Allylsilanes

85.56 mmol) in THF (17 mL) was added. After 5 min, a soln of (E)-127 (2 g, 7.19 mmol) in
dryTHF (47.9 mL) was added. The soln was stirred at ±788C for 5 min before being warmed
to 08C for 1 h. The soln was then diluted with 10% aq HCl (50 mL), extracted with EtOAc
(2 ” 50 mL), dried (MgSO 4), and the solvent was removed in vacuo; this afforded a crude
yellow oil. Chromatographic purification (silica gel, petroleum ether/EtOAc 1:0 to 1:4) afforded
hexenoic acid 129 as a clear oil (syn:anti 20:1); yield: 1.68 g (84%).
(2R,3R,4E)-3-(Dimethylphenylsilyl)-2-methylhex-4-enoic Acid (syn-125); Condition B;
Typical Procedure: [200]
For the preparation of LiHMDS, 1 M BuLi in hexane (1.53 mL, 1.53 mmol) was added to a
soln of freshlydistilled TMS 2NH (322 ìL, 1.53 mmol) in dryTHF (3 mL) at 08C. The soln was
stirred at 08C for 30 min, and was then added to a soln of 123 (200 mg, 0.76 mmol) in dry
THF (5.06 mL) at ±788C. After 10 min the yellow soln was treated with TBDMSCl (229.5 mg,
1.53 mmol) in HMPA (CAUTION: cancer suspect agent) (1 mL). The soln was allowed to
warm to rt over 5 h and then refluxed for 2 h before being diluted with 10% aq HCl
(50 mL). The soln was extracted with EtOAc (2 ” 50 mL), dried (MgSO 4), and concentrated
in vacuo to afford a crude yellow oil. This was redissolved in THF (10 mL), and then treated
with 10% aq HCl (2 mL) at rt and allowed to stir for 2 h. Further dilution with 5% aq HCl
(10 mL) and extraction with EtOAc (2 ” 20 mL), followed bydrying (MgSO 4), filtration,
and removal of solvent in vacuo resulted in isolation of a crude orange oil. Chromatographic
purification (silica gel, petroleum ether/EtOAc 1:0 to 1:4) afforded hexenoic acid
125 (syn:anti 16:1); yield: 150 mg (75%).
(2S,3S,4E)-3-(Dimethylphenylsilyl)-2-methoxyhex-4-enoic Acid (syn-129); Condition C;
Typical Procedure: [200]
To a soln of freshlydistilled TMS 2NH (890 ìL, 4.22 mmol) in dryTHF (8.44 mL) at 0 8C was
added 1 M BuLi in hexane (4.17 mL, 4.17 mmol). After 30 min at 08C, the soln was added to
a THF soln of (E)-127 (645 mg, 2.32 mmol) in dryTHF (9.3 mL) at ±788C. The yellow soln
was allowed to stir for 1 h at ±788C before addition of TMSCl (883 ìL, 6.96 mmol). The
soln was allowed to warm to rt over 14 h before being diluted with 10% aq HCl (50 mL).
The mixture was extracted with EtOAc (2 ” 50 mL), dried (MgSO 4), and concentrated in
vacuo to afford a crude yellow oil. Chromatographic purification (silica gel, petroleum
ether/EtOAc 1:0 to 1:4) afforded hexenoic acid 129 (syn:anti 23:1) as a clear oil; yield:
438 mg (68%).
4.4.40.31.3 Variation 3:
By the Eschenmoser Variant of the Claisen Rearrangement
Silanes containing both an amido and allyl functionality and with defined stereochemistryare
accessible bythe Eschenmoser variant of the Claisen rearrangement of ã-silylated
allyl alcohols. Thus, using N,N-dimethylacetamide dimethylacetal, the chiral allylsilane
(R)-132 has been prepared (Scheme 54). [203,204]
Scheme 54 Allylsilanes by the Eschenmoser Variant of the Claisen Rearrangement [203]
Me 3Si
131
OH
FOR PERSONAL USE ONLY
878 Science of Synthesis 4.4 Silicon Compounds
OMe
benzene, 90 o Me2N OMe
C, 13 h
95%
Me 2N
O
(R)-132
SiMe3
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG