Chemoenzymatic preparation of the enantiomers of β-tryptophan ethyl ester and the β-amino nitrile analogue

Racemic α-tryptophan was chemoselectively transformed into the enantiomers of β-tryptophan ethyl ester. The key step in achieving enantiopurity was the N-acylation of the 3-amino-4-(3-indolyl)butanenitrile intermediate with Candida antarctica lipase A (CAL-A). The enzymatic N-acylation of racemic β-tryptophan ethyl ester was also studied. CAL-A was highly (R)-enantioselective in the present kinetic resolutions, leading to a mixture of the butanamide product with an (R)-configuration and the unreacted starting material with an (S)-configuration at 50% conversion.