(6A-ALPHA,6B-BETA,12B-BETA,12C-ALPHA)-6A,6B,12B,12C-TETRAHYDROCYCLOBUTA[1,2-C-4,3-C']-DI[1]BENZOTHIOPYRAN-6,7-DIONE

Irradiation (350 nm) of 1-thiocoumarin in solution affords the title compound in 55% yield. This compound crystallizes in the chiral space group P6(5) with two molecules in the asymmetric unit. The absolute structure of the title compound was determined through the Flack parameter, x = 0.00(2) [Flack (1983). Acta Cryst. A39, 876-881]. The experimental data are compared to those of the corresponding coumarin dimer. The cyclobutane ring of the title compound shows a puckered conformation. The fused six-membered thiopyran rings adopt conformations between half-chair (H conformation) and twist-boat (S conformation). The total puckering amplitudes, Q, in the six-membered rings are small, indicating that these rings are rather flat.