C1([C@H](N)C)C=CC=CC=1.[C:10]([O:14][C:15]([N:17]1[CH2:22][C@H:21]([C:23]([O:25][CH3:26])=[O:24])[CH2:20][C@H:19]([C:27]([OH:29])=[O:28])[CH2:18]1)=[O:16])([CH3:13])([CH3:12])[CH3:11].CO.C(O)(=O)CC(CC(O)=O)(C(O)=O)O>O>[C:10]([O:14][C:15]([N:17]1[CH2:22][C@H:21]([C:23]([O:25][CH3:26])=[O:24])[CH2:20][C@H:19]([C:27]([OH:29])=[O:28])[CH2:18]1)=[O:16])([CH3:13])([CH3:11])[CH3:12](3S,5R)-1-(tert-Butoxycarbonyl)-5-(methoxycarbonyl)piperidine-3-carboxylic acid (1R)-1-phenylethylamine salt (20.0 g, 49.0 mmol), methanol (20 mL) and water (80 mL) were charged. A solution of citric acid (11.3 g, 58.8 mmol) in water (20.0 mL) was added dropwise at 20-30° C., and the mixture was stirred at the same temperature for 1.5 hr. The precipitated crystals were collected by filtration, washed with water (60 mL), and dried under reduced pressure at 50° C. to give (3S,5R)-1-(tert-butoxycarbonyl)-5-(methoxycarbonyl)piperidine-3-carboxylic acid as a white crystalline powder (13.5 g, yield 96.1%).
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