(Step 1) Synthesis of ethyl 3-(tert-butoxycarbonylamino)-3-methylbutanoate: ethyl 3-amino-3-methylbutanoate hydrochloride (10.5 g, 57.5 mmol) was dissolved in dichloromethane (100 mL). To this solution, triethylamine (8.19 mL, 5.95 g, 58.8 mmol) and di-tert-butyl dicarbonate (9.41 g, 43.1 mmol) were added sequentially and the reaction mixture was stirred at room temperature for 71.7 hours. After completion of the reaction, water was added to the mixture and extracted with dichloromethane. The organic layer was washed sequentially with water and saturated saline and dried over anhydrous sodium sulfate. The solvent was removed by distillation under reduced pressure and the resulting residue was purified by silica gel column chromatography (Biotage Japan Ltd., eluent: hexane/ethyl acetate = 98/2~50/50) to afford the title compound (10.4 g, yield: 73.7%) as an oil.1H-NMR (CDCl3) δ: 4.91 (1H, br s), 4.14 (2H. q, J = 7.1 Hz), 2.67 (2H, s), 1.43 (9H, s), 1.38 (6H, s), 1.26 (3H, t, J = 7.1 Hz).
