SYNTHESIS OF NEW 3-SUBSTITUTED 2H-THIOPYR AN DERIVATIVES VIA [4+2] CYCLOADDITION REACTIONS USING ACCEPTOR SUBSTITUTED ENAMINOTHIONES

3- and 4-Nitro acetophenones as well as 3- and 4-trifluoromethyl aceto phenones react with POCl3/DMF (Vilsmeier reagent) to give dimethylimin ium perchlorates 1a-d after addition of perchloric acid to the reactio n mixture. Substitution of the chloro atom in 1 by using sodium sulfid e nonahydrate occurs under mild conditions (in case of nitro compounds at -5-degrees-C) leading to enaminothiones 2a-d. Reactions with acrol eine and methylvinylketone in refluxing benzene give exclusively 2H-th iopyran derivatives 4a-h, which were isolated in good yields after spo ntaneous fast elimination of dimethylamine. In contrast, the introduct ion of 1-nitro-2-phenylethene as the dienophile allows stepwise reacti on to give stable adducts 3k and 3l, respectively, and also 3m under m ild conditions (reaction at room temperature). H-1 n.m.r. spectroscopi c data as well as the elimination of dimethylamine to 4k-m permit the elucidation of the structure of adducts 3k-m.