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4,5-Dichloro-1,2-dithiole-3-thione was found to undergo 1,3-dipolar cycloaddition reactions with internal and terminal alkynes. The combination of these reactions together with new transformations afforded versatile heterocyclic systems including 1,3-dithioles, imino-1,3-dithietanes, 2-iminothiophen-3-ones, thienothiopyranthiones, and others.

On heating with alkynes, the readily prepared 1,3-dithioles 3 undergo a new cycloaddition reaction and an unprecedented molecular rearrangement with loss of chlorine to give the first 7H-thieno[2,3-c]thiopyran-7-thiones 4 and 4H-thieno[3,2-c]thiopyran-4-thiones 5 whose structures were confirmed by X-ray diffraction. Unexpectedly, the different alkynes used to form 3 and to convert it into 4 and 5 were incorporated regiospecifically into the thiophene and thiopyran rings, respectively. [reaction: see text]

The reaction of readily available 3,4,5-trichloro-1,2-dithiolium chloride 1 and 4,5-dichloro-1,2-dithiole-3-thione 2 with o-substituted anilines provides a simple route to 1,2-dithiole-3-thiones and 3-imines, which can be cyclised to give new, more complex heterocyclic systems such as 1,2-dithiolo[4,3-b][1,4]benzoxazine of potential value in cancer chemotherapy