A THF solution (10 mL) of tert-butyl 2-oxopyrrolidine-1-carboxylate (500 mg, 2.70 mmol) was slowly added dropwise to a THF solution (5.4 mL, 5.4 mmol) of 1M lithium hexamethyldisilazane at -78 °C and the reaction mixture was stirred for 50 min at this temperature. Subsequently, a THF solution (5 mL) of ethyl chloroformate (0.27 mL, 2.84 mmol) was added dropwise to the above reaction solution at -78 °C. The reaction system was gradually warmed up to room temperature and stirring was continued for 4 hours. Upon completion of the reaction, the reaction solution was poured into saturated aqueous ammonium chloride solution and extracted with ethyl acetate. The organic phases were combined, washed with saturated brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography (eluent ratio: hexane/ethyl acetate=5:1 to 2:1) to afford 1-tert-butyl 3-ethyl 2-oxopyrrolidine-1,3-dicarboxylate (468 mg, 67% yield) as a liver-brown oily substance.1H-NMR (270 MHz, CDCl3) δ (ppm): 1.31 (3H, t, J=7.1 Hz) , 1.53 (9H, s), 2.16-2.46 (2H, m), 3.53 (1H, dd, J=7.4, 9.1 Hz), 3.66-3.75 (1H, m), 3.84-3.93 (1H, m), 4.24 (2H, dd, J=7.1, 14.3 Hz).ESI (LC-MS positive mode) m/z 258 (M +H).
