Abstract: Both 1, 5-hexadien-3-ones (7) and 1-acyl-1, 5-hexadienes (8) generally undergo intramolecular photochemical cyclization by way of 1, 5 (crossed) closure (see 2), as do other derivatives of 1, Shexadienes (1). Through study of the photochemistry of 14 members of series 7 and 8, summarized in Tables I and 11, two effects that modify this regiochemistry and favor a shifting to 1, 6 (straight) closure (see 3) were identified. These are alkyl ...