Regioselectivity of the Reactions of Heteroatom-Stabilized Allyl ...

696
difference between “hard” and “soft” termini of 574
is probably less than that between “hard” and “soft”
ends of gem-dichloroallyllithium and hence, 574 is
expected to be less regioselective than the dichloro
analogue. Furthermore, steric effects have also an
influence on the course of the reaction since the
trimethylsilyl group is sterically demanding (see gemdichloroallyllithium
section III.B for comparison). 304
2. R-Alkoxyallylsilanes (CdC−C(OR)(SiR3))
1-Lithio-1-(alkoxy)allyltrimethylsilane from 582
reacts with aldehydes to give both the R- 583 and
γ-products 585. However, transformation into the
titanium reagent leads to exclusive generation of
R-products 583 followed by Peterson elimination to
form the dienyl ether 584 while in the presence of
HMPA exclusively γ-attack to form 585 is observed.
The dienyl ether 584 can be easily hydrolyzed to form
the vinyl ketone 586 (Scheme 105). 305
Scheme 105
R-Silyl alcohols 587 undergo Brook rearrangement,
306 whereas the alkoxy anion in 588 attacks the
silyl group to form an R-alkoxy anion 589 (Scheme
106) 307,308 (see also section II.B).
Scheme 106
3. R-Silylallylamines (CdC−C(NR2)(SiR3))
R-Silylallylamines 590 furnish aza-Brook rearrangement
to products 591 and 592 upon treatment
with bases (Scheme 107). 307
Scheme 107
4. R-(Trimethylsilyl)allyl Sulfides (CdC−C(SiMe3)(SR))
The anion of (trimethylsilyl)alkylallyl or (trimethylsilyl)arylallyl
sulfide 593 undergoes regioselective
γ-addition with enones yielding predominantly the
1,4-adduct 595 even in the absence of HMPA (Scheme
108). 234 Furthermore, reaction with carbonyl compounds
takes place preferentially at the γ-carbon (cf.
594). 309 The reaction of lithiated 593 with epoxides
also proceeds with γ-attack giving alkoxides 596
Scheme 108
which can be converted into the corresponding alcohols
or tosylates 597 (Scheme 108). 78
5. Diethyl (Trimethylsilyl)crotylphosphonate
(CdC−C(SiMe3)(PO(OEt)2)
Lithiated diethyl (trimethylsilyl)crotylphosphonate
598 is generated in situ from diethyl crotylphosphonate
and reacts with ethyl chloroformate and ethyl
formate at the γ-carbon to both heterogroups to give
599 and 601, respectively, and upon hydrolysis the
corresponding phosphonates 600 and 602 (Scheme
109). 209
Scheme 109
B. Protected Cyanohydrins
1. Alkoxy Cyanohydrins (CdC−C(CN)(OR))
Protected cyanohydrins are valuable synthetic
intermediates 310,311 and act as acyl anion synthons.
Treatment of the allyl anions of 2-ethoxy-3-alkenenitriles
603 with alkyl halides 312 produces R-products
604 and displacement of the CN group by tributylstannyllithium
furnishes the γ-ethoxyallylstannanes
605 (Scheme 110). 313 R-Alkylation to give 607 is also
reported for trimethylsilyl cyanhydrins 606 (Scheme
110), 314,315 although more electron-attracting substit-