Synthesis and photochemistry of isothiocoumarins fused to an additional pyranone or thiopyranone ring.

Both 4H,7H-benzo[1,2-c:4,3-c']dithiin-4,7-dione (3) and 4H,7H-thiino[4,3-f][2]benzopyran- 4,7-dione (4) were synthesized from 1,2-bis(2-hydroxyethyl)benzene (7). On irradiation (lambda > 390 nm) in acetonitrile the symmetric tricycle 3 affords selectively the two-fold cis-cisoid-cis dimer 10 while the asymmetric tricycle 4 gives a 5:1 mixture of the cis-cisoid-cis dimers 11 and 12. On irradiation in benzene in the presence of 2,3-dimethylbut-2-ene 3 affords quantitatively a 3:2 mixture of the diastereomeric monocycloadducts 14c and 14t.