Chemoenzymatic synthesis of chiral carboxylic acids via nitriles

BACKGROUND: Enantiomerically pure 1,4‐benzodioxane‐2‐carboxylic acid derivatives are useful building blocks for the synthesis of pharmaceuticals and biologically active compounds whose interaction with their biological target (enzyme, receptor) depends very much on the absolute configuration of the chiral carbon at the 2‐position. The aim of the present work is to investigate the route to racemic nitriles and the subsequent selective enzymatic hydrolysis by nitrilase to optically active 1,4‐benzodioxane‐2‐carboxylic acid and 6‐formyl‐1,4‐benzodioxane‐2‐carboxylic acid. RESULTS: A range of microbial nitrilases from Rhodococcus, Alcaligenes and Pseudomonas strains have been prepared and screened for the desired biotransformations using a chiral high performance liquid chromatography (HPLC) analytical method. The nitrilase from Alcaligenes faecalis ATCC 8750 showed the highest and the nitrilase from Rhodococcus rhodochrous NCIMB 11216 the lowest activity towards 2‐cyano‐6‐formyl‐1,4‐benzodioxane. Lyophilised cells of Rhodococcus R 312 gave the (R)‐1,4‐benzodioxane‐2‐carboxylic acid with high enantioselectivity after 25% conversion. Excellent enantioselectivities for the hydrolysis of both 2‐cyano‐1,4‐benzodioxane as well as 2‐cyano‐6‐formyl‐1,4‐benzodioxane have been achieved and the absolute configuration of 1,4‐benzodioxane‐2‐carboxylic acid was determined to be R by comparison with the specific rotation of commercially available (R)‐1,4‐benzodioxane‐2‐carboxylic acid. CONCLUSIONS: This new nitrilase‐catalysed kinetic resolution of 2‐cyano‐ and 2‐cyano‐6‐formyl‐1,4‐benzodioxane opens a mild route to optically active 1,4‐benzodioxane‐2‐carboxylic acids. As the formyl functional group would be damaged in chemical nitrile hydrolysis, nitrilase‐catalysed hydrolysis solves this synthetic bottleneck and advances nitrilase biocatalytic tools for the preparation of more complex 1,4‐benzodioxane‐2‐carboxylic acids. Copyright © 2007 Society of Chemical Industry