Abstract: Irradiation of pentane solutions of bicyclo [2. ll] hexan-2-one (l), bicyclo [2.2. l] heptan-2-one (4), and camphor (5) leads to the formation of cycloolefinic aldehydes (7, 17, and 25) which react further under continued irradiation. In the case of 7 and 17, photocycloadditions of possible synthetic utility are observed, leading to the tricyclic oxetanes 10 and 18. Analogous oxetane formation from campholenic aldehyde (25), ...