The general procedure for the synthesis of benzyl (S)-(1-amino-3-methyl-1-oxobutan-2-yl)carbamate from CBZ-L-valine was as follows: to a solution of compound I (10 g, 39.84 mmol, 1 eq.) in DMF (100 mL) at 0 °C was added sequentially DIPEA (19.7 mL, 119.5 mmol, 3 eq. ) and HATU (18.17 g, 47.8 mmol, 1.2 eq.). The reaction mixture was stirred for 15 minutes. Subsequently, ammonium chloride (10.7 g, 199.2 mmol, 5 eq.) was added to the mixture and the reaction mixture was continued to be stirred at 23 °C for 16 hours. Upon completion of the reaction, the mixture was poured into ice-cold water (500 mL) and the organic components were extracted with EtOAc (3 x 500 mL). The combined organic layers were washed sequentially with aqueous ammonium chloride and brine, then dried over anhydrous sodium sulfate, filtered and concentrated to dryness. The crude product was purified by ethanol recrystallization to afford the target product (S)-(1-amino-3-methyl-1-oxobutan-2-yl)carbamic acid benzyl ester (9.8 g, 98% yield) as an off-white solid. The product was confirmed by 1H NMR (400 MHz, DMSO-d6): δ 7.35 (m, 6H), 7.12 (d, 1H, J = 9Hz), 7.01 (s, 1H), 5.03 (s, 2H), 3.80 (t, 1H, J = 8Hz), 1.95 (m, 1H), 0.85 (m, 6H).LCMS analysis: m/z = 251.2 [M + H], retention time RT = 2.81 min (program P1, column Y).
