Photocycloaddition of isocoumarins and isothiocoumarins to alkenes

On irradiation in the presence of tetrachloroethene (TCE), both isocoumarin s 3 and isothiocoumarins 4 afford in high yields the cis-fused cycloadducts 8 and 9, while only the oxacycles 3 undergo photocycloaddition to 2,3-dime thylbut-2-ene (TME) to give mixtures of cis- and trans-fused products 10 an d 11, respectively, in moderate yields. This higher efficiency in reacting with TCE as compared to TME for compounds 3 and 4 contrasts the behavior of simple cyclic enones, e.g., 5,5-dimethylcyclohex-2-enone (12), which is co nverted to bicyclooctanones about fifty times faster with TME than with TCE .