Photocyclization of coumarinoyl enamides revisited: [2+2+2] cycloreversion/cycloaddition mechanism

Abstract

The major and minor products for the photocyclization of coumarinoyl enamides were identified. Controlled and intermediate-trapping experiments indicate that the formation of the minor isomer involved a stepwise, radical [2+2+2] cycloreversion/cycloaddition mechanism. Deuterium labeling studies suggest that the photocyclization of both aroyl and alkenoyl enamides involved a [1,5] hydrogen shift as a key step, but different hydrogen atoms were migrated.