General procedure for the synthesis of (R)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid from formaldehyde and D-phenylalanine:
1. Preparation of (3R)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid: D-phenylalanine (50 g, 0.3 mol), concentrated hydrochloric acid (386 mL), and 37% wt formalin (113.7 mL) were mixed, and the reaction was carried out for 4 h at 95 °C with vigorous stirring. Upon completion of the reaction, the mixture was cooled to room temperature and stirring was continued for 2 hours. The reaction mixture was filtered and the precipitate was washed with cold water to afford (3R)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (25 g, 42% yield) in white solid form.
2. Esterification reaction: benzyl alcohol (64 g) and p-toluenesulfonic acid (26.9 g) were added to the above product, and the reaction was carried out under reflux in a Dean-Stark apparatus with benzene (400 mL) as solvent. After completion of the reaction, the solvent was removed in vacuum and the crude product was ground with ether to obtain a solid, which was subsequently recrystallized with water and methanol to give (R)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (28 g, 35% yield) in white solid form.
