Formaldehyde → (3S)-6,7-Dihydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid

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(S)-2-Amino-3-(3,4-dihydroxy-phenyl)-propionic acid (90 g) was suspended in 0.8 L water, then 18.2 mL concentrated sulfuric acid in 0.5 L additional water was added with stirring. Formaldehyde solution (136 mL, 37 wt %) was then added while stirring and the mixture was stirred 18 hours at rt. It was quenched by addition of 24.4 g NaOH in 0.5 L water, dropwise over 1 h while cooling. The resulting mixture was stored overnight at 4° C., and the solid that formed was collected by filtration, washed with hot water (70° C.), cooled again and collected by filtration. The solid was treated with 0.5 L MeOH and evaporated, then stored under vacuum overnight to give 65 g desired product. 1H NMR (400 MHz, MeOH-d4): 6.64 (s, 1H), 6.59 (s, 1H), 4.29-4.21 (m, 3H), 3.29-3.21 (m, 1H), 3.11-3.01 (m, 1H).

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