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. The reaction mixture was concentrated 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 x 15 mL) and dried to give the final D-p-hydroxyphenylglycine methyl ester hydrochloride (6.50 g, 100%) as a white solid with a melting point of 189-191 °C; specific optical rotation [α]20D = -120.9 (c 1, 1M HCl), literature value [α]25D = - 121.1 (c 1, 1M HCl); IR spectra (KBr) νmax/cm-1: 3339 (broad peak), 1740 (C=O), 1595; NMR hydrogen spectrum (300 MHz, DMSO-d6) δH: 3.68 (3H, single peak, OCH3), 5.11 (1H, single peak, C(2)H), 6.80 (2H, double peak. J=8.5 Hz, aromatic H), 7.25 (2H, double peak, J=8.5 Hz, aromatic H), 8.70 (3H, broad peak, NH3), 9.86 (1H, single peak, OH).
