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

Photocyclization of the heterocyclic 2,3-diones 1a-d with electron rich alk enes affords regio- and stereoselectively the 2+2 adducts 2, from Ib with b enzophenone 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 Ic the azepinone 6 is the only reaction product. The rmolysis of 4a generates the pyrono compound 5. Irradiation of the N-arylpy rrolediones le, f and ethylvinyl ether give furo[3,2-c]pyrrolones 7 thus ma king 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, r espectively. (C) 1999 Elsevier Science Ltd. All rights reserved.