Cyclization of Methyl-Coumalate-Derived Methyl 1‑Benzamido-6-oxo-1,6-dihydropyridine-3-carboxylates: Assembly of the [1,2,4]Triazolo[1,5‑a]pyridine Ring System

An efficient three-step synthesis of a series of fused bicyclic s-[1,2,4]­triazolo­[1,5-a]­pyridines 1 was accomplished utilizing novel intermediates derived from inexpensive, commercially available hydrazides A and methyl coumalate B. A significant feature of this approach was the formation of a dihydrazide intermediate 2, bypassing the need for oxidative N–N bond formation in the 1,2,4-triazole synthesis. Further purification of the dihydrazides 2, beyond simple isolation, proved to be unnecessary owing to the impurity rejection afforded by the crystalline oxadiazolium salts 3. Additionally, the prepared oxadiazolium perchlorate salts showed excellent moisture stability, an unusual feature in compounds of this type.