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The reaction of 3-aryl-5-benzoyl-2-imino-2,3-dihydro-1,3,4-thiadiazoles (6) with dimethyl acetylenedicarboxylate (4b) gave the corresponding thiazole (8) and benzoyl cyanide through sulfurane (B) by 1,3-dipolar cycloaddition and cis- (9) and trans-vinyl (10) compounds through zwitterion (C) by simple addition. Two types of addition reactions competed each other and the ratio of the two [R=B/C] depended solely on the solvent polarity (ET). Dibenzoylacetylene behaved similarly. Several iminothiazolines and iminothiadiazolines reacted with activated acetylenes and the selectivity for the two types of addition reactions for each system was shown to be affected by quite subtle balance of electron-withdrawing ability of each heterocycle.

3-Substituted 2-nitrosoimino-2,3-dihydrobenzothiazoles (1) were reduced with lithium aluminum hydride to give the corresponding thiazolone azines and bis[o-(N-substituted N-formylamino)phenyl] disulfides as major products. Reactions of 1 with some substituted diazomethanes gave the corresponding unsymmetrical azine N-monoxides (16) or azines (17) depending on the structure of the diazomethane.