The chemistry of sulfines. 13. 2-Thiabicyclo[2.2.1]hept-5-ene and its S-oxides and 3-alkyl derivatives: sulfine and sulfene cyclopentadiene Diels-Alder adducts. Conversion of the cyclopentadiene-sulfine adducts into 2-oxa-3-thiabicyclo[3.3.0]oct-7-enes, novel bicyclic sultenes