Photocycloaddition of 2-methyl-1,4-naphthoquinone to enol esters. A photochemical approach to benzofuran derivatives

Naphthoquinone 1 undergoes facile photocycloaddition with vinyl acetate and isopropenyl acetate to give cyclobutane derivatives in excellent yields. The photocycloaddition reactions were found to be completely regioselective; in the case of vinyl acetate, a mixture of epimeric acetates 2 and 3 was obtained and in the case of isopropenyl acetate, a single adduct 7 was formed. Treatment of the photoadducts with dilute sulfuric acid resulted in skeletal rearrangement giving naphthofurans 4 and 8.