Enamine chemistry—XXIII: The [4+2] cycloaddition reactions between enaminothiones and electrophilic olefins and acetylenes

The enamino-thione, 1c, reacts with acrylonitrile and 2-chloroacrylonitrile at room temperature to give 3,4-dihydro-4-(1-pyrrolidinyl)-2$\text{H}$-thiopyrans, 4 and 5, respectively. The reaction between 1-aryl-3-(1-pyrrolidinyl) (piperidino)-apropene-1-thiones, 1a-c (1d-f), and dimethyl acetylenedicarboxylate gives 4-(1-pyrrolidiny)(piperidino)-4$\text{H}$-thiopyrans, 6a-c (6d-f). Compounds 1a-c (1d-f) and ethyl propiolate produce 2-(1-pyrrolidinyl) (piperidino)-2$\text{H}$-thiopyrans, 8a-c (8d-f), and a new type of rearrangement is observed. The 2$\text{D}$-thiopyran, 9, is formed from 1b and ethyl 3$\text{D}$-propiolate, which elucidates the mechanism. ¹H and ¹³C NMR data of 6 and 7 are discussed.