(Z)-.alpha.-(trimethylsilyl) .alpha.,.beta.-unsaturated esters. Their stereoselective conversion into .alpha.,.beta.- and .beta.,.gamma.-unsaturated esters and .beta.,.gamma.-unsaturated ketene acetals

Deprotonation of methyl(Z)-α-(trimethylsilyl) α,β-unsaturated esters with lithium diisopropylamide (LDA) or with lithium hexamethyldisilazide (LHMDS) in the presence of hexamethylphosphoramide (HMPA) as an activator, followed by protonation of the intermediate dienolates with methanol, produces stereoselectivity the desilylated (E)-3-alkenoic esters. Trapping the dienolates with chlorotrimethylsilane instead of methanol and then treatment of the resultant ketene acetals with aqueous hydrochloric acid affords (E)-α-(trimethylsilyl)-β,γ-alkenoic esters in 98% isomeric purities. In the absence of HMPA, (Z)-α-(trimethylsilyl)-α,β-alkenoic esters undergo a Michael-type addition with LDA to furnish, after methanol-mediated elimination of the diisopropylamine moiety, (E)-α-(trimethylsilyl)-α,β-alkenoic esters. In contrast to the behavior with the corresponding Z esters, deprotonation of the E esters with LDA does not require an activator. Treatment of the dienolate intermediates formed with chlorotrimethylsilane yields O-methyl-C,O-bis(trimethylsilyl)ketene acetals, and alkylation furnishes (E)-α-alkyl-β,γ-unsaturated esters. Protodesilylation of the latter compounds with tetra-n-butylammonium fluoride followed by hydrolytic workup provides trisubstituted 2-alkenoates