General procedure for the synthesis of D-p-hydroxyphenylglycine methyl ester hydrochloride from methanol and (R)-2-amino-2-(4-hydroxyphenyl)acetic acid: under nitrogen protection and -15 °C, thionyl chloride (1.5 mL, 33.0 mmol) was slowly added dropwise to anhydrous methanol (25 mL) in (R)-4-hydroxyphenylglycine (5.00 g, 29.91 mmol) ) suspension while stirring. The reaction mixture was gradually warmed to room temperature and stirred continuously overnight. Subsequently, the yellow solution was refluxed for 30 min. Upon completion of the reaction, the solvent was removed by concentration under reduced pressure to give D-p-hydroxyphenylglycine methyl ester hydrochloride in the form of an off-white solid. The hydrochloride was stirred in ether (15 mL), filtered, washed with ether (2 × 15 mL) and dried to give the final D-p-hydroxyphenylglycine methyl ester hydrochloride (6.50 g, 100%) in white solid form with a melting point of 189-191 °C; [α]20D = -120.9 (c 1 , 1M HCl), literature value [α]25D = -121.1 (c 1 , 1M HCl); IR spectrum (KBr) νmax/cm-1: 3339 (broad peak, OH), 1740 (C=O), 1595 (aryl ring); 1H NMR (300 MHz, DMSO-d6) δ: 3.68 (3H, s, OCH3), 5.11 (1H, s, CH), 6.80 (2H, d, J = 8.5 Hz. H), 7.25 (2H, d, J = 8.5 Hz, aryl H), 8.70 (3H, broad peak, NH3+), 9.86 (1H, s, OH).
