Formation of pyrazoles from 1,3,4-oxadiazolium salts

Abstract

1,3,4-Oxadiazolium salts react with ethyl cyanoacetate in the presence of triethylamine to yield 1-substituted 3-aminopyrazole-4-carboxylic esters. An open-chain intermediate has been isolated and a mechanism is proposed involving initial attack at C-2 of the oxadiazole ring. The reaction was extended to the synthesis of 3-aminopyrazole-4-carbonitriles, ethyl pyrazole-4-carboxylates, 4-acylpyrazoles, and a 3-acetoxypyrazole from oxadiazolium salts and malononitrile, β-ketonic esters, β-diketones, and ethyl malonate, respectively. Application of these reagents to oxadiazolo[3,2-a]pyridinium salts gave derivatives of pyrazolo[1,5-a]pyridine.