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 (λ > 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. Similarly, 4 gives a 3 ∶ 2 mixture of 15c and 15t on irradiation in benzene in the presence of the same alkene, while in the presence of tetrachloroethene 17 is obtained selectively, all these monocycloadducts resulting from chemoselective addition of the alkene to the thiinone C-C double bond of 4, respectively.