Four-Component Bicyclization Approaches to Skeletally Diverse Pyrazolo[3,4‑<i>b</i>]pyridine Derivatives

A novel four-component bicyclization strategy has been established, allowing a flexible and practical approach to 37 examples of multicyclic pyrazolo­[3,4-<i>b</i>]­pyridines from low-cost and readily accessible arylglyoxals, pyrazol-5-amines, aromatic amines, 4-hydroxy-6-methyl-2<i>H</i>-pyran-2-one, and cyclohexane-1,3-diones. The polysubstituted cyclopenta­[<i>d</i>]­pyrazolo­[3,4-<i>b</i>]­pyridines were stereoselectively synthesized through a microwave-assisted special [3+2+1]/[3+2] bicyclization with good control of the spatial configuration of exocyclic double bonds. The novel [3+2+1]/[2+2+1] bicyclization resulted in 17 examples of unreported pyrazolo­[3,4-<i>b</i>]­pyrrolo­[4,3,2-<i>de</i>]­quinolones. Reasonable mechanisms for forming two new types of multicyclic pyrazolo­[3,4-<i>b</i>]­pyridines are also proposed