[C:1]([O:5][C:6]([NH:8][C@@H:9]([CH2:13][C:14]1[CH:19]=[CH:18][C:17]([N+:20]([O-:22])=[O:21])=[CH:16][CH:15]=1)[C:10](O)=[O:11])=[O:7])([CH3:4])([CH3:3])[CH3:2].C[N:24]1CCOCC1.ClC(OCC(C)C)=O.N>CN(C=O)C>[NH2:24][C:10](=[O:11])[C@@H:9]([NH:8][C:6](=[O:7])[O:5][C:1]([CH3:4])([CH3:3])[CH3:2])[CH2:13][C:14]1[CH:19]=[CH:18][C:17]([N+:20]([O-:22])=[O:21])=[CH:16][CH:15]=1To a 0° C. solution of 2-(S)-tert-butoxycarbonylamino-3-(4-nitrophenyl)-propionic acid and N-methylmorpholine (1.1 mL, 9.65 mmol) in DMF (10 mL) is added dropwise iso-butyl chloroformate (1.25 mL, 9.65 mmol). The mixture is stirred at 0° C. for 20 minutes after which NH3 (g) is passed through the reaction mixture for 30 minutes at 0° C. The reaction mixture is concentrated and the residue dissolved in EtOAc, washed successively with 5% citric acid, water, 5% NaHCO3, water and brine, dried (Na2SO4), filtered and concentrated in vacuo to a residue that is triturated with a mixture of EtOAc/petroleum ether to provide 2.2 g (74% yield) of the desired product as a white solid.
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