Silyl Ethers - Thieme Chemistry

can occur from a carbon atom or from oxygen. Thus, while it is usually very difficult to
silylate a tertiary alcohol with tert-butyldiphenylsilyl chloride or trifluoromethanesulfonate,
intramolecular rearrangement of a hydroxylated tert-butyldiphenylsilane can provide
a means of accomplishing this protection. An example of tert-butyldiphenylsilyl migration
is seen in the rearrangement of hydroxylated silane 91 to silyl ether 92 mediated
by 1,8-diazabicyclo[5.4.0]undec-7-ene as the base (Scheme 45). [92]
Scheme 45 Rearrangement of a Hydroxylated tert-Butyldiphenylsilane
to a tert-Butyldiphenylsilyl Ether [92]
TBDPS
O OAc
OH
91
OAc
DBU, CH2Cl2
rt, 6 h
90%
O OAc
OAc
OTBDPS
92
The severe steric crowding that attends a tertiary tert-butyldiphenylsilyl ether such as 92
may prompt migration of the silyl residue to a less congested site, if one is available. For
example, exposure of 93 to sodium hydroxide is sufficient to cause an internal rearrangement
of the tertiary silyl ether to yield the secondary tert-butyldiphenylsilyl ether 94
(Scheme 46); [92] the primary tert-butyldimethylsilyl ether remained unaffected under
these reaction conditions.
Scheme 46 Rearrangement of a Tertiary tert-Butyldiphenylsilyl Ether to
a Secondary tert-Butyldiphenylsilyl Ether [92]
O
OH OAc
OTBDPS
93
OTBDMS
NaOH, t-BuOH
H2O, rt, 3 h
70%
TBDPSO OAc
O
OH
94
OTBDMS
(4S,5R)-5,6-Diacetoxy-4-(tert-butyldiphenylsiloxy)-4-methylhex-1-en-3-one (92): [92]
To a soln of enone 91 (150 mg, 0.31 mmol) in CH 2Cl 2 (0.5 mL) at rt was added DBU (10 mg).
The mixture was stirred for 6 h and then quenched with aq NH 4Cl and extracted with
Et 2O. The ethereal layer was washed with brine and dried (MgSO 4). After removal of solvent
under reduced pressure, the residue was chromatographed (silica gel, Et 2O/hexanes
1:10) to afford enone 92 as a clear oil; yield: 135 mg (90%).
(2R,3S,4S,5R)-2-Acetoxy-1-(tert-butyldimethylsiloxy)-4-(tert-butyldiphenylsiloxy)-5,6epoxy-3-methylhexan-3-ol
(94): [92]
A soln of epoxide 93 (200 mg, 0.35 mmol) in 1 M NaOH/t-BuOH (1:6, 1.0 mL) was stirred at
rt for 3 h. The mixture was quenched with aq NH 4Cl and extracted with Et 2O. The ethereal
layer was dried (MgSO 4) and concentrated. The residue was chromatographed (silica gel,
Et 2O/hexanes 1:4) to afford tertiary alcohol 94 as a clear oil; yield: 140 mg (70%).
Cleavage
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404 Science of Synthesis 4.4 Silicon Compounds
4.4.17.5.4 Method 4:
Cleavage of tert-Butyldiphenylsilyl Ethers with
Tetrabutylammonium Fluoride
In common with other silyl ethers, tert-butyldiphenylsilyl ethers are readily cleaved with
tetrabutylammonium fluoride in tetrahydrofuran. [93] The reagent shows no selectivity for
tert-butyldiphenylsilyl ethers in different structural environments. For example, the final
step in a synthesis of (+)-isobretonin A (96) involves deprotection of both the phenolic and
the secondary tert-butyldiphenylsilyl ether of 95 with tetrabutylammonium fluoride
(Scheme 47). [94]
White, J. D.; Carter, R. G., SOS, (2002) 4, 371. 2002 Georg Thieme Verlag KG