Synthetic utility of a heterocyclic o-aminoaldehyde: synthesis of pyrazolopyridopyrimidines, pyrazolonaphthyridines, and pyrazolopyrimidonaphthyridinones

A series of various polysubstituted pyrazolo[3,4-h][1,6]naphthyridines, benzo[b]pyrazolo[3,4-h][1,6]naphthyridines, and pyrazolo[4',3':5,6]pyrido[4,3-d]pyrimidines were synthesized by Friedlander condensation of 4-amino-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbaldehyde (o-aminoaldehyde) with active methylene compounds. The o-aminoaldehyde was functionalized to o-aminonitrile and o-aminocarboxamide with malononitrile and cyanoacetamide, respectively, which on condensation with monosubstituted ureas and acetic anhydride, respectively, afforded pyrazolo[3,4-h]pyrimido[4,5-b][1,6]naphthyridines.