[CH3:1][O:2][C:3](=[O:17])[CH2:4][CH:5]([NH:9][C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])[C:6](O)=[O:7].CCN(CC)CC.ClC(OCC)=O.[BH4-].[Na+].OS([O-])(=O)=O.[Na+]>C1COCC1.CO>[CH3:1][O:2][C:3](=[O:17])[CH2:4][CH:5]([NH:9][C:10]([O:12][C:13]([CH3:15])([CH3:14])[CH3:16])=[O:11])[CH2:6][OH:7]To a solution of DL-2-tert-butoxycarbonylamino-succinic acid 4-methyl ester (5 g, 0.02 mol) in dry THF (100 mL) at −10° C. Et3N (3.1 mL, 0.022 mol) was added, followed by ethyl chloroformate (2.1 mL, 0.022 mol). After 10 min, NaBH4 (2.27 g, 0.06 mol) was added and then MeOH was dropped into the mixture over a period of 20 min at 0° C. The reaction mixture was stirred for 1 hour at 0° C. and for 2 hours at room temperature then neutralized with sat. aq. NaHSO4. The organic solvent was removed and the product was extracted with AcOEt three times. The combined organic phases were washed consecutively with sat. aq. NaHSO4, water, sat. aq. NaHCO3, water and dried over anh. Na2SO4. The solvent was evaporated and the residue was purified by flash chromatography (n-hexane/AcOEt 5:1), affording the title compound (1.63 g, 35%).
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