10 ml pear shaped flask under nitrogen was added 0.37 ml (1.60<br />
mmol) of tert-butyldimetiiylsUyl trifluoromethanesulfonate (TBDMSOTf) dropwise. After<br />
30 minutes the starting material had been consumed (TLC) so die reaction was poured onto<br />
25 ml saturated NaHC03 solution, which was then extracted with 3x25 ml ether. The<br />
organic extracts were washed with brine, dried over MgS04, and concenmated under<br />
vacuum to afford a clear oil (0.84 g, >100%). Flash column chromatography (9:1<br />
hexanes:ethyl acetate, 20 g sUica gel) gave 0.65 g (99.4%) of die desired product as a clear<br />
oil. This corresponds to a 43% yield over the five steps from the aldol product 88. No<br />
attempt was made to obtain an elemental analysis of this compound.<br />
^H (200 MHz, relative to TMS at 0.00 ppm) 5 7.25 (d, J=8.8, 2H), 6.88 (d J=8.8,<br />
2H), 4.45 (d, 1=10.1, IH), 4.38 (d, 1=10.1, IH), 3.81 (s, 3H), 3.67 (d of d,<br />
1=6.1,.3.0, IH), 3.50 (d of d, 1=9.0,4.9, IH), 3.24 (d of d, 1=11.4, 7.0, IH),<br />
3.21 (d of d, 1=9.5,4.2, IH), 3.11 (d of d, 1=9.5,7.1, IH), 1.93 (m, 2H), 0.99<br />
(d, 1=6.8, 3H), 0.94 (d, 1=7.0, 3H), 0.88 (s, 9H), 0.07 (s, 3H), 0.05 (s, 3H).
13C (75MHz) 5 159.09,130.03,129.15,113.74,76.37,72.72,72 29 55 27<br />
39.61, 38.09, 26.09, 18.40, 15.28, 14.90, 14.55, -3.70, -4.12.<br />
IR 2955, 2930, 2856, 1612, 1513, 1463, 1249, 1038, 835, 774cm-l.<br />
TLC Rf = 0.40 (95:5 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D=+7.84° (c=0.0430 g/ml, CH2CI2).<br />
(2'R,3'S,4'S,4R,5S)-3-(5'-r4"-MPthoxv-phenv11-n.Pth^vy.<br />
2'.4'-dimethyl-3'-r-hntyldimethy1-sUvloxv-pp.ntannyl)-Sphenyl-4-methvl-2-oxazolidinonp<br />
(Ql)<br />
MPMO^<br />
TBDMSO<br />
C3iH45N06Si f.w. = 551 86<br />
To 0.068 g (0.16 mmol) of the aldol 88, 0.047 ml (0.40 mmol) 2,6-lutidine, and<br />
0.5 ml CH2CI2 stirring under a nitrogen atmosphere in a 5 ml pear shaped flask was added<br />
0.055 ml (0.24 mmol) of TBDMSOTf. After 2 minutes the reaction mixmre was quenched<br />
by addition to a separatory funnel containing 10 ml ether and 10 ml water. The layers were<br />
separated and the aqueous layer extracted witii 3x5 ml ether. The organic phases were<br />
combined and extracted with 10 ml brine, and tiien dried over MgS04. Concentration<br />
under vacuum, followed by flash column chromatography (10 g silica gel, 8:2<br />
hexanes:etiiyl acetate) afforded 0.070 g (0.13 mmol, 82%) of die desked product as a<br />
colorless oil.<br />
1H (300 MHz, relative to TMS at 0.00 ppm) 5 7.42-7.26 (m, 7H), 6.89-6.81 (m,<br />
2H), 5.04 (d, 1=7.1, IH), 4.53 (p, 1=6.7, IH), 4.41 (d, J=14.0, IH), 4.40 (d,<br />
J=14.0, IH), 4.04-3.96 (m, 2H), 3.67 (s, 3H), 3.59 (d of d, 1=9.2,5.8, IH),<br />
3.19 (d of d, 1=9.2,5.6, IH), 2.02-1.92 (m, IH), 1.21 (d, J=7.1, 3H), 1.03 (d,<br />
J=7.1, 3H), 0.90 (s, 9H), 0.82 (d, 1=6.6, 3H), 0.070 (s, 3H), 0.068 (s, 3H) .<br />
13c (50 MHz) 5 175.87, 158.99, 152.37, 133.31, 130.68, 129.12, 128.53 [two<br />
carbons], 112.54, 113.63, 78.42, 75.47, 72.59, 71.69, 55.04, 54.89, 41.65,<br />
38.80, 26.05, 18.30, 15.15, 14.81, 14.10, -3.85, -3.90.<br />
119
IR 3065, 3034, 2932, 2856, 1783, 1698, 1613, 1513 cm-l.<br />
TLC Rf = 0.60 (8:2 hexanes:ethyl acetate).<br />
(2S,3R,4S)-3-f-Butvldimethvlsilvlnxy-5-(4'-mpthnYyphenvDmethoxv-7.,4-dimethvl-1<br />
-pentanol (92^<br />
MPMO OTBDMS<br />
C2iH3804Si f.w. = 382.68<br />
To 0.36 g (0.87 mmol) of the p-silyloxy ester 96 and 5 ml THF stming at 0°<br />
(ice/water bath) under a nitrogen atmosphere in a 25 ml pear shaped flask was added 2.2 ml<br />
DIBAL-H (2.2 mmol, 1.0 M solution in toluene) dropwise. After 30 minutes, the reaction<br />
mixture was quenched by the careful addition of 2 ml methanol, followed by 5 ml water.<br />
The resulting grey gel was filtered tiirough a one inch pad of celite, foUowed by 4x25 ml<br />
ether washes. The filtrate was washed witii 25 ml brine, dried over MgS04, and<br />
concentrated under vacumm to afford 0.32 g (0.83 mmol, 96%) of the desired product as a<br />
clear oil, >90% pure by ^H NMR. Flash column chromatography (20 g silica gel, 8:2<br />
hexanes;ethyl acetate) afforded 0.30 g (0.78 mmol, 90%) of the pure alcohol.<br />
IH (300 MHz, relative to TMS at 0.00 ppm) 5 7.25 (d, J=8.7, 2H), 6.87 (d, J=8.7,<br />
2H), 4.44 (d, 1=13.4, IH), 4.27 (d, 1=13.4, IH), 3.80 (s, 3H), 3.74 (d of d,<br />
1=5.7,2.9, IH), 3.51 (m. 3H), 3.27 (d of d, 1=9.1,7.1, IH), 2.03 (m, 2H), 1.87<br />
(m, IH), 0.96 (d, 1=7.0, 3H), 0.88 (s, 9H), 0.86 (d, 1=7.0, 3H), 0.06 (s, 3H),<br />
0.04 (s, 3H).<br />
13C (50 MHz) 5 159.07, 130.54, 129.19, 113.70, 74.72, 72.66, 72.61, 65.98, 55.21,<br />
38.86, 37.53, 25.99, 18.26, 15.04, 11.89, -4.25.<br />
IR 3428, 2961, 2926, 2849, 1613, 1514, 1249 cm-i.<br />
OPTICAL ROTATION [a]D= -5.29° (c=0.2180 g/ml, CH2CI2).<br />
TLC Rf=0.25 (hexanes).<br />
120
(lR,2R,2'S,3'S.4'SVN-(r-HYH..vy,r_^henv1prnp.n-0'.<br />
yl)-5-(4'-methoxyphpnynmethoyy-^-r-butvldimpthYlsUyloxv-2,4-HimPthyl-1-pp.nty1aminp(0^)<br />
OTBDMS<br />
MPMO' ^Y^^'^Nr^'^H OH<br />
C3oH48N04Si f.w = 515.89<br />
To 0.070 g (0.13 mmol) of die silylated aldol product 91 and 2 ml ether stirring in<br />
a 5 ml pear shaped flask at -78° under a nitrogen atmosphere was added 0.0038 g (0.1<br />
mmol) LiAlH4. After warming to room temperamre (2 hours) the reaction was quenched<br />
by the addition of 3 drops water, followed by 3 drops IN NaOH and 6 more drops water.<br />
MgS04 was added and the solution filtered through a medium fritted funnel, with a 10 ml<br />
ether wash. Concentration under vacuum foUowed by flash column chromatography (10 g<br />
siUcal gel, 8:2 hexanes:ethyl acetate) afforded 0.04 g (0.078 mmol, 60%) of the amino<br />
alcohol.<br />
iH (300 MHz, relative to CHCI3 at 7.26 ppm) 5 7.49-7.46 (m, 2H), 7.32-7.23 (m,<br />
5H), 6.90-6.87 (m, 2H), 5.95 (d of d, 1=10.5,4.0, IH), 4.92 (d of d, 1=12.7,4.3,<br />
IH), 4.86 (s, IH), 4.72 (br q, 1=6.9, IH), 4.66 (s, IH), 4.47 (d of d,<br />
1=12.8,11.6, 2H), 4.43-4.34 (M, IH), 4.99 (d, J=9.6, IH), 3.81 (s, 3H), 3.75 (d<br />
of d, 1=9.8,9.8, IH), 3.43 (d of q, 1=9.6,6.9, IH), 1.85-1.80 (m, IH), 1.15 (d,<br />
J=6.9, 3H), 0.97 (d, 1=7.2, 3H), 0.94 (s, 9H), 0.88 (d, J=7.2, 3H), 0.15 (s,<br />
3H), 0.10 (s, 3H).<br />
i^C (50 MHz) 5 177.05, 159.59, 141.56, 130.06, 127.95, 126.81, 125.82, 113.94,<br />
78.75, 74.32, 73.44, 71.75, 70.84, 56.25, 55.24, 42.04, 37.14, 26.28, 18.53,<br />
17.38, 16.96, 9.43, -3.25, -3.52.<br />
IR 3386, 2956, 2931, 2856, 1614, 1514, 1250, 1046 cm-i.<br />
TLC Rf=0.24 (8:2 hexanes:ethyl acetate).<br />
121
(2S,3R,4S)-5-(4'-MpthnvYDhenvnmprhoxv-2.4-HimPfhYi-<br />
13-Dentanpdir>1 (Q^i)<br />
MPMO<br />
C15H24O4 f.w. = 268.39<br />
DUsobutylaluminum hydride (13.4 ml of a 1.5 M solution in toluene, 20.1 mmol)<br />
was added dropwise over a 15 minute period to 1.5 g (5.0 mmol) of the p-hydroxy ester<br />
and 10 ml CH2CI2 stming at 0° under nitrogen. The reaction mixture was warmed to room<br />
temperature, and aUowed to stir for an additional 30 minutes. The reaction was carefully<br />
quenched by the slow addition of 3.5 ml anhydrous methanol, and then 20 ml of ether,<br />
followed by 1.6 ml water, were added. After stirring for an additional 15 minutes the<br />
milky suspension was filtered through a 3 cm pad of ceUte and concentrated under vacuum<br />
to afford 1.2 g (89%) of a viscous clear oil which solidified upon standing. This solid was<br />
then chromatographed (20 g sUica gel, 6:4 hexanes:ethyl acetate) to afford 1.1 g (82%) of a<br />
white soUd. This soUd could be further purified by crystaUization from edier:pentane to<br />
give fluffy white needles (0.97 g, 72%).<br />
1H (200 MHz, relative to TMS at 0.00 ppm) 5 7.24 (d, J=8.6, 2H), 6.89 (d, J=8.6,<br />
2H), 4.46 (s, 2H), 4.16 (s, 1H[0H, variable]), 3.81 (s, 3H), 3.78-3.66 (m, 3H),<br />
3.60 (d of d, 1=9.1, 4.1, IH), 3.45 (t, 1=9.1, IH), 2.76(br. s, 1H[0H, variable]),<br />
1.98 (m, IH), 1.72 (m, IH), 0.98 (d, J=7.0, 3H), 0.77(d, J=6.9, 3H).<br />
13c (50 MHz) 5 159.23, 129.29, 113.77, 78.90, 76.17, 73.09, 67.37, 67.30, 55.14,<br />
36.25, 35.75, 13.03, 8.63.<br />
IR 3422, 2967, 1645, 1514, 1248 cm'l.<br />
MP 53-56°<br />
TLC Rf = 0.17 (1:1 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D=+37.9° (c=0.0391 g/ml, CH2CI2).<br />
ANALYSIS Calcd: C, 67.12; H, 9.02<br />
Found: C, 67.01; H, 9.19.<br />
122
(2R,3S,4R)-Methyl3-hvdrnvY-5-(4'-mpthnvYp>^^nYl)<br />
methoxy-2.4-dimetiiylppntanoate(Q5)<br />
MPMO<br />
'OMe<br />
C16H24O5 f.w. = 296.40<br />
To a 25 mL round bottomed flask containing 1.08 g (2.40 mmol) of the<br />
recrystalUzed aldol product, 5 ml CH2CI2, and 6 ml metiianol stirring at 0° under nitrogen<br />
was added dropwise a freshly prepared solution of sodium medioxide (prepared by the<br />
addition of 0.06 g{2.64 mmol} of clean sodium to 6 ml metiianol), and die mixture was<br />
allowed to stir for 10 minutes. The reaction mixmre was tiien added to 25 ml of 10%<br />
aqueous NaHC03 and 15 ml ether, the layers separated, and the aqueous phase extracted<br />
with 25 ml CH2CI2. The combined organic extracts were tiien washed witii 10 ml brine,<br />
dried over MgS04, and concentrated under vacuum. Rash column chromatography (60 g<br />
silica gel, 8:2 hexanes:etiiyl acetate) afforded 0.58 g (2.0 mmol, 81.5%) of die p-hydroxy<br />
ester plus 0.44 g (-100%) of the recovered crystallme chiral auxiUary (oxazolidinone ) 86.<br />
iH (200 MHz, relative to TMS at 0.00 ppm) 5 7.24 (d, J=8.8, 2H), 6.87 (d, J=8.8,<br />
2H), 4.44 (s, 2H), 3.89 (d of d of d, 1=8.2, 4.3, 3.9, IH), 3.80 (s, 3H), 3.70 (s,<br />
3H), 3.63 (d of d, 1=9.2, 4.3, IH), 3.62 (d, J=4.3, 1H[0H, variable]), 3.51 (d of<br />
d, 1=9.2, 6.7, IH), 2.61 (d of q, J=7.0, 3.9, IH), 1.88 (m, IH), 1.19 (d, J=7.0,<br />
3H), 0.90 (d, 1=7.0, 3H).<br />
13C (50 MHz) 5175.86, 159.13, 129.68, 129.15, 113.66, 75.86, 74.26, 72.99,<br />
55.07, 51.59, 42.34, 35.62, 13.78, 9.64.<br />
IR 3483, 2952, 1734, 1613, 1514 cm"!.<br />
TLC Rf = 0.58 (1:1 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D= +6.72° (c=0.2233 g/ml, CH2CI2).<br />
ANALYSIS Calcd: C, 64.83; H, 8.17<br />
Found: C, 64.00; 8.33.<br />
123
(2S,3R,4S)-Methvl3-(r-hntyldimethy1)dlvloxv-5-(4'metiioxyphenvDmethoxY-9,4-dimethylppnft^n^^t-<br />
("6)<br />
MPMO OTBDMS<br />
-COgMe<br />
C22H3805Si f.w. = 410.69<br />
To 0.46 g (1.6 mmol) of the p-hydroxy ester 95, 0.37 ml 2,6-lutidine, and 2 ml<br />
CH2CI2 stirring under a nitrogen atmosphere in a 5 ml round bottomed flask at room<br />
temperaturewas added 0.55 ml (2.4 mmol) of r-butyldimethylsilyltrifluoromethanesulfonate<br />
dropwise. This mixture was stirred for 30 minutes, then added to a separatory funnel<br />
containing 10 ml ether and 10 ml saturated aqueous NaHCOs. The phases were separated,<br />
the aqueous layer back extracted with 3x10 ml ether, and the organic layers combined and<br />
washed with 10 ml brine. The organic solution was then dried over MgS04 and<br />
concentrated under vacuum. The cmde material was subjected to flash chromatography (20<br />
g siUca gel, 95:5 hexanes;ethyl acetate) to afford 0.63 g (1.5 mmol, 98%) of the desired<br />
product as a clear oil.<br />
IH (200 MHz, relative to TMS at 0.00 ppm) 5 7.26 (d, J=8.7, 2H), 6.87 (d, J=8.7,<br />
2H), 4.44 (d, 1=12.0, IH), 4.36 (d, 1=12.0, IH), 4.05 (d of d, 1=6.0,4.5, IH),<br />
3.81 (s, 3H), 3.64 (s, 3H), 3.50 (d of d, 1=9.1,4.8, IH), 3.24 (d of d, 1=9.1,7.3,<br />
IH), 2.65 (d of q, 1=7.0,4.5, IH), 1.91 (m, IH), 1.13 (d, J=7.0, 3H), 0.97 (d,<br />
J=6.9, 3H), 0.86 (s, 9H), 0.03 (s, 3H), -0.03 (s, 3H).<br />
13C (50 MHz) 5 175.76, 159.02, 130.73, 129.05, 113.66, 74.51, 72.60, 71.97,<br />
55.20, 51.49, 42.76, 38.78, 25.96, 18.26, 14.42, 11.44, -4.33, -4.39.<br />
IR 2954, 2856, 1736, 1612, 1513, 1460, 1249 cm-i.<br />
OPTICAL ROTATION [a]D = -11-28° (c=0.2753 g/ml, CH2CI2).<br />
TLC Rf=0.59 (8:2 hexanes;ethyl acetate).<br />
124
(2S,3RAS)-3-(r-ButvlHimpthynsUvloYY-^-(^'methoxvphpnYl)mPthr^vY-2.4-d^mp.fhY1p^nt-1_y]<br />
methanesulfonate (Q7)<br />
MPMO OTBDMS<br />
^OSOgMe<br />
C22H4o06SSi f.w. = 460.77<br />
To 0.43 g (1.1 mmol) of the alcohol 92, 0.23 ml (1.7 mmol), and 2.5 ml CH2CI2<br />
stirring in a 10 ml pear shaped flask under a nitrogen atmosphere at 0° (ice/water bath) was<br />
added 0.11 ml (1.4 mmol) metiianesulfonyl chloride. After stirring for 45 minutes at 0° die<br />
reaction mixture was added to a separatory funnel containing 10 ml ether and 10 ml<br />
saturated aqueous NaHC03 solution. The layers were separated and die aqueous layer<br />
washed with 3x10 ml ether. The organic phases were combined and washed with 15 ml<br />
brine, and then dried over MgS04 and concentrated under vacuum to afford 0.51 g (1.1<br />
mmol, 98%) of the desfred product in >90% purity (^H NMR). Flash column<br />
chromatography afforded 0.47 g (92%) of the mesylate as a clear oil.<br />
iR (200 MHz, relative to TMS at 0.00 ppm) 5 7.25 (d, J=8.6, 2H), 6.87 (d, 1=8.6,<br />
2H), 4.43 (d, 1=15.6, IH), 4.36 (d, 1=15.6, IH), 4.11-4.04 (m, 2H), 3.80 (s,<br />
3H), 3.73 (d of d, 1=6.2,2.6, IH), 3.46 (d of d, 1=9.1,5.4, IH), 3.27 (d of d,<br />
1=9.1,6.5, IH), 2.94 (s, 3H), 2.10 (d of q, 1=6.9,2.6, IH), 2.00-1.94 (m, IH),<br />
0.95 (d, J=7.0, 3H), 0.94 (d, J=6,8, 3H), 0.89 (s, 9H), 0.06 (s, 3H), 0.05 (s,<br />
3H).<br />
13C (50 MHz) 5 159.05, 130.54, 129.12, 113.67, 72.75, 72.66, 72.57, 72.13, 55.18,<br />
38.15, 37.19, 36.18, 25.96, 18.29, 14.43, 10.95, -4.19, -4.26.<br />
IR 2954, 2934, 2856, 1613, 1514, 1359, 1249, 1177 cm-^.<br />
OPTICAL ROTATION [a]D = +0.40° (c=0.2391 g/ml, CH2CI2).<br />
TLC Rf=0.29 (8:2 hexanes;ethyl acetate).<br />
125
(3S,5S)-3,5-DimethYl-4-f-butvldimpthvlsUvlnyYtetrahvdropvran<br />
(QR)<br />
^<br />
'%<br />
OTBDMS<br />
Ci3H2802Si f.w = 244.50<br />
To 0.20 g (0.45 mmol) of the mesylate 97 and 5 ml acetone in a 50 ml round<br />
bottomed flask was added sodium iodide untU the solution was samrated. The flask was<br />
fitted witii a water cooled condensor and immersed in a 65° oU badi. The mixture was<br />
allowed to reflux for 6 hours, then cooled and dissolved in 100 ml ether. This orange<br />
solution was washed with 50 ml saturated aqueous NaHC03, and this aqueous layer was<br />
extracted with 3x15 ml ether. The combined organic layers were then washed with 50 ml<br />
brine and dried over MgS04, and then concentrated under vacuum to afford 0.44 g of a<br />
mixture of die pyran and MPM-I (87 % of a 1:1 mixture) as die only organic products<br />
(accordmg to ^H NMR). Altiiough column chromatography partially purified this product,<br />
the MPM-I streaked badly on siUca gel, and thereby contaminated die pyran. This pyran<br />
was found to be extremely volatile, and losses occurred upon high vacuum treatment.<br />
IH (200 MHz, relative to CHCI3 at 7.26 ppm) 5 3.80 (d of d, 1=8.2,4.1, IH), 3.58 (d<br />
of d, 1=6.9,3.9, IH), 3.49-3.38 (m, 2H), 3.12 (d of d, 1=6.7,4.3, IH), 1.94-<br />
1.80 (m, IH), 1.79-1.68 (m, IH), 0.91 (d, J=6.9, 3H), 0.90 (d, 1=6.7, 3H), 0.90<br />
(s, 9H), 0.04 (s, 6H).<br />
13C (50 MHz) 5 75.15, 70.83, 70.59, 35.00, 34.06, 25.82, 18.09, 14.52, 11.96, -<br />
4.54, -4.75.<br />
IR 2963, 2758, 1610, 1513, 1464, 1382, 1360, 1251 cm-i.<br />
1 'y/i
(2S,3R,4S)-3-Hydroxy-5-(4'-mpthr)YYphenvnmp.thr>YY-9^/idimethvloent-l-vl<br />
methanesulfonate (QQ)<br />
MPMO OH OSOoMe<br />
C16H26O6S f.w. = 346.48<br />
Methanesulfonyl chloride (0.11 ml, 1.4 mmol) was added dropwise to a solution of<br />
the diol (0.38 g, 1.4 mmol), triethylamine (0.24 ml, 1.7 mmol) and CH2CI2 (3 ml) stirring<br />
in a 10 ml pear shaped flask at 0° under nitrogen. After 30 minutes the reaction was poured<br />
onto 10 ml of saturated NaHC03, the layers separated, and the aqueous layer back<br />
extracted with 3x10 ml ether. The organic phases were combined, dried over MgS04, and<br />
concentrated under vacuum to afford 0.49 g (>100%) of the desUed product as a faintly<br />
yeUow gum, contaminated with less than 10% of other products (bis-mesylate, diol). In<br />
practice this cmde product was used directiy m the next step. It could, however, be<br />
purified by flash chromatography (6:4 hexanes:ethyl acetate). No attempt was made to<br />
obtain an elemental analysis of this compound.<br />
IH (300 MHz, relative to TMS at 0.00 ppm) 5 7.24 (d, J=8.7, 2H), 6.88 (d, J=8.7,<br />
2H), 4.46 (s, 2H), 4.28 (d of d, 1=9.5,8.4, IH), 4.10 (d of d, 1=9.5,6.4, IH),<br />
3.92 (t, 1=1.5, IH [OH, variable]), 3.81 (s, 3H), 3.63 (d of d, 1=9.2,2.3, IH),<br />
3.60 (d of d, 1=9.1,3.8, IH), 3.45 (t, 1=9.2, IH), 3.01 (s, 3H), 2.09 (m, IH),<br />
1.96 (m, IH), 0.92 (d, J=6.9, 3H), 0.77 (d, 1=6.9, 3H).<br />
13C (50 MHz) 6 159.30, 129.32 (two carbons), 113.80, 76.23, 74.85, 73.12, 72.68,<br />
55.16, 36.80, 35.24, 34.54, 12.79, 8.42.<br />
IR 3456, 2970, 1613, 1514, 1353, 1248, 1175cm-l.<br />
TLC Rf = 0.27 (1:1 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D=+21.2° (c=0.0327 g/ml, CH2CI2).<br />
127
(2S,3R,4S)-l-Iodo-5-(4'-methoxyphpnYl)m.th^.Y o ^<br />
dimetiivl-3-ppntanr.] (100^<br />
MPMO<br />
C15H22O3I f.w. = 378.28<br />
A 25 ml round bottomed flask fitted with a water cooled condensor was charged<br />
with die cmde mesylate 99 (0.78 g, 2.24 mmol), 10 ml acetone (freshly distiUed from<br />
activated 4A molecular sieves), 1.68 g (11.2 mmol) sodium iodide (dried at 100° under<br />
vacuum [~1 mm Hg] for 3 hours), and 0.1 ml ethyl dUsopropylamine. The flask was<br />
placed in an oil batii and heated to reflux for ~ 2 hrs, with careful monitoring by TLC.<br />
After die starting material was consumed the mixture was cooled to room temperature and<br />
added to 50 ml 5% Na2S203. The mixmre was extracted with 3x50 ml ethyl acetate, and<br />
die combined organic extracts washed with brine, dried over MgS04, and concentrated<br />
under vacuum to afford a yellow oil. Flash chromatography (8:2 hexanes:ethyl acetate, 20<br />
g sUica gel) afforded 0.50 g (1.33 mmol, 59.5%) of the desired product as a clear oil. No<br />
attempt was made to obtain an elemental analysis of this compound.<br />
iR (300 MHz, relative to TMS at 0.00 ppm) 5 7.24 (d, J=8.6, 2H), 6.88 (d, J=8.6,<br />
2H), 4.49 (d, 1=11.4, IH), 4.42 (d, 1=11.4, IH), 3.80 (s, 3H), 3.63 (d of d,<br />
1=8.8,2.6, IH), 3.58 (d of d, 1=9.1,3.9, IH), 3.46 (t, 1=8.9, IH), 3.36 (d of d,<br />
1=9.5,7.5, IH), 3.18 (d of d, 1=9.5,6.6, IH), 1.88 (m, 2H), 1.00(d, 1=6.8, 3H),<br />
0.78 (d, 1=7.0, 3H).<br />
13C (75 MHz) 6 159.30, 129.44, 129.34, 113.84, 77.88, 76.03, 73.16, 55.21,<br />
38.66, 35.98, 13.29, 13.11, 12.84.<br />
IR 3481, 2962, 2932, 2860, 1612, 1513, 1459, 1248, 1085, 1035, 820 cm-l.<br />
TLC Rf = 0.41 (8:2 hexanes:ethyl acetate).<br />
OPTICAL ROTATION [a]D=+46.4 (c=0.0221 g/ml, CH2CI2).<br />
128
(4S,5R,6S)-7-(4'-MethnvYp>^.nvimethnvY)-d/;-H^TTif-thvl5<br />
(r-butyldimethvl).ilYlr.vY-1 (3"-(2'"-fnmPthYic^V~<br />
ethoxy)methoxy)php.nvl-1 -hppt«nr^n^ (i n^ ^<br />
MPMO OTBDMS<br />
SEMO<br />
C35H5806Si2 f.w. = 631.11<br />
To a 10 ml pear shaped flask containmg 1.07 ml (1.12 M solution in hexane, 1.20<br />
mmol) lithium diisopropylamide and 0.21 ml (1.20 mmol) hexamethylphosphoric triamide<br />
stirring at 0° under a nitrogen atmosphere was added 0.42 g (1.20 mmol) of the imine as a<br />
solution in 0.6 ml THF, followed by a 0.6 ml THF wash. This bright yellow solution was<br />
stirred at 0° for 10 minutes, then cooled to "78° and stirred for an additional 30 minutes.<br />
The neat iodide was then added dropwise, followed by 3x0.5 ml THF washes. The<br />
mixture was allowed to warm to room temperature over the course of three hoiu^s, and<br />
maintained at room temperature for an additional seven hours. The reaction was quenched<br />
by die addition of 10 ml pH 4 buffer and the layers separated. The aqueous layer was<br />
extracted with ether (3x20 ml) and the organic extracts combined, tiien washed with 20 ml<br />
brine, dried over MgS04, and concentrated under vacuum to afford a yellow oil. This<br />
cmde imine was dissolved in 10 ml ether, and 10 ml pH 4 buffer were added. After<br />
stirring die biphasic mixture vigorously for four hours at room temperature die hydrolysis<br />
was complete so the phases were separated and the aqueous layer extracted with 3x20 ml<br />
ether. The combined organic extracts were washed widi brine, dried over MgS04, and<br />
concentrated under vacuum to afford an orange oil. Flash chromatography (20 g silica gel,<br />
98:2 hexanes:ethyl acetate) afforded 0.592 g (78.2%) of the ketone as a slightiy yellow oil.<br />
Repeating tiie chromatography could purify tiiis material further, to a clear oil.<br />
IH (300 MHz, relative to CHCI3 at 7.26 ppm) 5 7.59 (t, 1=1.9, IH), 7.56 (d of t,<br />
1=7.8,1.2, IH), 7.35 (t, 1=7.8, IH), 7.24 (d, 1=8.8, 2H), 6.85 (d, 1=8.8, 2H),<br />
5.25 (s, 2H), 4.41 (d, 1=6.3, IH), 4.38 (d, 1=6.3, IH), 3.79 (s, 3H), 3.76 (d of<br />
d, 1=9.5,7.1, 2H), 3.53 (m, 2H), 3.25 (d of d, 1=9.1, 7.5, IH), 2.98 - 2.88 (m.<br />
129
130<br />
2H), 1.96-(m. IH), 1.82 (m,4H), 1.68 -1.57 (m,-3H), 0.98 - 0.89 (m 8H) 0 88<br />
(s, 9H), 0.04 (s, 3H), 0.03 (s, 3H), 0.00 (s, 9H).<br />
13C (75 MHz) 5 200.05, 159.01, 157.63, 138.48, 130.88, 129.55, 129.12, 121.42,<br />
120.75, 115.50, 113.68, 92.89, 77.35, 72.76, 72.61, 66.40, 55.23, 38.04,<br />
37.04, 35.98, 29.31, 26.13, 18.41, 18.02, 15.06, 14.25, -1.43, -3.71, -4.06.<br />
IR 2955, 2856, 1685, 1583, 1509, 1248, 1091, 1039, 836, 771, 689, 669'cm-l.<br />
TLC Rf = 0.18 (95:5 hexanes:ethyl acetate).<br />
ANALYSIS Calcd: C, 6.62; H, 9.26<br />
Found: C, 66.57; H, 9.15.<br />
(1S .4S .5R.6R)-7-(4'-Methoxvphenynmethoxv-4.6-<br />
dimethyl-1 -(3"-r2"'-trimethylsilvlethoxv1 methoxy)<br />
phenyl-5-r-butyldimethvlsiloxV-1 -heptanol (109)<br />
MPMO OTBDMS<br />
SEMO<br />
C35H6o06Si2 f.w. = 633.13<br />
To a 5 ml pear shaped flask contaming 0.5 ml THF, 0.05 ml (0.05 mmol of a 1.0<br />
M solution in THF) BH3 solution, and 0.006 g (0.023 mmol) of Corey's (R)-<br />
oxaboroUdine (prepared from (R)-(+)-2-(Diphenylhydroxymethyl)pyrtolidine42 stining at<br />
0° under a nitrogen atmosphere was added 0.057 g (0.09 mmol) of the ketone dropwise as<br />
a solution in 0.3 ml THF, followed by 2x0.3 ml THF washes. The mixture was<br />
maintained at 0° for one hour, at which time TLC analysis indicated that die reaction was<br />
still incomplete. Another 0.05 ml BH3 solution (1.0 M in THF, 0.05 mmol) was added,<br />
and the reaction aUowed to stir at 0° for an additional 30 minutes. The completed reacnon<br />
was dien quenched by die addition of 1 ml methanol, followed by 5 ml saturated aqueous<br />
NaHC03 solution. The phases were separated, the aqueous phase back extracted with<br />
3x10 ml ether, the combined organic extracts washed widi brine, dried over MgS04, and<br />
concentrated under vacuum. Flash column chromatography (20 g silica gel, 9 .^ - •
hexanesiethyl acetate gradient) to afford 0.0423 g (74.2%) of the desi.^ alcohol as<br />
approximately a 12:1 mixture of epimers (75 MHz I3c integrations).<br />
Spectra for separate diastereomers were discemed by comparing the spectra of the<br />
product from this reaction with the spectra of a 1:1 mixture of diastereomers obtained from<br />
reduction of the ketone with sodium borohydride.<br />
IH Major isomer: (300 MHz, relative to CHCI3 at 7.26 ppm) 6 7.21 (m, 3H), 6.98<br />
(t, 1=1.8, IH), 6.92 (d of d, 1=7.9,2.0, 2H), 6.83 (d of t, 1=8.6,2.0, 2H),'5.21<br />
(s, 2H), 4.58 (t, 1=5.8, IH), 4.41 (d, 1=11.7, IH), 4.36 (d, 1=11.7, 1H),'3.79<br />
(s, 3H), 3.76 (d of d, 1=8.4,8.3, 2H), 3.50 (d of d, 1=9.0,4.7, IH), 3.43 (d of d,<br />
1=6.1,3.1, IH), 3.20 (d of d, 1=8.9,7.5, IH), 2.01 (br s, 1H[0H, variable]),<br />
1.95-1.88 (m, IH), 1.83-1.71 (m, IH), 1.68-1.52 (m. 2H), 1.36-1.25 (m, 2H),<br />
0.96 (d of d, 1=8.4,8.3, 2H), 0.90 (d, 1=6.7, 3H), 0.844 (s, 9H), 0.82 (d, 1=6.4,<br />
3H), 0.00 (s, 9H), -0.01 (s, 3H), -0.04 (s, 3H).<br />
Minor isomer: (300 MHz, relative to CHCI3 at 7.26 ppm) 5 7.21 (m, 3H), 6.96<br />
(t, 1=1.8, IH), 6.95 (d of d, 1=7.9,2.0, 2H), 6.87 (d of t, 1=8.6,2.0, 2H), 5.21<br />
(s, 2H), 4.58 (t, 1=5.8, IH), 4.41 (d, 1=11.7, IH), 4.36 (d, 1=11.7, IH), 3.80<br />
(s, 3H), 3.76 (d of d, 1=8.4,8.3, 2H), 3.42-3.52 (m, 2H), 3.21 (d of d,<br />
1=8.9,7.5, IH), 2.01 (br s, 1H[0H, variable]), 1.95-1.88 (m, IH), 1.83-1.71 (m,<br />
IH), 1.68-1.52 (m. 2H), 1.36-1.25 (m, 2H), 0.96 (d of d, 1=8.4,8.3, 2H), 0.90<br />
(d, 1=6.7, 3H), 0.836 (s, 9H), 0.82 (d, 1=6.4, 3H), 0.00 (s, 9H), -0.02 (s, 3H), -<br />
0.05 (s, 3H).<br />
13C Major isomer: (75 MHz) 5 159.003, 157.626, 146.684, 130.820, 129.413,<br />
129.128, 119.102, 115.020, 113.724, 113.671, 92.85, 77.383, 74.505, 72.708,<br />
72.577, 66.166, 55.202, 37.811, 37.341, 36.192, 30.639, 26.111, 18.389,<br />
18.010, 15.274, 14.395, -1.43, -3.773, -4.055.<br />
Minor isomer: (75 MHz) 6 159.003, 157.648, 146.505, 130.880, 129.413,<br />
129.102, 119.128, 115.117, 113.819, 113.671, 92.876, 77.383, 74.884, 72.776,<br />
72.577, 66.166, 55.202, 37.516, 37.241, 36.277, 30.879, 26.111, 18.389,<br />
18.010, 15.150, 14.072, -1.43, -3.773, -4.055.<br />
IR 3447, 2954, 2929, 2956, 1611, 1513, 1248, 1089, 1036, 859, 836, 773 cm'l.<br />
TLC Rf = 0.26 (8:2 hexanes:ethyl acetate).<br />
ANALYSIS Calcd: C, 66.41; H, 9.56<br />
Found: C, 66.73; H, 9.45.<br />
131
(lS,4S,5R,6S)-7-(4'-MethoxyDhenvnmptbnvv-i,me.,hnvy-<br />
4,6-dimethyl-l-(3"-(trimethylethoxvmpthnvv-)Dhenv1)-S-(rbutvldimethynsUvlnxYhpptane<br />
(109)<br />
MPMO OTBDMS OMe<br />
SEMO<br />
C3oH6206Si2 f.w. = 647.16<br />
To a suspension of potassium hydride (0.13 g, 1.13 mmol) and 1.5 ml THF<br />
stining in a 5 ml pear shaped flask at 0° (ice/water bath) under a nitrogen atmosphere was<br />
added 0.28 g (0.45 mmol) of the alcohol 110 as a solution in 1 ml THF. This suspension<br />
was allowed to warm to room temperature over 1 hour, then re-cooled to 0°. Methyl iodide<br />
(0.07 ml, 1.13 mmol) was added, and the reaction was maintained at 0° for 2 hours, after<br />
which die reaction was quenched by die careful addition of 1 ml methanol. The mixture<br />
was then added to a separatory funnel containing 10 ml etiier and 10 ml water and the<br />
layers were separated. The aqueous phase was washed with 3x10 ml ether, and then the<br />
organic layers were combined and washed witii 10 ml brine. The ether solution was then<br />
dried over MgS04 and concentrated under vacuum to afford a mixture of the product and<br />
mineral oU. Rash column chromatography (20 g siUca gel, 9:1 hexanes:ethyl acetate)<br />
afforded 0.22 g (0.34 mmol, 75%) of the methyl etiier as a colorless oU.<br />
IH (300 MHz, relative to CHCI3 at 5 7.26 ppm) 5 7.26-7.22 (m, 3H), 6.96-6.83 (m,<br />
5H), 5.22 (s, 2H), 4.38 (s, 2H), 4.00 (d of d, 1=7.2,5.7, IH), 3.80 (s, 3H), 3.77<br />
(d of d, 1=9.1,7.6, 2H), 3.51-3.45 (m, IH), 3.41 (d of d, 1=6.4,2.8, IH), 3.23-<br />
3.17 (m, IH), 3.20 (s, 3H), 1.92-1.75 (m, 2H), 1.58-1.49 (m, 2H), 1.32-1.23<br />
(m, 2H), 0.96 (d of d, 1=9.1,9.1, 2H), 0.91 (d 1=6.9, 3H), 0.84 (s, 9H), 0.83 (d,<br />
1=8.1, 3H), 0.00 (s, 9H), -0.03 (s, 3H), -0.06 (s, 3H).<br />
^3C (75 MHz) 5 159.01, 157.66, 144.29, 130.93, 129.34, 129.12, 119.98 115^0U<br />
114.48, 113.68, 92.96, 84.22, 77.31, 72.86, 72.60, 66.21, 56-71, 5 -26^ 7.91,<br />
36.56, 36.30, 30.84, 26.14, 18.41, 18.03, 15.23, 14.17, -1.41, -3.77, -4.07.<br />
ANALYSIS Calcd: C, 66.83; H, 9.67<br />
Found: C, 66.57; H, 9.76.<br />
132
(2S,3R,4S,7S)-7-MethnYY-2.4-dimp.thYl-7-(3'-triniPthYisilylethoxymethnxY)phPnyl-3-(NhnfYlHim.thY])<br />
silyloxv-l-hp.ptanni(in)<br />
OTBDMS<br />
SEMO<br />
C28H5405Si2 f.w. = 527.00<br />
To the 0.14 g (0.21 mmol) of the ether 102, 2 ml CH2CI2, and 0.4 ml pH 4 buffer<br />
stirring in a 10 ml pear shaped flask at room temperature was added 0.60 g (0.26 mmol)<br />
2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The solution immediately turned dark<br />
green. After 30 minutes the reaction mixmre was added to a separatory funnel containing<br />
20 ml ether and 20 ml saturated aqueous NaHC03 solution. The layers were separated and<br />
the aqueous layer was extracted with 3x10 ml ether. The organic layers were combined<br />
and washed witii 3x25 ml of the bicarbonate solution, doUowed by 25 ml brine. The<br />
organic phase was then dried over MgS04 and concentrated under vacuum. Flash column<br />
chromatography afforded 0.96 g (0.18 mmol, 86%) of the alcohol 111 as a white wax.<br />
Spectra for separate diastereomers were discemed by comparing die spectra of die<br />
product from this reaction with tiie spectra of a 1:1 mixture of diastereomers<br />
iR (200 MHz, relative to CHC13 at 5 7.26 ppm, 'natural* 7S isomer) 5 7.29-7.20 (m,<br />
IH), 6.96-6.88 (m, 3H), 5.21 (s, 2H), 4.01 ( d of d, 1=5.6, 4.3, IH), 3.76 (d of<br />
d, 1=8.2,7.5, 2H), 3.56 (d, 1=5.2, 2H), 3.45 (d of d, 1=5.2,4.3, IH), 3.20 (s,<br />
3H), 2.53 (br s, lH[-OH, variable]), 1.90-1.28 (m, H), 0.99-0.82 (m, 8H), 0.86<br />
(s, 9H), 0.07 (s, 3H), 0.01 (s, 3H), -0.01 (s, 9H).<br />
iH (200 MHz, relative to CHC13 at 5 7.26 ppm, 'unnatural' 7R isomer) 5 7.29-7.20<br />
(m, IH), 6.96-6.88 (m, 3H), 5.21 (s, 2H), 4.01 ( d of d, 1=5.6, 4.3, IH), 3.76 (d<br />
of d, 1=8.2,7.5, 2H), 3.56 (d, 1=5.2, 2H), 3.45 (d of d, 1=5.2,4.3, IH), 3.19 (s<br />
3H), 2.53 (br s, lH[-OH, variable]), 1.90-1.28 (m, H), 0.99-0.82 (m, 8H), 0.87<br />
(s, 9H), 0.06 (s, 3H), 0.00 (s, 3H), -0.01 (s, 9H).<br />
133
13C<br />
13C<br />
CD<br />
(50 MHz, 'natural' 7S isomer) 6 157.64, 144 12 129 .. ion<br />
114.49, 92.91, 84.21, 80.74 66 17 56 65 VR '^' ^^^1^'<br />
26.04, 18.26, 17.99, 16.08, 14.95,-145 '3 99'Vof''^^^^^<br />
(50 MHz, 'unnatural' 7R isomer) 8 157.6^, 143 95 no ..no<br />
114.39, 92.91, 84.11, 80.63, 66.03, 56.65 37^3 ^3^^^ '''''^<br />
26.04, 18.26, 17.99, 15.95, 14.59, -1.45, -3 99 4 14 ' ^'''^ ^'''^^<br />
(ethanol) Positive cotton effects at 268 and 271 nm<br />
C28H5203Si2 f.w. = 524.98<br />
OTBDMS<br />
SEMO<br />
C28H5203Si2 f.w. = 524.98<br />
To a solution of DMSO (0.052 ml, 0.72 mmol) and 2 ml CH2CI2 stimng in a 5 ml<br />
round bottomed flask at -78° under a nitrogen atmosphere was added the oxalyl chloride<br />
(0,032 ml, 0,32 rmnol) dropwise. After this mixture was stin-ed for 10 minutes 0.096 g<br />
(0.181 mmol) of the alcohol 111 was added dropwise as a solution in 0.5 ml CH2CI2,<br />
with 2x0.3 ml CH2CI2 washes. This solution was maintained at -78° for 20 minutes, and<br />
t en 0.25 ml (1.81 mmol) triethylamine was added dropwise. The resulting suspension<br />
was stirred at -78° for 15 minutes and tiien it was warmed to room temperature over 10<br />
minutes and stirred at room temperamre for 10 more minutes. The reaction mixture was<br />
t en added to a separatory funnel containing 20 ml ether, 10 ml water, and 10 ml saturated<br />
ous NaHC03. The layers were separated and the aqueous phase was extracted with<br />
nil ether. The organic phases were combined and washed with 25 ml brine, and then<br />
^ed over MgS04 and concentrated under vacuum to afford 0.095 g (0.18 mmol, 100%)<br />
of the aldehyde as a clear oil, >95% pure by ^H NMR.<br />
134
IH (300 MHz, relative to CHCI3 at 7.26 ppm, 1:1 mixture of C7 epimers) 5 9.72 (d of<br />
d, 1=1.7,1.1, IH), 7.24 (t, 1=7.6, IH), 6.97-6.88 (m, 3H), 5.22 (s, 2H), 4.00 (t,<br />
1=6.9, IH), 3.76 (d of d, 1=7.8,7.4, 2H), 3.72-3.68 (m, IH), 3.20 (s, 1.5H),<br />
3.19 (s, 1.5H), 2.54-2.46 (m, IH), 1.90-1.82 (m, IH), 1.70 (q, 1=7.9, IH),<br />
1.67-1.52 (m, 2H), 1.38-1.12 (m, IH), 1.03 (d, 1=7.1, 1.5H), 1.02 (d, 1=7.1,<br />
1.5H), 0.96 (d of d, 1=7.8, 7.4, 2H), 0.90-0.82 (m, 3H), 0.84 (s, 4.5H), 0.83 (s,<br />
4.5H), 0.04 (s, 3H), -0.01 (s, 9H), -0.02 (s, 1.5H), -0.03 (s, 1.5H).<br />
(2S.8S.9R.10R)-7-Hvdroxv-13-methoxv-l-(4'methoxyphenyl)methoxy-2,4.4,8.10-pentamethyl-13-(3"-(trimethylsilylethoxvmethoxy)phenyl-9-(rbutyldimethyl)silyloxy-5-tridecanone<br />
(113)<br />
MPMO OTBDMS OMe<br />
C45H7808Si2 f.w. = 803.41<br />
SEMO<br />
To a solution of ketone 31 (0.075 g, 0.27 mmol) and 1 ml THF stirting in a 5 ml<br />
round bottomed flask at -78° under a nitrogen atmosphere was added 0.24 ml (0.27 mmol)<br />
LDA. This solution was maintained at -78° for 15 minutes, dien the aldehyde 112 (0.095<br />
g, 0.18 mmol) was added dropwise as a solution in 0.5 ml THF, followed by 3x0.3 ml<br />
THF washes. After 10 minutes at -78° the reaction mixture was added to a separatory<br />
funnel containing 20 ml ether, 10 ml water, and 10 ml samrated aqueous NaHCOs- The<br />
layers were separated and the aqueous phase was extracted widi 3x25 ml etiier. 1 e<br />
organic phases were combined and washed with 25 ml brine, and dien dned over MgSU<br />
and concentrated under vacuum. Flash column chromatography (20 g silica gel so v<br />
gradient hexanes to 9:1 hexanes:ethyl acetate) afforded 0.122 g (0.15 mmol, 85%)<br />
aldol product 113 as a clear oil.<br />
135
IH (300 MHz, relative to CHCI3 at 7.26 ppm, 1:1 mixture of C13 epimers,<br />
indeterminate mixture of C7 epimers) 5 7.26-7.22 (m, 3H), 6.95-6.85 (m 5H)<br />
5.22 (s, 2H), 4.39 (s, 3H), 4.02 (t, 1=6.8, IH), 3.80 (s, 3H), 3.77 (d of d,<br />
1=8.4,7-4, 2H), 3.59-3.55 (m, IH), 3.57 (d, 1=2.9, IH), 3.20 (s, 1.5H), 3.20 (s,<br />
1.5H), 3.16 (d of d, 1=6.1,2.4, 2H), 2.66 (d of d, 1=17.7,8.7, IH), 2.48'(d of t,'<br />
1=17.7,3.7, IH), 0.96 (d of d, 1=8.4,7.4, 2H), 0.87-0.79 (m, 15H), 0.85 (s,<br />
4.5H), 0.84 (s, 4.5H), 0.10 (s, 1.5H), 0.09 (s, 1.5H), 0.01 (s, 1.5H), 0.00 (s,<br />
9H), -0.01 (s, 1.5H).<br />
(2S.8S.9R.10R)-13-Methoxy-l-(4'-methoxvphenvlV<br />
methoxy-2,4,4,8,10-pentamethyl-13-(3"-(trimethylsilylethoxymethoxy)phenyl-9-(f-butyldimethyl)sUyloxy-5,7-tridecanedione<br />
(115)<br />
MPMO<br />
C45H7608Si2 f.w. = 801.39<br />
SEMO<br />
136<br />
To a solution of DMSO (0.043 g, 0.61 mmol) and CH2CI2 d ml) in a 10 ml round<br />
bottomed flask under a nitrogen atmosphere at -78° was added oxalyl chloride (0.027 ml,<br />
0.31 mmol) dropwise. This reaction mixture was maintained at -78° for 10 minutes, and<br />
then the P-hydroxy ketone 113, in 0.5 ml CH2CI2, followed by 2x0.3 ml CH2CI2 washes<br />
were added dropwise. This mixture was stirred at -78° for 20 minutes, then 0.21 ml (1.5<br />
mmol) trietiiylamine was added. The reaction mixmre was stirred at -78° for an additional<br />
10 minutes, then it was wanned to room temperature oyer 10 minutes and allowed to sur at<br />
room temperamre for 10 minutes. The reaction mixture was dien added to a separatory<br />
funnel containing 20 ml ether and 20 ml saturated aqueous NaHC03 and the layers<br />
u A ifh ^^O'^ ml ether and dien the organic layers<br />
separated. The aqueous layer was washed with 3x25 ml etner, diiu<br />
were combined and washed witii 25 ml brine. The organic solution was dned over
137<br />
MgS04, concentrated under vacuum, and submitted to flash chromatography (20 g silica<br />
gel, 93:7 hexanes:ediyl acetate) to afford 0.064 g (0.079 mmol, 53%) of the desired<br />
compound as a clear oU. iR NMR analysis suggests that this is a 1:2 mixture of die p<br />
diketone and its various enol forms.<br />
iR (300 MHz, relative to CHCI3 at 7.26 ppm, 1:1 mixture of C13 epimers, mixture of<br />
keto/enol tautomers, diagnostic peaks only) 5 5.22 (s, 2H), 3.80 (s, 3H), 3.21 (s,<br />
1.5H), 3.20 (s, 1.5H), 1.13 (s, 1.5H), 1.13 (s, 1.5H), 0.80 (s, 4.5H), 0.78 (s,<br />
4.5H), 0.00 (s, 9H).
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mm^m::^i<br />
140