Step A: Synthesis of (S)-2-(((benzyloxy)carbonyl)amino)-3,3-dimethylbutanoic acid; to a methanolic (200 mL) suspension of L-tert-leucine (11.88 g, 90.7 mmol) was sequentially added triethylamine (26.56 mL, 190 mmol) and N-(benzyloxycarbonyloxy)succinimide (24.88 g, 99.8 mmol). The reaction mixture was stirred at room temperature for 14 hours. After completion of the reaction, methanol was removed by rotary evaporator under reduced pressure to give a viscous light yellow oil. The oily substance was dissolved in ethyl acetate (100 mL) and the organic layer was washed sequentially with 1 M hydrochloric acid (15 mL) and saturated saline. The organic layer was dried over anhydrous magnesium sulfate and filtered, and the filtrate was concentrated under reduced pressure by rotary evaporator to afford the title compound (S)-2-(((benzyloxy)carbonyl)amino)-3,3-dimethylbutanoic acid as an oil (24 g, quantitative yield).1H-NMR (CDCl3) δ: 7.43-7.36 (5H, m), 5.36 (1H, d, J = 9.4 Hz) , 5.12 (2H, br s), 4.20 (1H, d, J = 9.6 Hz), 1.02 (9H, s).LRMS: m/z 266 [M + H]+, 264 [M - H]-, 529 [2M - H]-.
