Step 5: To a solution of 1H-pyrazolo[3,4-b]pyridine-3-carboxylic acid (V) (0.39 g, 2.4 mmol) in anhydrous methanol (10 mL) was slowly added concentrated sulfuric acid (4 drops) and the reaction was refluxed under nitrogen protection for 6 hours. Upon completion of the reaction, the reaction solution was cooled to room temperature and the solvent was subsequently removed by evaporation under reduced pressure. The residue was partitioned between dichloromethane and saturated sodium bicarbonate solution. The organic layer was separated, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The crude product was purified by fast silica gel column chromatography (elution gradient: 100% dichloromethane → 100:3 dichloromethane:methanol) to afford methyl 1H-pyrazolo[3,4-b]pyridine-3-carboxylate (VI) as a white solid (382 mg, 2.16 mmol, 90% yield).1H NMR (CDCl3) δ ppm: 4.08 (s, 3H) 7.38 (m, 1H), 8.63 (dd, J = 8.10, 1.51 Hz, 1H), 8.72 (dd, J = 4.62, 1.41 Hz, 1H); ESIMS m/z: 178.2 ([M + H]+).
![1H-Pyrazolo[3,4-b]pyridine-3-carboxylicacid(9CI)](/CAS/GIF/116855-08-4.gif)
![Methyl 1H-pyrazolo[3,4-b]pyridine-3-carboxylate](/CAS/GIF/916325-83-2.gif)