Squaric esters as peptide stapling reagents.

We report that squaric esters can serve as bifunctional reagents for selective peptide stapling reactions. Formation of the squaric amide staple occurs under mild conditions with amine-containing side chains. We show that short resin-bound peptides are readily stapled on solid phase and that stapling can occur at various relative positions along the peptide and with various amine tether lengths (e.