Synthesis of New 3-Substituted 2H-Thiopyran Derivatives via Cycloaddition Reactions using Acceptor Substituted Enaminothiones

Article abstract of DOI:10.1002/prac.19943360510

3- and 4-Nitro acetophenones as well as 3- and 4-trifluoromethyl acetophenones react with POCl3/DMF (Vilsmeier reagent) to give dimethyliminium perchlorates 1a-d after addition of perchloric acid to the reaction mixture.Substitution of the chloro atom in 1 by using sodium sulfide nonahydrate occurs under mild conditions (in case of nitro compounds at 5 deg C) leading to enaminothiones 2a-d.Reactions with acroleine and methylvinylketone in refluxing benzene give exclusively 2H-thiopyran derivatives 4a-h, which were isolated in good yields after spontaneous fast elimination of dimethylamine.In contrast, the introduction of 1-nitro-2-phenylethene as the dienophile allows stepwise reaction to give stable adducts 3k and 3l, respectively, and also 3m under mild conditions (reaction at room temperature). 1H n.m.r. spectroscopic data as well as the elimination of dimethylamine to 4k-m permit the elucidation of the structure of adducts 3k-m.