Heterodiene syntheses. Part XVI. A new [4 + 2] cycloaddition pathway in the reaction of 2-oxoindolin-3-ylideneacetophenones with ethyl vinyl ether

Abstract

In addition to dihydropyran[(2) and (3)] and dihydrofuran (4) derivatives, the reaction of (1-acetyl-2-oxoindolin-3-ylidene)acetophenones with ethyl vinyl ether gives 1-acetyl-3-aroyl-5-ethoxy-2a,3,4,5-tetrahydrobenz[cd]indol-2(1H)-ones. Their formation is rationalized in terms of a Diels–Alder reaction between the vinyl ether and the styrenic diene system of the oxoindolylidene derivatives, followed by a hydrogen shift and rearomatization. The configuration and conformation of the new adducts were determined by n.m.r. spectroscopy and confirmed by X-ray analysis.