[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.C(OC(Cl)=O)C(C)C.[BH4-].[Na+]>COCCOC>[CH2:1]([O:8][C:9](=[O:23])[CH2:10][C@H:11]([NH:12][C:13]([O:15][C:16]([CH3:18])([CH3:17])[CH3:19])=[O:14])[CH2:20][OH:21])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1To a solution of Boc-L-aspartic acid-4-benzyl ester (100 g, 309 mmol) in DME (1.8 L) at −20° C. was added NMM (34 mL, 309 mmol) then dropwise iso-butylchloroformate (40 mL, 309 mmol) and the mixture was stirred at −20° C. for 20 min. The reaction mixture was filtered and the filtrate was cooled to −20° C. NaBH4 (17.5 g, 463 mmol) was added portionwise at −20° C. The mixture was allowed to warm and stirred at rt for 1 h. The mixture was quenched with a 20% citric acid solution and then extracted with AcOEt. The organic layer was washed with NaHCO3 solution, H2O and brine, dried (MgSO4), filtered and concentrated. The product was used without further purification for the next step.
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