C([O:3][C:4]([C:6]1[C:7]([OH:24])=[C:8]2[CH:14]=[N:13][N:12]([CH2:15][C:16]3[CH:21]=[CH:20][C:19]([O:22][CH3:23])=[CH:18][CH:17]=3)[C:9]2=[N:10][CH:11]=1)=[O:5])C.[OH-].[Na+].Cl>C(O)C>[OH:24][C:7]1[C:6]([C:4]([OH:5])=[O:3])=[CH:11][N:10]=[C:9]2[N:12]([CH2:15][C:16]3[CH:21]=[CH:20][C:19]([O:22][CH3:23])=[CH:18][CH:17]=3)[N:13]=[CH:14][C:8]=124-Hydroxy-1-(4-methoxybenzyl)-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid ethyl ester was dissolved in ethanol (20 mL), and a sodium hydroxide solution (5 M, 6 mL) was added thereto. The resulting mixture was stirred at 70° C. for 2 hours, and a hydrochloric acid solution (5 M, 6 mL) was added thereto under ice-cooling. The precipitate was collected by filtration and washed with water. The obtained solid was suspended in a methanol-toluene solvent mixture and concentrated under reduced pressure to give 4-hydroxy-1-(4-methoxybenzyl)-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid (2.89 g, 98%) as a colorless powder.
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