Silyl Ethers - Thieme Chemistry

(1R,7S,8S,10R,14R)-7-Hydroxy-14-isopropyl-8-(4-methoxybenzyloxy)-7,11-dimethyl-3-
(triisopropylsiloxymethyl)bicyclo[8.4.0]tetradeca-2,11-dien-5-yn-4-one (11): [31]
To a soln of 10 (3.2 mg, 4.7 ìmol) in MeOH (1 mL) was added, at 258C, PPTS (1.18 mg,
4.7 ìmol), and the mixture was allowed to stir at rt for 30 min. The reaction was quenched
by the addition of sat. NaHCO 3 soln (1 mL), then extracted with Et 2O (2 ” 10 mL). The combined
organic extracts were dried (Na 2SO 4) and concentrated. The crude product was purified
by flash chromatography (silica gel, EtOAc/hexane 15:85) to provide alcohol 11 as a
colorless oil; yield: 2.7 mg (94%).
rac-(3R,4aS,5R,6S,6aS,12aS,12bR)-3-[Dimethyl(phenyl)silyl]-4a,5-epoxy-6,8-dihydroxy-3methyl-1,2,3,4,4a,5,6,6a,12a,12b-decahydrobenzo[a]anthracene-7,12-dione
(13): [32]
One drop of 1 M HCl was added to a soln of 12 (200 mg, 0.375 mmol) in MeOH (4 mL) and
CH 2Cl 2 (2 mL) at 08C. The mixture was stirred for ca. 0.5 h at 08C and then extracted with
CH 2Cl 2 (50 mL). The organic phase was washed with ice-cold H 2O (2 ” 20 mL), dried
(Na 2SO 4), and the solvent was removed under reduced pressure to afford epoxy alcohol
13 as an unstable, yellow oil containing some (5%) aromatization product; yield: 156 mg
(90%).
4.4.17.1.6 Method 6:
Cleavage ofTrimethylsilyl Ethers under Basic Conditions
One of the mildest methods for cleaving a trimethylsilyl ether is through its exposure to
potassium carbonate in methanol, conditions which will not effect cleavage of any other
silyl ether. The selective deprotection of the ciguatoxin precursor 14 bearing three different
silyl ethers, as well as a benzyl ether and an epoxide, exemplifies an application of
this method (Scheme 7). [33]
Scheme 7 Selective Cleavage of a Trimethylsilyl Ether with Base [33]
O
O
TMSO
O
FOR PERSONAL USE ONLY
376 Science of Synthesis 4.4 Silicon Compounds
H H
O
O OTBDPS
H H
BnO
14
OTBDMS
O
O
O
K2CO3, MeOH
0 oC, 1.5 h
96%
H H
O
HO O OTBDPS
H H
BnO
15
OTBDMS
The most widely used method for cleaving silyl ethers is through the use of tetrabutylammonium
fluoride in tetrahydrofuran. This reagent can be used for cleaving trimethylsilyl
ethers, but is not selective with respect to silyl ethers in general. Tetrabutylammonium
fluoride is a sufficiently basic reagent to cause elimination and other side reactions with
base-sensitive compounds, and it will sometimes promote migration of a silyl group to a
neighboring free hydroxy group; however, esters are not usually cleaved with this reagent,
as deprotection of the benzoate 16 reveals (Scheme 8). [34]
White, J. D.; Carter, R. G., SOS, (2002) 4, 371. 2002 Georg Thieme Verlag KG