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-Büchi 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- ...
