Late-Stage Modification of Halotryptophan-Containing Peptides Via Negishi Cross-Coupling

Late-stage functionalization of peptides provides powerful opportunities for the rapid diversification of bioactive sequences. Among the available strategies, Negishi cross-coupling represents a versatile and orthogonal method for the installation of alkyl substituents under mild conditions. We describe the applicability of Negishi cross-coupling to increasingly complex bromotryptophan-containing peptides. While conditions effective for bromo-l-tryptophan proved unsuitable for peptide substrates, leading to incomplete conversion and pronounced debromination, systematic optimization enabled efficient cross-coupling of the model tripeptide Ac-Ala-Trp(6-Br)-Gly-NH₂. The optimized protocol afforded good to excellent conversions and isolated yields while maintaining operational simplicity. A key limitation arising from peptide solubility was addressed through the yet underexplored use of DMSO and DMSO/DMF solvent systems in Negishi cross-couplings, which enabled efficient reaction of peptides bearing either 6- or 7-bromo-l-tryptophan as well as unprotected residues such as aspartic acid and serine. Application to more complex pentapeptides revealed solvent-dependent reactivity: poor solubility in DMF resulted in reduced conversion and increased debromination, whereas DMSO-containing solvent systems significantly improved both solubility and catalytic performance. These results demonstrate that Negishi cross-coupling can be adapted for the late-stage functionalization of bromotryptophan-containing peptides with functional group tolerance in structurally complex peptide substrates.