Versatile three component coupling for the synthesis of pyrazolopyridines and other pyrido fused systems

A new unexpected reaction of 1-(4-fluorophenyl)-2-(4pyridyl)ethanone 2 with 3-aminopyrazole, involving loss of a 4-pyridylmethyl group, afforded 4,6-bis (4-fluorophenyl)-5-(4-pyridyl)-1H-pyrazolo[3,4-h]pyridine 1 in good yields. Investigation of the features of this cyclization led to the development of a new three component coupling for the synthesis of pyrazolo[3,4-b]pyridines, from a ketone, an aldehyde and a 3-aminopyrazole in the presence of acid catalysis and a protic solvent. The study of the scope of this reaction showed its remarkable versatility and its usefulness in the preparation of other bicyclic systems, such as thieno[3,2-b]pyridines, pyrrolo[3,2-b]pyridines, imidazo[4,5-b]pyridines, furo[2,3-b]pyridines, isoxazolo[5,4-b]pyridines and, isothiazolo[5,4-h]pyridines. The three component nature of this reaction makes it suitable for the preparation of combinatorial libraries since scaffold decoration can be easily achieved in one step.