Photochemical reactions. Part III. Thermal generation of photoenols and their derivatives from substituted 1,2-dihydrobenzocyclobutenes

Abstract

On heating, 1,2-dihydrobenzocyclobuten-1-ol and its derivatives undergo an electrocyclic, conrotatory ring opening to form the corresponding o-quinonedimethide species in which the oxygen substituent always adopts the (E)-configuration. The dimethide species can be trapped with a variety of dienophiles, including maleic anhydride and quinones. These trapping experiments prove that the o-quinonedimethide species produced thermally are identical with the photoenols formed during the irradiation of o-alkylaromatic ketones. The rate of ring opening of substituted benzocyclobutenols depends on the electronic nature of the substituents as well as their size.