ELECTRONIC CONTROL OF STEREOSELECTIVITY IN PHOTOCYCLOADDITION REACTIONS .4. EFFECTS OF METHYL SUBSTITUENTS AT THE DONOR OLEFIN

The photocycloaddition reaction of benzaldehyde with a series of methyl-substituted cycloalkenes (1-methyl- and 1,2-dimethylcyclopentene, 1-methyl- and 1,2-dimethylcyclohexene, and 5-methyl-2,3-dihydrofuran) was investigated and compared with the results of the corresponding parent hydrocarbons. In all cases, the incorporation of methyl groups led to inversion of stereoselectivity (e.g. for cyclopentene, endo/exo = 61:39; for 1,2-dimethylcyclopentene, endo/exo = 20:80). No influence on regioselectivity could be observed for the alicyclic cases 1-methylcyclopentene and 1-methylcyclohexene, whereas 5-methyl-2,3-dihydrofuran gave only one regioisomer. The reactive conformations of the intermediate 1,4-triplet biradicals are used to rationalize the observed diastereoselectivity.