General procedure for the synthesis of benzyl (S)-4-amino-2-((tert-butoxycarbonyl)amino)-4-oxobutanoate from benzyl bromide: proceeding with reference to BIOORG. MED. CHEM., 6 (1998) 1185-1208. N-[(1,1-dimethylethoxycarbonyl)amino]-L-aspartic acid (20.7 g, 89.1 mmol, 1 eq.) was dissolved in methanol (500 mL) and cesium carbonate (15.97 g, 49 mmol, 0.55 eq.) was added. After the reaction was complete, the solvent was evaporated to give a white solid. The solid was dissolved in N,N-dimethylformamide (200 mL) to form a suspension. Benzyl bromide (11.6 mL, 98 mmol, 1.1 eq.) was added slowly and dropwise to the suspension and the mixture was stirred at room temperature overnight. At the end of the reaction, the solvent was evaporated under reduced pressure, diluted with water (300 mL) and the mixture was extracted with ethyl acetate (200 mL). The organic phase was washed with brine (50 mL) and the solvent was removed under reduced pressure to give the crude product. The crude product was suspended in n-hexane (160 mL), filtered and dried under vacuum to give 14.68 g of white solid in 51% yield. Product characterization data: melting point 113°C-115°C. 1H NMR (DMSO-d6) δ: 7.35 (6H, m); 7.13 (1H, d, J = 7.9 Hz); 6.94 (1H, br s); 5.10 (2H, s); 4.39 (1H, q, J = 7.4 Hz); 2.6-2.4 (2H, m); 2.03 ( 2H, t, J = 7.3 Hz); 1.37 (9H, s).
