General procedure for the synthesis of (S)-3-(((benzyloxy)carbonyl)amino)-2-((tert-butoxycarbonyl)amino)propionic acid from (R)-3-amino-2-((tert-butoxycarbonyl)amino)propionic acid and benzyl chloroformate: N2-(tert-butoxycarbonyl)-L-2,3-diaminopropionic acid (3.8 g, 0.018 mol, 1 eq.) was dissolved in a 10% sodium carbonate aqueous solution (2.2 eq.) and 1,4-dioxane (38 mL) in a solvent mixture. Benzyl chloroformate (3 mL, 0.020 mol, 1.1 eq.) was added dropwise to this solution at 25 °C and the reaction mixture was stirred at the same temperature for 3 hours. After completion of the reaction, the mixture was poured into water (100 mL) and washed with ether (100 mL). Subsequently, the pH was adjusted to 2 by adding 37% hydrochloric acid (6 mL) to the aqueous phase and extracted with ethyl acetate (100 mL). The organic layer was separated, washed with brine and dried over anhydrous sodium sulfate. The solvent was removed by concentration under reduced pressure to give a colorless oil, which was further dried under vacuum to give a white foamy product. The yield was 93% with a yield of 5.9 g. The product was detected by thin layer chromatography (silica gel, unfolding agent was dichloromethane/methanol/acetic acid, v/v/v) with Rf value and confirmed by 1H NMR (DMSO-d6) spectrum: δ 12.6 (1H, broad single peak); 7.35 (5H, multiple peaks); 6.94 (1H, double peak); 5.0 (2H, single peak); 4.1 (2H, multiple peaks); 1.4 (9H, single peak).
