Chemo-enzymatic synthesis of enantiomerically pure (R)-2-naphthylmethoxyacetic acid

Enantiomerically pure (R)-2-naphthylmethoxyacetic acid (2-NMA) was synthesized from 2-naphthaldehyde via an integrated chemo-enzymatic procedure. The one-pot, successive use of SnBr₂-TMSCN and AcBr worked effectively to give a racemic cyanohydrin acetate. Lipase from Burkholderia cepacia then mediated the highly enantioselective hydrolysis of the (S)-enantiomer of the racemate, leaving the (R)-acetate with an e.e. of >99.9%. The resulting product of this enzyme-catalyzed hydrolysis, an (S)-cyanohydrin, spontaneously decomposed into naphthaldehyde, the starting material of this synthetic route, which could be recycled. The hydration of nitrile to amide as well as the hydrolysis of the acetate was performed with a microorganism, Rhodococcus rhodochrous, under very mild conditions without any loss of the enantiomeric purity. The amide group was hydrolyzed with nitrosylsulfuric acid, and the product was isolated as an α-hydroxy ester. The α-hydroxyl group was methylated with diazomethane-silica gel and the final task, hydrolysis of the ester, was accomplished under conditions as mild as neutral pH with an esterase from Krebsiella oxytoca to give enantiomerically pure 2-NMA.