Oxime Ligation via in situ Oxidation of <i>N</i>‑Phenylglycinyl Peptides

Mild conditions for oxime ligations via in situ generation of α-imino amide intermediates are reported. The evaluation of a variety of <i>N</i>-terminal <i>N</i>-phenylglycine residues revealed that a metal-free, chemoselective oxidation was possible using oxygen as the only oxidant in buffer at pH 7.0. Moreover, selective unmasking of an inert residue by addition of potassium ferricyanide is demonstrated. These simple and mild conditions, which can be fine-tuned by the electronic properties of the <i>N</i>-phenylglycine residue, offer unique advantages over conventional approaches for oxime ligations