D,L-phenylalanine (50 g, 3.03 mol) was used as raw material to prepare its suspension. Concentrated hydrochloric acid (325 mL) and 37% formaldehyde solution (110 mL) were added to the suspension under vigorous stirring and heated to a mild reflux condition and maintained for 30 min. Subsequently, additional formaldehyde (50 mL) and concentrated hydrochloric acid (110 mL) were added and the reaction continued to heat for 4 hours. Upon completion of the reaction, the reaction mixture was cooled to room temperature and the solid product was collected by filtration. The solid was washed with methanol (30 mL) and dried to afford the target compound 3-carboxy-1,2,3,4-tetrahydroisoquinoline in a yield of 64.09 g (98.9% yield) with a melting point of 295-298 °C (decomposition temperature >326 °C) (Literature reference: Julian PL, J. Chem. Soc. 1948, 70, 180-183) as an enantiomeric mixtures. Its NMR hydrogen spectrum (DMSO-d6) data are as follows: δ 3.10-3.18 (1H, m, CH), 3.28-3.34 (1H, m, CH), 4.27 (2H, s, CH2), 4.39 (1H, m, CH), 7.25-7.27 (4H, m, 4*ArH), 9.89 (1H, brs, exchangeable, NH) , 10.06 (1H, brs, exchangeable, OH). Mass spectral data: m/z 177.0 (M+H)+.
