[CH2:1]([O:8][C:9](=[O:23])[CH2:10][C@@H:11]([C:20](O)=[O:21])[NH:12][C:13]([O:15][C:16]([CH3:19])([CH3:18])[CH3:17])=[O:14])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.CN1CCOCC1.ClC(OCC(C)C)=O>C(COC)OC>[C:16]([O:15][C:13]([NH:12][C@H:11]([CH2:20][OH:21])[CH2:10][C:9]([O:8][CH2:1][C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1)=[O:23])=[O:14])([CH3:18])([CH3:19])[CH3:17]N-(tert-Butoxycarbonyl)-L-aspartic acid 4-benzyl ester (30 g) was dissolved in dimethoxyethane (90 mL) and the solution was cooled to −5° C. N-Methylmorpholine (10.32 mL) was added followed by isobutyl chloroformate (12.7 mL) in such a way to keep the temperature below −10° C. The mixture was aged for 0.5 hour. The solids were quickly filtered and washed with dimethoxyethane (90 mL). To the filtrate cooled to −50° C. was carefully added sodium borohydride (4.4 g) as a solution in water (45 mL) in such a way to keep the temperature between −30° C. and −15° C. After all the hydride had been added, water (500 mL) was added in such a way to maintain the temperature below −15° C. The suspension was filtered, the solid washed with water (400 mL) and dried to yield benzyl (3S)-3-[(tert-butoxycarbonyl)amino]-4-hydroxybutanoate.
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