2-Thiabicyclo[3.2.0]hepta-3,6-dienes. 3. Desulfuration and sulfuration of 2-thiabicyclo[3.2.0]hepta-3,6-dienes and x-ray crystal structure of 3a,6,7,8,9,9a-hexahydro-3a,5-dimethylthieno[3,2-b][2]benzothiophene-2,3-dicarbonitrile