Solid‐Phase Synthesis of Macrocyclic Peptides via Side‐Chain Anchoring of the Ornithine δ‐Amine

Cyclic peptides represent a popular class of macrocyclic drug candidates and therefore their solid phase synthesis has attracted much attention. In this contribution we present an efficient method of side‐chain anchoring for ornithine and lysine residues to be used in the standard Fmoc‐based synthesis of cyclic peptides via on‐resin cyclization. We demonstrate that the side chain of ornithine and lysine protected with N‐Boc‐group can efficiently be converted to the isocyanate which is then immobilized on Wang‐type resin in almost quantitative yield. We further show the synthesis of four biologically active cyclic peptides employing the side chain ornithine anchoring. Our method is at least on a par with the previously reported methodologies in terms of yield and the purity of the final products and is arguably operationally more straightforward.