A series of 1,3,6-trisubstituted and 1,3,5,6-tetrasubstituted pyrazolo [3,4-b] pyridines 5 have been synthesized by series of reactions on 1-phenyl-3-carboxylate pyrazolone to obtain o-aminoaldehyde, which undergo facile condensation with various a-methylene ketones, nitriles, and esters, furnish fused pyridine derivative in good yield.