Silyl Ethers - Thieme Chemistry

Scheme 8 Cleavage of a Trimethylsilyl Ether with Tetrabutylammonium Fluoride [34]
O
H
O
O
O
H
Bn
TBAF, THF, 0 oC 98%
OTMS OBz
O
OH
16 17
H
O
O
O
H
(1R,3S,4R,5R,6S,8R,9S)-5-Benzyloxy-9-(tert-butyldimethylsiloxy)-8-[2-(tert-butyldiphenylsiloxy)ethyl]-3-{(2Z,4S,5R)-6-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]-4,5-epoxyhex-2-enyl}-2,7dioxabicyclo[4.4.0]decan-4-ol
(15): [33]
To a soln of 14 (5.2 mg, 5.50 ìmol) in MeOH (1 mL) was added K 2CO 3 (an excess amount) at
08C, and the mixture was stirred at the same temperature for 1.5 h. The mixture was diluted
with Et 2O, and H 2O was added. The aqueous layer was extracted repeatedly with
EtOAc. The combined organic layers were washed with brine, dried (MgSO 4), filtered,
and concentrated in vacuo. The residue was purified by column chromatography (silica
gel, hexane/EtOAc 3:1) to afford 15 as a colorless oil; yield: 4.6 mg (96%).
[2S-(2á,3aâ,3bâ,6aâ,9aá,9bá,10á,11aâ)]-5-(Benzoyloxymethyl)-2-benzyl-2,9b-epoxy-6ahydroxy-11a-isopropenyl-8,10-dimethyl-3a,3b,6,6a,9a,10,11,11a-octahydro-7H-azuleno[5,4-e]-1,3-benzodioxol-7-one
(17): [34]
To a stirred soln of silyl ether 16 (8 mg, 12.4 ìmol) in THF (1 mL) at 08C was added 1.0 M
TBAF in THF (24 ìL, 24.8 ìmol). After being stirred at 08C for 10 min, the mixture was
poured into sat. aq NH 4Cl (5 mL) and extracted with EtOAc (3 ” 2 mL). The combined organic
extracts were dried (MgSO 4), filtered, and concentrated in vacuo. Purification by flash
chromatography (EtOAc/hexanes 2:8) gave alcohol 17; yield: 6.9 mg (98%).
4.4.17.2 Triethylsilyl Ethers
Triethylsilyl (TES) ethers are more stable and more resistant to acidic cleavage than trimethylsilyl
ethers. They are often used where exhaustive silylation of a polyol is desired,
but where a per(trimethylsilyl ether) is too labile for purification. The volatility of a triethylsilyl
ether is only slightly less than that of a trimethylsilyl ether; however, the increased
size of the substituents at silicon in a triethylsilyl ether can make this silylation of a tertiary
alcohol quite sluggish, especially with chlorotriethylsilane.
Formation
FOR PERSONAL USE ONLY
4.4.17 Silyl Ethers 377
4.4.17.2.1 Method 1:
Silylation ofAlcohols with Chlorotriethylsilane
Triethylsilyl ethers are most frequently prepared using chlorotriethylsilane (TESCl) [35] in
the presence of a base such as imidazole, pyridine or 4-(dimethylamino)pyridine. [36] A typical
silylation with this reagent is that of alcohol 18 carried out in dimethylformamide at
room temperature to give triethylsilyl ether 19 (Scheme 9). [37]
OBz
Bn
for references see p 410
White, J. D.; Carter, R. G., SOS, (2002) 4, 371. 2002 Georg Thieme Verlag KG