Step 4: To an aqueous solution (20 mL) containing NaOH (0.88 g, 22 mmol) was added 3-methyl-1H-pyrazolo[3,4-b]pyridine (IV) (0.4 g, 3 mmol). The suspension was heated at 80°C until a clarified solution was formed. Subsequently, an aqueous solution (180 mL) of KMnO4 (1.73 g, 11 mmol) was added slowly over a period of 2 hours while keeping the solution heated at 80°C. The solution was heated at 90°C for 2 hours. The reaction mixture was continued to be heated at 90°C for 2 hours until the complete disappearance of the feedstock was confirmed by thin layer chromatography (TLC). The solution was cooled to 70°C and filtered through a diatomaceous earth pad. The solids were washed with boiling water. The filtrates were combined and cooled to 0°C and acidified with concentrated HCl to pH=2. The acidified solution was extracted with n-butanol (2 x 10 mL). The n-butanol layer was concentrated under reduced pressure to give a white residue. The residue was dissolved in dichloromethane (DCM), a minimum amount of methanol (MeOH) was added to aid solubilization, and then filtered. The filtrate was concentrated to afford 1H-pyrazolo[3,4-b]pyridine-3-carboxylic acid (V) as a white solid (390 mg, 2.39 mmol, 81% yield).1H NMR (CDCl3) δ ppm: 7.37 (dd, J = 8.10, 4.52 Hz, 1H), 8.47 (dd, J = 7.54, 1.88 Hz, 1H), 8.62 (dd, J = 7.54, 1.88 Hz, 1H), 8.63 (dd, J = 7.54, 1.88 Hz, 1H). 8.62 (dd, J = 4.52, 1.32 Hz, 1H), 14.37 (brs, 1H).
![1H-Pyrazolo[3,4-b]pyridine,3-methyl-(9CI)](/CAS/GIF/116834-96-9.gif)
![1H-Pyrazolo[3,4-b]pyridine-3-carboxylicacid(9CI)](/CAS/GIF/116855-08-4.gif)