To a solution of (S)-1-BOC-pyrrolidine-3-carboxylic acid (863 mg, 4.01 mmol) and K2CO3 (829 mg, 6.00 mmol) in anhydrous DMF (8 mL) was added iodomethane (300 μL, 4.82 mmol). The reaction mixture was stirred at room temperature for 4 hours. After completion of the reaction, water was added and extracted three times with ether (Et2O). The organic phases were combined, dried with anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure. The residue was purified by fast column chromatography (eluent: n-hexane/ethyl acetate = 3:1) to afford the target product (S)-1-Boc-3-carboxypyrrolidinium methyl ester (916 mg, 100%) as a colorless oil. The product was confirmed by 1H NMR (400 MHz, CDCl3): δ 3.70 (3H, s), 3.65-3.28 (4H, m), 3.10-2.98 (1H, m), 2.18-2.05 (2H, m), 1.45 (9H, s). lRMS (ESI, [M + Na]+) showed the molecular ion peak as 252.1.
