New β-substituted benzo- and naphthofuryl derivatives (5, 6 and 7) of o-divinylbenzene, were synthesized and irradiated in order to form annelated bi-cyclo[3.2.1]octadienes. While 2-[2-(2-vinylphenyl)ethenyl]benzo[b]furan (5) upon irradiation gives the bicyclo[3.2.1]octadiene derivative (12) in 65% yield, 2-[2-(2-vinylphenyl)ethenyl]naphtho[2,1-b]furan (6) and 2-[2-(2-vinylphenyl)ethenyl]na-phtho[1,2-b]furan (7), undergo cis-trans-isomerization but not form intramole-cular photocycloaddition products. The mechanism of the intramolecular [2+2] photocycloaddition is explained via intermediate (17) which was proved by the formation of products (18) and (19), by irradiation in methanol and deuteromethanol.

U.v. irradiation of acetaldehyde in the presence of styrene or methylstyrenes yields oxetans in a reaction which is both regio- and stereo-selective; benzaldehyde also reacts, but in a less selective manner.