REACTION OF 2H-BENZIMIDAZOLE-2-THIONE WITH DIMETHYL ACETYLENEDICARBOXYLATE

The reaction of dimethyl acetylenedicarboxylate (DMAD) with 2H-benzimidazole-2-thione in methanol has been investigated. When run for prolonged reaction times, the exclusive product of this reaction, in high yield, is methyl 4-oxo-4H-[1, 3]thiazino[3, 2-o]benzimidazole-2-carboxylate. This is also the exclusive product after short reaction times in the presence of a catalytic amount of sodium methoxide. The structure of the product was confirmed by its hydrolysis and decarboxylation to the same product obtained from the reaction of ethyl propiolate with 2H-benzimidazole-2-thione. When reactions with DMAD are run for shorter periods of time in methanol, without added base, mixtures of three products can be obtained. Each of these compounds was isolated and characterized. One of them, 2-(carbomethoxymethylene)-3-oxo-2H, 3H-thiazolo[3, 2-a]benzimidazole, rearranges in methanol to the isomeric thiazinone, and this rearrangement can be catalyzed by methanolic sodium methoxide. © 1979, American Chemical Society. All rights reserved.