General procedure for the synthesis of BOC-4-oxo-L-proline methyl ester from N-Boc-trans-4-hydroxy-L-proline methyl ester:
(a) To a solution of methyl (2S,4R)-1-Boc-4-hydroxypyrrolidine-2-carboxylate (40 g, 157 mmol) in dichloromethane (DCM, 500 mL) was added Days-Martin oxidizer (DMP, 80 g, 189 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 4 h. Upon completion of the reaction, the reaction was quenched with sodium bisulfite (NaHSO3) solution. The reaction mixture was extracted with dichloromethane (DCM) and the organic phase was washed with aqueous sodium bicarbonate (NaHCO3) solution. Concentration of the organic phase afforded the title intermediate (2S)-1-tert-butoxycarbonyl-4-oxopyrrolidine-2-carboxylic acid methyl ester (33 g, 89% yield) as a colorless oil.
1H NMR (400 MHz, CDCl3): δ (ppm) 1.51 (s, 9H), 2.550-2.597 (d, 1H, J = 18.8Hz), 2.876-2.944 (t, 1H, J = 13.6Hz), 3.895 (s, 3H), 3.766-3.895 (m, 1H), 4.696- 4.814 (m, 1H).
