The effects of methyl group substitution on the photochemistry of the 1, 5-hexadien-3-one system has been examined, with particular emphasis on the intramolecular cycloaddition reaction to form the bicyclo [2.1. 1] hexan-2-one system. Monosubstituted compounds 12, 13, 15, 17, and 18 and disubstituted compounds 14, 16, and 19 were studied. These structural changes were found to have profound effects on the photochemical reactions ...