Selective Synthesis of New Tetracyclic Coumarin‐fused Pyrazolo[3,4‐b]pyridines and Pyrazolo[3,4‐b]pyridin‐6(7H)‐ones

A three-component reaction for the synthesis of new coumarin-fused tetracyclic system from 4-hydroxycoumarin, aldehydes, and 5-aminopyrazoles/5-aminoisoxazole is described. In the presence of acetic acid, 4,7-dihydro-1H-pyrazolo[3,4-b]pyridines (4) and pyrazolo[3,4-b]pyridines (5) were obtained in acetonitrile and dimethylsulfoxide medium, respectively. The reaction gave rise to 4,5-dihydro-1H-pyrazolo[3,4-b]pyridin-6(7H)-ones (6) in acetic acid–ethanol combination system, which involved the C–O bond cleavage. 4-Hydroxy-6-methyl-2H-pyran-2-one and acenaphthylene-1,2-dione were also examined, affording the corresponding C–O bond cleavage products. Mechanism indicates that the reaction is reversible in acetic acid–ethanol combination system.