Synthesis of 1,3,4-thiadiazolines from 1,2-dithiole-3-thiones

The thiocarbonyl group of 4,5-dichloro-1,2-dithiole-3-thione reacts as a 1,3-dipolarophile towards diaryl nitrile imines by 1,3-dipolar cycloaddition followed by opening of the dithiole ring with loss of sulfur to give 5-methylene-1,3,4-thiadiazolines; this reaction together with nucleophilic displacement (before or after cycloaddition) of the selectively reactive 5-chlorine atom provides a rapid access to stable 5-methylene-1,3,4-thiadiazolines.