Photocyclisation of enamides. Part I. Photochemical reactions of N-acyl enamines

Abstract

Irradiation of ethereal solutions of the N-benzoyl enamines (8a–d) of 2-tetralone with a low pressure mercury lamp afforded benzo[a]phenanthridones (10a–d) as the major products along with the acyl-migration product (9). The structure and stereochemistry of the photoproducts have been established. Similar irradiation of the corresponding N-acetyl enamines (1) and (5a–d), however, afforded only acyl-migration products which on hydrolysis provided 2-acetyl ketones in comparable yields. The transformation of the trans-photoproduct (10b) into the cis-isomer (12) was observed when the former was heated with selenium at an elevated temperature.