Experimental demonstration of base-catalyzed interconversion of isomeric betaine intermediates in the Corey-Chaykovsky epoxidation.

The collapse of hydroxysulfonium salts has been examined as a model for the epoxidation of aldehydes. The anti diastereomer reacted with retention of stereochemistry and no crossover, while the syn diastereomer gave crossover products along with cis and trans epoxides. Deprotonation and reprotonation on the carbon of the alpha-hydroxy sulfonium ylide is presumed responsible for production of the trans epoxide. This reaction pathway has been proposed to explain losses of enantioselectivity but never directly observed.