Treatment of 2-methoxycarbonyl- (or 3-cyano-)allyl acetoacetates with a tertiary amine or triphenylphosphane afforded α-methylene γ-substituted δ-keto esters (or nitrile) in satisfactory yields. Various bases and nucleophiles were tested to elucidate the mechanism. The results of the study strongly suggest an intermolecular pathway involving a SN2′-decarboxylation-SN2′ cascade.