It was found that in [2+2] intramolecular photocycloaddition the isomer ratio in system II is different from system I. Compounds 1 and 20 cyclize with high stereoselectivity to give in high yield 3 and 21 respectiveIy. The mechanistic consequences are discussed. The mechanism of [2+2] photocycloaddition of double bonds to cycloalkenones was proposed by Corey(') in 1964. It is generally agreed (*) that in most cases a triplet enone will form a diradical intermediate I-:ia an exciplex(3). The role of the exciplex was questioned lately based on triplet state lifetime measurements(4). The triplet diradical will cyclize to the products after spin inversion to the singlet stste. Prediction of the stereo and the regioselectivity of the cycloaddition reaction is still unsatisfactory. Scheme I
&-p+!-J\
)
)f$-Jy II Scheme II It was proposed(5s6) that when system I is irradiated, it forms the 1,4-diradical III. The first bond is formed between the olefin and carbon-p of the excited cyclohexenone. Rotation around the single bond in the 1,4-diradical intermediate was found to be much faster than cyclization. Therefore z or c olefins lead to the same product mixture. Corollary, any attempt to transfer structural information from the olefin to the photoadduct will