REGIOCHEMISTRY AND STEREOCHEMISTRY OF INTRAMOLECULAR [2+2] PHOTOCYCLOADDITION OF CARBON CARBON DOUBLE-BONDS TO CYCLOHEXENONES

The intramolecular [2 + 2] photocycloaddition mechanism of alkenes tethered by a three-methylene chain to cyclohexenones has been studied. It was found that the reversion from a 1,4-diradical intermediate to starting material is slow relative to the rate of photoadduct formation. Only ''straight'' closure was observed in the systems studied. For compounds 1-3 and 5, the assumption that the first bond is formed between the beta-carbon of the enone and C-4' leading to a 1,4-diradical is supported. For the formation of compounds 6-11, possible mechanisms are discussed.