3-Formylchromone Enamines as a Simple, Facile Route to Novel Pyrazolo[3,4-b]pyridines and the Synthetic Utility of the Latter

Abstract: One-pot and facile preparations of 6-(2-hydroxy-5-R-benzoyl)-4-methyl-2-R1-pyrazolo[3,4-b]pyridines 4a-o are described, using the reaction of 3-formyl chromones 1 with 5-amino-1-R1-pyrazoles 2. An enamine-intermediate 2-ethyloxy-6-R-3-(3-methyl-1-phenylpyrazol-5-ylaminomethylene)chroman-4-one 3 was isolated at lower temperatures. Acyloxy-derivatives 5 of compounds 4 were obtained by acylation with acid chlorides or acid anhydrides. Coumarins 6 substituted at the 3- and 4-positions were prepared from the pyrazolo[3,4-b]pyridines 4 by condensation reactions and hydrazones 7 were formed from their reaction with 2,4-dinitrophenyl hydrazine. Reactions under microwave irradiation proceeded significantly faster and with high yields