Methanol (50 mL) was cooled to -5°C in a thermostat bath and thionyl chloride (5 mL, 68.9 mmol) was added slowly and dropwise. Subsequently, 3,4-dihydroxy-L-phenylalanine (10.0 g, 50.7 mmol) was added in batches and the reaction mixture was stirred for 5 min after the addition was completed. The reaction system was gradually warmed up to room temperature and subsequently heated to 50 °C with continuous stirring for 14 hours. Upon completion of the reaction, the reaction solution was concentrated under reduced pressure to afford methyl (S)-2-amino-3-(3,4-dihydroxyphenyl)propionate hydrochloride (14.3 g, 57.7 mmol, quantitative yield), the product was in the form of oil.1H-NMR (400 MHz, CD3OD) δ: 3.04 (dd, 1H, J = 7.4, 14.5 Hz), 3.13 (dd, 1H, J = 5.8, 14.5 Hz), 3.13 (dd, 1H, J = 5.8, 14.5 Hz). J = 5.8, 14.5 Hz), 3.84 (s, 3H), 4.22-4.25 (m, 1H), 6.58 (dd, 1H, J = 2.2, 8.0 Hz), 6.69 (d, 1H, J = 2.1 Hz), 6.77 (d, 1H, J = 8.0 Hz). esims (m/z): 212.7 ([M + H]+). 423.2 ([2M + H]+), 210.2 ([M - H]-), 241.1 ([M + Cl]-).
