Chemoenzymatic Approach to (S)-1,2,3,4-Tetrahydroisoquinoline Carboxylic Acids Employing D-Amino Acid Oxidase

Optically pure 1,2,3,4-tetrahydroisoquinoline carboxylic acids constitute an important class of building blocks for the synthesis of natural products and synthetic pharmaceuticals. However, redox deracemization of racemic 1,2,3,4-tetrahydroisoquinoline carboxylic acids as an attractive method is still challenging for the lack of suitable oxidoreductases. Herein, a D-amino acid oxidase from Fusarium solani M-0718 ( Fs DAAO) with broad substrate scope and excellent enantioselectivity was exploited through genome mining, and applied for the kinetic resolution of a number of racemic 1- and 3-carboxyl substituted tetrahydroisoquinolines to yield the corresponding ( S )-enantiomers with excellent enantiomeric excess ( ee ) values (up to >99%). By using Fs DAAO in combination with ammonia-borane in one pot, deracemization of these racemic carboxyl-substituted tetrahydroisoquinolines was achieved with conversions up to >98% and >99% ee . Preparative-scale deracemization of racemic 1,2,3,4-tetrahydroisoquinoline-1-carboxylic acid and 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid was also demonstrated with good isolated yields (82% and 73%, respectively) and ee >99%. Our study provides an effective method for the synthesis of enantiomeric pure 1,2,3,4-tetrahydroisoquinoline carboxylic acids. This method is expected to provide access to chiral carboxyl-substituted 1,2,3,4-tetrahydroquinolines and 1,2,3,4-tetrahydro-ß-carbolines.