PHOTOCHEMICAL [2+2] CYCLOADDITIONS .4. CYCLOADDITION OF 2-CYCLOPENTENONE TO SOME (OMEGA-1)-ALKEN-1-OLS - EVIDENCE FOR REGIOSELECTIVITY DUE TO HYDROGEN-BONDING

Irradiation (lambda > 295 nm) of the title ketone in the presence of an (omega-1)-alken-1-ol (omega = 3,4,5) gave mixtures of the head-to-head and head-to-tail [2 + 2] cycloadducts in good yields. Both the regioisomer composition and the reaction efficiency were very sensitive to the solvent employed and to the alkenol concentration used. In hexane the head-to-head regioisomer was the major product at low alkenol concentration while in methanol and in hexane at high alkenol concentration the head-to-tail adduct predominated. It is proposed that this change in regioselectivity may be due to hydrogen-bonding effects either between the enone and the alkenol prior to reaction, or in the intermediate biradicals for-med in the reaction.