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-b]­pyridines from low-cost and readily accessible arylglyoxals, pyrazol-5-amines, aromatic amines, 4-hydroxy-6-methyl-2H-pyran-2-one, and cyclohexane-1,3-diones. The polysubstituted cyclopenta­[d]­pyrazolo­[3,4-b]­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-b]­pyrrolo­[4,3,2-de]­quinolones. Reasonable mechanisms for forming two new types of multicyclic pyrazolo­[3,4-b]­pyridines are also proposed.