The reactivity of (Z)-5-acetyl-3-aryl-2,3-dihydro-2-[(thioacyl)methylene]- 1,3,4-thiadiazoles: A surprising base-induced conversion into 3-(N-arylamino)thiophenes.

We report the reactivity shown by two classes of rarely investigated heterocycles, the 5-acetyl-2,3-dihydro-3-phenyl-2-(phenylmethylene)-1,3,4-thiadiazoles and the 5-alkanoyl-3-aryl-2,3-dihydro-2-[(thioacyl)methylene]-1,3,4-thiadiazoles. In both cases, strong bases promote cleavage of the thiadiazole ring with loss of thiocyanate anion, generating N-arylketeneimines and N-aryl(thioacyl)keteneimines, respectively. These very reactive species undergo either nucleophilic addition or [4+2] cycloaddition reactions involving the thioacyl function. The most surprising result was found in the reactions of the 5-acetyl-3-aryl-2,3-dihydro-2-[(thioacyl)methylene]-1,3,4-thiadiazoles which afford 3-(arylamino)thiophenes as the main products. This serendipitous transformation, which seems to be rather general, probably involves an intermediate step in which the acetyl group of the substrates is converted into ketene.