Late-stage modification pathways are undeniably valuable for incorporating diverse functionalities into peptide sequences to enable the construction of peptide-based natural products and serve as promising therapeutic leads.^[1] Despite the general appeal of electroorganic synthesis,^[2] electrochemistry is scarcely employed as a modification approach for peptides. Nonetheless, electro-organic methods, especially oxidation reactions, are highly desirable to avoid toxic reagents, minimize step count, and tune conditions by varying the oxidation potential. Reported methods that leverage the reactivity of compounds bearing amide and carbamate functionalities—relevant motifs for the modification of peptides—include the anodic oxidation of proline derivatives designed by Kim et al.^[3] and the anodic functionalization of polylactams, demonstrated by Aubé, Moeller, and coworkers.^[4]

Building off these technologies, we recently described an operationally simple electroauxiliary-assisted anodic functionalization of glutamine—an underexplored handle for site-specific peptide modification and a residue in which few orthogonal protecting group strategies are available. Leveraging judiciously designed glutamine N,S-acetal electroauxiliaries, we prepared a variety of electroactive peptide substrates using conventional Fmoc-SPPS and showcased mild and selective N,O-acetal formation in the presence of several functionalized side-chains. The tunable and sequential oxidation was also demonstrated using a pentapeptide bearing electronically-distinct 4-methoxythiophenyl and 4-nitrothiophenyl electroauxiliaries.^[5]

Despite its advantages for tunable oxidations, the 4-nitrothiophenyl group was incompatible with standard Fmoc deprotection conditions in iterative Fmoc-SPPS, limiting incorporation of this auxiliary exclusively onto the peptide N-terminus. Additionally, our original oxidation conditions were largely incompatible with unprotected Tyr, Trp, and Cys residues. We recently addressed these barriers by introducing Fmoc-SPPS compatible second-generation building blocks bearing the easily oxidizable 2,4-dimethoxythiophenyl and less oxidizable 2,4-dichlorothiophenyl electroauxiliaries, which successfully shifted the selectivity window toward lower oxidation potentials. The modified tunable oxidation method was exemplified by the sequential functionalization of a segment of involucrin—a transglutaminase substrate protein—bearing the newly designed electroauxiliaries.^[6]