Conjugate addition of keten silyl acetals to α,β-unsaturated carbonyl compounds in acetonitrile gave a quantitative yield of the corresponding methyl (3-trialkylsiloxyalk-2-enyl)acetates; subsequent site-specific electrophilic substitution yielded the corresponding 2-substituted 3-(alkoxycarbonylmethyl)alkanones. These novel addition and sequential alkylation reactions could be applied to a simple synthesis of methyl jasmonate, methyl didehydrojasmonate, and methyl dihydrojasmonate.