An Unequivocal Synthesis of 2‐Aryl Substituted 3‐Amino‐2,4,5,7‐tetrahydro‐6H‐pyrazolo[3,4‐b]pyridin‐6‐ones

The reaction between pyridones (1) and substituted hydrazines 4 can afford two different regioisomeric pyrazolo[3,4-b]pyridin-6-ones 2 and 3 depending on the initial substitution of the methoxy group and the direction of the cyclization. In the case of phenylhydrazine 4 (R3 = Ph), we have clearly shown that the treatment of pyridones 1a–d with 4 (R3 = Ph) in MeOH at temperatures below 140°C yields, independently of the nature and position of the substituents present in the pyridone ring, the open intermediates 7a–d. When the reaction is carried at 140°C under microwave irradiation, the corresponding 2-aryl substituted pyrazolo[3,4-b]pyridines 3a–d are always formed. We have experimentally determined, using DSC techniques, the activation energies of the two steps involved in the formation of 3: a) substitution of the methoxy group present in pyridones 1 with phenylhydrazine 4 (R3 = Ph) to afford intermediates 7 and b) cyclization of intermediates 7 to yield pyrazolopyridines 3. The results obtained, 15 and 42 kcal⋅mol−1 respectively, are in agreement with the experimental findings.