Synthesis Example 1: L-methionine (0.5 g, 1 eq.) was suspended in a 100 mL round-bottomed flask fitted with a magnetic stirrer, methanol (30 mL, concentration 0.1 M) was added and cooled to 0°C under nitrogen atmosphere. Thionyl chloride (0.5 mL, 2 equiv.) was added slowly and dropwise to the above suspension, followed by heating and refluxing the reaction for 16 hours. After completion of the reaction, the solvent was removed by rotary evaporator to give a light yellow solid. The solid was ground with hot ether and the ether solution was removed by filtration to give a white solid L-methionine methyl ester hydrochloride (0.65561 g, 98% yield). The product was further purified by vacuum drying.1H NMR (D2O, 500 MHz) δH: 4.24 (1H, m, CH), 3.79 (3H, s, OCH3), 2.63 (2H, t, CH2), 2.25 (1H, m, CH2), 2.16 (1H, m, CH2), 2.06 (1H, s, SCH3). 13C NMR (D2O, 125 MHz) δC: 170.58 (C=O), 53.64 (OCH3), 51.70 (CH), 28.71 (CH2), 28.41 (CH2), 13.85 (SCH3). IR (neat, vmax, cm-1): 2880.8/2676.2 (CH3, CH2, CH), 2016.2, 1742.2 (C=O, ester), 1483.6, 1443.5, 1227, 1194.7, 1149.8, 1079.5 (C-O). HRMS m/z (+ESI): C6H14NO2S, calculated mass = 164.074 (error = 0.07 ppm).
