[C:1]([O:5][C:6]([N:8]1[CH2:13][CH2:12][CH:11]([N:14]2[C:18]3=[N:19][C:20]([O:25][CH3:26])=[N:21][C:22]([O:23]C)=[C:17]3[CH:16]=[N:15]2)[CH2:10][CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2].[OH-].[Na+].O1CCOCC1.Cl>O.C(OCC)(=O)C>[C:1]([O:5][C:6]([N:8]1[CH2:13][CH2:12][CH:11]([N:14]2[C:18]3[N:19]=[C:20]([O:25][CH3:26])[NH:21][C:22](=[O:23])[C:17]=3[CH:16]=[N:15]2)[CH2:10][CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2]A mixture of 4-(4,6-dimethoxy-pyrazolo[3,4-d]pyrimidin-1-yl)-piperidine-1-carboxylic acid tert-butyl ester (1 mmol), 2 M aqueous sodium hydroxide (1 mL) and dioxane (10 mL) is heated at 40° C. for 5 h. The mixture is allowed to cool, and is then neutralized to approximately pH 7 by the addition of 1 M aqueous hydrochloric acid. Ethyl acetate and water are added. The organic layer is washed with water and brine, dried (magnesium sulfate), filtered, evaporated, and purified by flash chromatography on silica gel to give 4-(6-methoxy-4-oxo-4,5-dihydro-pyrazolo[3,4-d]pyrimidin-1-yl)-piperidine-1-carboxylic acid tert-butyl ester.
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