(1) Carbonyldiimidazole (10.2 g, 63.1 mmol) was added to a suspension of 2-(tert-butoxycarbonylamino)acetic acid (10 g, 57.1 mmol) in tetrahydrofuran (100 mL) and the reaction was stirred at room temperature for 2 hours. Subsequently, magnesium chloride (5.4 g, 57.1 mmol) and monoethyl malonate potassium salt (9.7 g, 57.1 mmol) were sequentially added to the reaction system and stirring was continued for 1 h at 60 °C. The reaction was completed by filtration. After completion of the reaction, the insoluble material was removed by filtration, and the filtrate was diluted with ethyl acetate (200 mL), washed with 1N hydrochloric acid, saturated aqueous sodium bicarbonate and saturated aqueous sodium chloride in turn, dried over anhydrous sodium sulfate, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography [eluent: n-hexane-ethyl acetate (2:1)] to afford ethyl 4-(tert-butoxycarbonylamino)-3-oxobutanoate (11.9 g, 48.5 mmol, 85% yield) as a colorless oil.1H-NMR (CDCl3) δ: 1.29 (3H, t, J = 7.4 Hz), 1.45 (2H, s) , 3.87 (2H, s), 3.96 (3H, s), 4.21 (2H, q, J = 7.2 Hz), 7.08 (1H, d, J = 8.7 Hz), 7.39 (9H, s), 3.49 (2H, s), 4.14 (2H, d, J = 5.8 Hz), 4.21 (2H, q, J = 7.4 Hz), 5.15-5.24 (1H, br).
