The reactions of LR (Lawesson’s Reagent) with 4-oxocarboxylic acid derivatives I, N-acyl aminoacid derivatives III, and N-acyl-N′-ethoxycarbonylhydrazines V, smoothly produce substituted thiophenes II, thiazoles IV, and 1,3,4-thiadiazole-2(3H)-thiones VI.
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Thomsen et al. (1983) studied this question.