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 P 4 S 10 1 or Lawesson's reagent 2 , gave mainly the spirodimers 3'phenyl-2'-thoxo-1',3'-thiazolidino2,3-d]-spiro6',7' diaryl-5,5'-perhydrothiapyrano]-3-phenyl-1,3-thiazolidin-4-thiones ( 3a and b ) beside, 5-(3-bromo-4-methoxyphenylmethylene)-3-phenyl-1,3-thiazolidin-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 stereo-chemistry of obtained compounds have been studied.