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A facile one-pot synthesis of 2-(prop-2-yn-1-ylidene)-2,3-dihydro-1,4-thiazepines

An unprecedented method for the synthesis of 2-(prop-2-yn-1-ylidene)-2,3-dihydro-1,4-thiazepines from N-(2,4-pentadiynyl) beta-enaminones is reported. Upon treatment with Lawesson's reagent, N-(2,4-pentadiynyl) beta-enaminones were thionated to furnish N-(2,4-pentadiynyl) beta-enaminothiones, which immediately underwent 7-exo-dig cyclization to generate 2-(prop-2-yn-1-ylidene)-2,3-dihydro-1,4-thiazepines in one-pot. A general trend was observed for various N-(2,4-pentadiynyl) beta-enaminones and cyclization proceeded with broad functional group tolerance. This operationally easy method may provide quick access to a library of functionalized 1,4-thiazepines in the field of pharmaceutical chemistry.