Enamine chemistry. Part XVII. Reaction of αβ-unsaturated acid chlorides with enamines. Further mechanistic investigations. Effect of triethylamine on the reaction path

Abstract

αα′-Annulation of enamines of cyclic ketones occurs on treatment with αβ-unsaturated acid chlorides in boiling benzene, to give bridged bicyclic ketones. Further evidence supports the contention that this occurs by initial N-acylation of the enamine followed by a [3,3]sigmatropic rearrangement. In the presence of triethylamine the course of the reaction is changed. C-Acylation of the enamine occurs, leading to a tetrahydrochromanone. This reaction has been shown to involve a vinylketen intermediate. The evidence available indicates that vinylketens react with enamines by a two-stage mechanism to give a zwitterionic intermediate, rather than by concerted [2 + 2] or [4 + 2] cycloaddition.