30.0 g (232 mmol) of L-pyroglutamic acid was suspended in 100 mL of anhydrous ethanol and 20 mL (274 mmol, 1.2 eq.) of SOCl2 was slowly added dropwise at 0 °C. After the dropwise addition, the reaction mixture was stirred at 0 °C for 15 h. The reaction mixture was then allowed to warm up naturally to room temperature, resulting in a clarified solution. After completion of the reaction, all volatile components were removed under reduced pressure. The residue was dissolved in 500 mL of ethyl acetate and treated sequentially with K2CO3 and MgSO4 with stirring, removing the desiccant by filtration after each treatment. The solution was filtered through a small amount of silica gel and concentrated under reduced pressure to give 36.0 g (230 mmol, 99% yield) of the target product, ethyl (S)-5-oxopyrrolidine-2-carboxylate, in the form of a yellowish viscous oil. 2.22-2.48 (m, 3H, H-3α, H-4), 2.07-2.22 (m, 1H, H-3β), 1.22 (t, 3H, J = 7.1 Hz, ester CH3).13C-NMR (75 MHz, CDCl3): δ (ppm) = 178.37 (lactam CO), 172.10 (ester CO), 61.58 ( ester CH2), 55.59 (CH), 29.33 (CH2, C-4), 24.77 (CH2, C-3), 14.11 (ester CH3).
