A concise preparation of the non-proteinogenic amino acid l-kynurenine

A concise and practical preparation of the non-proteinogenic amino acid l-kynurenine is reported. The synthetic approach is scalable and provides ready access to this valuable amino acid in either l- or d-stereochemistry starting from l- or d-tryptophan, respectively. In the optimized procedure, two discreet oxidation steps are applied sequentially to convert the tryptophan indole ring into the keto-aniline moiety contained within the kynurenine side chain.