General Papers ARKIVOC 2007 (xiv) 1-11 ISSN 1424-6376 Page 1 © ARKAT USA, Inc. Unusual reactivity of thiosemicarbazides towards 2,3-diphenylcyclopropenone: synthesis of new pyridazinethiones and 1,2,4-triazolo[4,3-b]pyridazinethiones Ashraf A. Aly,* Alaa A. Hassan, Mohsen A.-M. Gomaa, and Essmat M. El-Sheref Chemistry Department, Faculty of Science, El-Minia University, El-Minia, A. R. Egypt E-mail: ashraf160@yahoo.com Abstract 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-diphenyl- cyclopropenone. 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. Keywords: Thiosemicarbazides, 2,3-diphenylcyclopropenone, pyridazinethiones, 1,2,4-triazolo- [4,3-b]-pyridazinethiones, [3+3]-cycloaddition Introduction Thiosemicarbazides are easily cyclized by the action of acids, bases or oxidants, therefore they are useful versatile building blocks for the preparation of heterocyclic ring systems. Some time ago, we investigated the reactions of thiosemicarbazides with π-deficient compounds. As a result we synthesized many heterocyclic ring systems such as thiazoles, thiazines, thiadiazoles, thiadiazines, pyrazines and indazoles. 1,2 Cyclopropenones undergo several interesting cycloaddition reactions and they may be useful starting materials for a variety of compounds. 3,4 They are also reactive towards dipolar reagents and compounds having a reactive π-system. 5 The reaction pattern is sometimes complex, but recent extensive investigations have established the utility of these molecules as building blocks for the construction of larger molecules. It is expected that the use of cyclopropenones as C 3 synthetic blocks will find a broader applicability in the field of synthetic chemistry in the future. 2,3-Diphenylcyclopropenone (1) can be represented by the resonance structures 1a-c (equivalent