Synthesis and stereochemistry of thiapyranothiazoles as Diels-Alder adducts obtained from spirodimers of 1,3-thiazolidines with cinnamic acid and its ester

Thiation of 5-Arylmethylene-3-phenyl-2-thioxo-1,3-thiazolidin-4-ones (1a and b) with either P4S101 or Lawesson's reagent(2), gave mainly the spirodimers 3'phenyl-2 thoxo-1',3'-thiazolidino(2,3-d)-spiro(6',7' diaryl-5,5' perhydrothiapyrano]-3-phenyl-1,3-thiazolidin-4-thiones (3a and b) beside, 5-(3-bromo-4-methoxyphenylmethylene)-3-phenyl-1,3thiazolidira-2,4-dithione (2b) as a mixture with 3b. 2b was allowed to react with ethyl cinnamate as a dienophile producing 4b and 5b. Moreover, prolonged heating of either 3b or 3a with ethyl cinnamate gave a mixture contains 40% of 4b and a mixture of 4a and 5a respectively. Furthermore, the dimer 3b reacted with cinnamic acid in glacial acetic acid to give the Diels-Alder-adduct 6b and 7b. Structures and stereochemistry of obtained compounds have been studied.