Unusual reactivity of thiosemicarbazides towards 2,3-diphenylcyclopropenone: synthesis of new pyridazinethiones and 1,2,4-triazolo[4,3-b] pyridazinethiones

New pyridazinethiones and 1,2,4-triazolo[4,3-b]pyridazinethiones have been obtained during the reaction of thiosemicarbazide and its 1,4-disubstituted derivatives with 2,3-diphenylcyclopropenone. The reaction of this cyclopropenone with two equivalents of thiosemicarbazide afforded the corresponding 1,2,4-triazolo[4,3-b] pyridazinethiones. However, the reaction of N-substituted hydrazino derivatives of thiosemicarbazides with the cyclopropenone occurs with stoichiometric amounts of the starting materials to produce pyridazinethiones. The reaction mechanism, in both cases, was described as a formal [3+3]-cycloaddition.