SYNTHETIC PHOTOCHEMISTRY .1. GENERATION OF BENZOCYCLOALKENONES BY THE RING-OPENING OF BENZOCYCLOBUTENOLS - EVIDENCE FOR AN ORTHO-QUINODIMETHANE INTERMEDIATE

Photolysis of benzocyclobutenols (4) constitutes a new synthetic approach to the benzocycloalkenones (5). A singlet excited state is implicated in these reactions. Quenching by oxygen leads to the hydroxyperoxides (15). Incorporation of deuterium provides evidence for the intermediary of dienols; these may evolve by either (i) direct protonation producing the ketones (5b) or (ii) an intramolecular hydrogen shift forming the enols of the ketones (5a). The general character of these photolyses is emphasized by the study of several benzocyclobutenols (16) and (17), naphthocyclobutenols (22), and methyl ether derivatives (12).