Photocycloaddition of 23-Dihydro-2,2-dimethyl-4H-thiopyran-4-one (a 4-Thiacyclohex-2-enone) to bonafide Triplet Quenchers. A Contradiction?

摘要

2,3-Dihydro-2,2-dimethyl-4H-thiopyran-4-one undergoes regiospecific photocycloaddition to 2-chloroacrylonitrile as well as regio- and stereospecific photocycloaddition to ethylidene-malononitrile. On irradiation in the presence of either 2,3-di-methylbuta-l,3-diene or isoprene, the same enone affords mainly [2+2]-cycloadducts and only minor amounts of (stepwise) [2+4]-cycloadducts, whereas quantitative quenching is observed in the presence of 2,5-dimethylhexa-2,4-diene. In contrast, the corresponding pyran is almost quantitatively quenched in the presence of any of the enenitriles or dienes mentioned.