Pyrazolopyridines reduction

Pyrazolopyridines isomeric to those described previously have been obtained by other methods. Thus, the derivative (558) was formed by Raney nickel reduction of the 4-nitrosopyrazole (557) (7UHC1035), and the pyrazolo[3,4-c]pyridine derivative (560) was prepared from the azide (559) (79CC627). 

A new method of preparing pyrazolopyridines from A -azinylpyridinium iV-aminides has been developed by Alvarez-Builla et al. <02SL1093> and features a radical addition to a pyridine. Initially, attempts to convert aminide 142 into pyrazolopyridine 143, through the slow addition of tris(trimethylsilyl)silane and AIBN to a refluxing acetonitrile - benzene solution of the two components, was thwarted by a competitive reduction of the N-N bond. Indeed, aminopyridine 144 was formed as the major product in 60% yield. However, through the simple expedient of adding potassium carbonate to the solution of aminide 142, that reduction pathway was almost completely shut down and the yield of pyrazolopyridine 143 was elevated from 2% to 56% (Scheme 39). 

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