Regio- and stereoselective photocycloadditions of heterocyclic 2,3-diones — Evidence for an unexpected 1,2-aroyl migration

Abstract Photocyclization of the heterocyclic 2,3-diones 1a-d with electron rich alkenes affords regio- and stereoselectively the 2+2 adducts 2 , from 1b with benzophenone as photosensitizer also the Paterno-Buchi adduct 3 is obtained. Similarly, with phenylethyne the cycloadducts 4 are formed in moderate to low yields, in case of 1c the azepinone 6 is the only reaction product. Thermolysis of 4a generates the pyrono compound 5 . Irradiation of the N-arylpyrrolediones 1e, f and ethylvinyl ether give furo[3,2-c]pyrrolones 7 thus making evident an unexpected 1,2-benzoyl migration. Structural elucidation of all ring systems described is based upon X-ray analyses of 2d, 5 and 7f , respectively.