[C:1]([O:5][C:6]([NH:8][C@@H:9]([C:19](O)=[O:20])[CH2:10][O:11][CH2:12][C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)=[O:7])([CH3:4])([CH3:3])[CH3:2].ClC(OCC(C)C)=O.[BH4-].[Na+].C(O)(=O)C>C1COCC1.C1COCC1.O>[C:1]([O:5][C:6]([NH:8][C@@H:9]([CH2:19][OH:20])[CH2:10][O:11][CH2:12][C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)=[O:7])([CH3:4])([CH3:3])[CH3:2]N-(tert-Butyloxycarbonyl)-O-benzyl-D-serine 10 (7.56 g, 25.63 mmol) was dissolved in dry THF and cooled to -20° C. under argon atmosphere. To this cold stirred solution was added TEA (3.03 g, 30 mmol) and isobutyl chloroformate (4.08 g, 30 mmol). The stirring was continued for 30 min at -20° C. under argon atmosphere. The reaction mixture was filtered immediately under a blanket of argon, the precipitate was washed with dry THF (50 ml). The combined filtrate was added slowly into a cold (0° C.) solution of NaBH4 (7.4 g, 200 mmol) in THF/water (80:20, 200 ml) during 10 min period. After the addition, the reaction mixture was stirred for 2 h at 0° C. and the pH adjusted to 7 with acetic acid. The solution was evaporated to dryness, partitioned between ethyl acetate/water (300:150 ml) and extracted in ethyl acetate. The organic extract was washed with brine (100 ml), dried over anhydrous sodium sulfate and evaporated to dryness. The crude product was purified by flash column chromatography over silica gel using CH2Cl2 →EtOAc as the eluent. The pure product was pooled together and evaporated to dryness to give 6.68 g (92%) of the pure product as an oil. 1HNMR (CDCl3): δ1.41 (s, 9H, Boc), 3.60-3.70 (m, 4H), 3.82 (d, 2H), 4.53 (s, 2H, OCH2Ph), 5.20 (bs, 1H, NH) and 7.30-7.40 (m, 5H, Ph).
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