Stereochemistry of the thermal electrocyclic ring opening of α-cyano-cis-stilbene oxide

Abstract

Whereas α-cyano-trans-stilbene oxide undergoes conrotatory ring opening to a carbonyl ylide which combines stereospecifically with dimethyl fumarate, the cis-oxiran produces mixtures of stereospecific and nonstereospecific cycloadducts depending on the dimethyl fumarate concentration; a geometrical isomerisation of the carbonyl ylide formed by conrotation and, probably, a disrotatory ring opening of the oxiran are responsible for the nonstereospecific course.