Stereoselectivity in Photocycloaddition of Bicyclic Enones to Olefins

Stereoselectivity in photocycloaddition of bicyclic enones 1,2, 5, and 6 to 2-butene and cyclohexene is discussed. In the cases of 1 and 5, having a five-membered ring fused to the double bond of monocyclic enones, from two to four kinds of cycloadducts were always obtained regardless of the ring size of the enone moieties, and, therefore, stereoselectivity is relatively low. On the other hand, in the cases of 2 and 6, having a six-membered ring fused to the double bond of monocyclic enones, the formation of cis-anti-trans cycloadducts proceeded stereoselectively. This remarkable effect of fused ring size on stereoselectivity in photocycloaddition is ascribed to the degree of non-bonded hydrogen interaction in 1,4-diradical intermediates and can be associated with differing flexibility and rigidity of cyclohexane and cyclopentane rings.