Reaction of N-substituted acetohydrazides with 2-substituted cinnamonitriles. Competitive cyclizations to pyrazolo[3,4-b]pyridinones and [1,2,4]triazolo[1,5-a]pyridinones
A novel synthesis of pyrazolo[3,4-b]pyridinones 9 from 2′-acyl-2-cyanoacetohydrazide 5 and arylidenecyanoacetates 6 is described. In the reaction, an alternative cyclization leading to [1,2,4]triazolo[1,5-a]pyridinones takes place. Compounds 9 were isolated from the reaction mixture as the corresponding piperidinium salts due to the high stability of the heterocyclic anion. Acidification with dilute hydrochloric acid yielded the neutral pyrazolo[3, 4-b]pyridinone. Depending on the reaction conditions, the corresponding intermediate dihydropyridinone 12 and pyrazolo derivatives 16 were also obtained.