Facile Preparation of optically Pure (3S)- and (3R)- 1, 2, 3, 4-Tetrahydroisoquinoline-3-carboxylic Acid

Abstract

Optically pure (3S)-1, 2, 3, 4-tetrahydroisoquinoline-3-carboxylic acid (1) was easily obtained by fractional crystallization of its benzyl ester (2b) p-toluenesulfonate, which was prepared from partially racemized 1 hydrochloride, followed by catalytic debenzylation. Similarly, (3R)-1 was prepared by the same procedure. The degree of racemization during the Pictet-Spengler reaction using optically active phenylalanine was determined by 1H-nuclear magnetic resonance (1H-NMR) spectroscopy of the corresponding methyl ester (2a), derived from the reaction product (1), in the presence of a chiral shift reagent.