Heterocyclization of Acetamidrazones. I. Synthesis of 1,2,4‐triazolo[4,3‐ a ]pyridines via ring closure of 6‐(2‐acylhydrazino)pyridine intermediates

The reaction of N 1 ‐acyl‐2‐ethoxycarbonylacetamidrazones with ethyl ethoxymethylenecyanoacetate (EMCA) has been examined. The acetamidrazone 1a reacts with EMCA in boiling dimethyl sulphoxide to give the 1,2,4‐triazolo[4,3‐ a ]pyridin‐3(2 H )‐one 2 in excellent yield. Similarly from the amidrazones 1b‐h the 1,2,4‐triazolo[4,3‐ a ]pyridines 3b‐h were obtained. When the reaction between the amidrazones and EMCA was performed in ethanolic solution, the 6‐(2‐acylhydrazino)‐pyridines 4 were isolated. Ring closure of 4 afforded the 1,2,4‐triazolo[4,3‐ a ]pyridines.