Stereochemistry of cycloaddition of dimethylketen to cis- and trans-but-2-ene

Abstract

Dimethylketen, generated in situ by two different routes, has been shown to react with cis- and trans-but-2-enes at 100° to form cis- and trans-1,1,2,3-tetramethylcyclobutanones. The cis-olefin reacts with at least 97·5% retention of stereochemistry while the trans-isomer reacts half as quickly to give only 65–80%trans-product. The differences in behaviour of the olefins are interpreted in terms of a coplanar approach of the reagents resulting in a much looser transition state for the addition of trans- than of cis-but-2-ene.