One-Pot Synthesis of d‑Halotryptophans by Dynamic Stereoinversion Using a Specific l‑Amino Acid Oxidase

Tryptophan (Trp) derivatives constitute important building blocks found in many natural products, peptides, and drugs. Accordingly, there is a high demand for suitable biocatalytic pathways providing selective derivatization of Trp like C–H functionalization or C^α-oxidation. The specific l-amino acid oxidase RebO from the actinomycete L. aerocolonigenes was harnessed for chemoenzymatic oxidation of substituted Trp derivatives. An array of potential substrates was tested, and several Trp derivatives are being accepted with reasonable turnover. The highest selectivity was observed for 7-halotryptophan being converted about 35-fold faster than nonsubstituted Trp. This selectivity is also useful for establishing a colorimetric halogenase high-throughput assay. RebO was also employed in dynamic stereoinversion in the presence of an ammonia–borane complex to provide access to non-native d-configured Trp analogues. Optimized reaction conditions yielded ∼70% d-amino acid with >98% ee for halotryptophans on an analytical scale. Dynamic stereoinversion preceded by enzymatic halogenation in a sequential one-pot reaction cascade provided d-configured 5- or 7-bromotryptophan with improved conversion of approximately 90%, >92% ee, and good isolated yields.