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Step 1: Synthesis of DL-proline methyl ester hydrochloride Thionyl chloride (15.7 mL, 217.4 mmol) was slowly added dropwise to a mixed solution containing D/L-proline (5.0 g, 43.47 mmol) and methanol (50 mL) at 0 °C, and the dropwise process lasted for about 15 minutes. The reaction mixture was stirred at room temperature for about 16 hours. Subsequently, the solvent was removed by vacuum concentration to give a colloidal product. The colloidal product was ground with n-pentane, decanted and dried to give the final DL-proline methyl ester hydrochloride as an off-white solid (6.25 g, 87% yield). The product characterization data are as follows: 1H NMR (400 MHz, DMSO-d6) δ 1.87-2.04 (m, 3H), 2.21-2.28 (m, 1H), 3.22-3.26 (m, 2H), 3.75 (s, 3H), 4.34 (t, J = 7.8 Hz, 1H);. IR (film) υ 3513, 3130, 2597, 1740, 1632, 1452, 1402, 1245, 1178, 1046, 764 cm-1; MS 130 (M + 1).