[NH2:1][C@@H:2]1[CH2:7][N:6]([C:8]([O:10][C:11]([CH3:14])([CH3:13])[CH3:12])=[O:9])[CH2:5][C@H:4]([C:15]([O:17][CH3:18])=[O:16])[CH2:3]1.[CH:19](=O)[CH:20]([CH3:22])[CH3:21].C(O)(=O)C.C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+]>CO>[CH3:19][CH:20]([CH3:22])[CH2:21][NH:1][C@@H:2]1[CH2:7][N:6]([C:8]([O:10][C:11]([CH3:12])([CH3:13])[CH3:14])=[O:9])[CH2:5][C@H:4]([C:15]([O:17][CH3:18])=[O:16])[CH2:3]11-tert-Butyl 3-methyl (3R,5S)-5-aminopiperidine-1,3-dicarboxylate (1.83 g), isobutyraldehyde (0.78 ml) and acetic acid (0.49 ml) were dissolved in methanol (50 ml), and the mixture was stirred at room temperature for 30 min. Sodium triacetoxyborohydride (3.80 g) was added to the reaction mixture, and the mixture was stirred at room temperature for 7 hr. The reaction mixture was concentrated under reduced pressure, the concentrate was basified with aqueous sodium bicarbonate, and extracted with ethyl acetate. The extract was washed with water and saturated brine, and dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure. The residue was subjected to silica gel column chromatography, and a fraction eluted with ethyl acetate-hexane (1:1)-ethyl acetate 100%-ethyl acetate-methanol (9:1) was concentrated under reduced pressure to give the object product (1.42 g) as an oil.
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