Cycloadditions of Ketene Diethyl Acetal and 2-Methylene-1,3-dioxepane to Electrophilic Alkenes

Abstract

Reactions of strong electron-rich donor alkenes with strong electron-poor acceptor alkenes give a variety of products, including cyclobutanes, cyclohexanes composed of two alkenes of one kind and one of the other, open chain adducts, and homopolymers of the alkenes. We examined cycloadditions of ketene diethyl acetal and 2-methylene-1,3-dioxepane to alkenes containing from one to four acceptor groups. The products and rates, in particular the structure of the cyclohexane adducts produced, are correlated with the ionic homopolymerizability of the alkenes, zwitterion stabilization, and gem-dialkyl cyclization effects. These concepts are extended to enamines and enolate anions as other electron-rich alkenes. 2-Methylene-1,3-dioxepane is the ketene acetal of choice for the preparation of many cyclobutanes, presumably due to the instability of the cationic end of the initial zwitterion from this cyclic ketene acetal relative to those from acyclic ketene acetals.