Synthesis and reactions of [1,2,4]triazolo[4,3-a]pyridinium-3-aminides and [1,2,4]triazolo[4,3-a]pyrimidinium-3-aminides: evaluation of the scope and mechanism of a new type of heterocyclic rearrangement

1,2,4-Triazolo[4,3-a]pyridinium betaines 6a, b have been prepared by treating 2-pyridyl-4-phenylthiosemicarbazides 4a, b with dicyclohexylcarbodiimide, and related compounds 6c–f have been prepared through the intermediate S-methyl thiosemicarbazides 5c–f. Reaction of the pyrimidin-2-yl thiosemicarbazide derivatives 8a–c with dicyclohexylcarbodiimide (DCC) gave the 1,2,4-triazolo[4,3-a]pyrimidinium betaines 7a–c, but attempted thermal cyclization of the free bases derived from methiodides 8j–o gave a series of enamines 11. The structure of one such enamine 11a was elucidated by degradation with 2 mol dm–3 HCI which afforded a mixture of the amine 9 and the ketone 10. Treatment of the thiosemicarbazide 8d with dicyclohexylcarbodiimide gave the pyridinium betaine 14a, and a related compound 14b was prepared by reaction of the salt 12b with diazabicyclo[5.4.0]undec-7-ene (DBU). The crystal and molecular structure of the maleate salt of 14b was determined by X-ray crystallography. It was established that the betaine 14a could be converted into an enamine derivative 11a by heating it in toluene.