Ethoxyethylidene protecting group prevents N-overacylation in aminooxy peptide synthesis

We report herein an improved synthetic route for the preparation of homogenous aminooxy peptides suitable for oxime ligation. Aminooxyacetic acid (Aoa) was protected with 1-ethoxyethylidene group (Eei) then incorporated either using PyBOP or as N-hydroxysuccinimidyl ester at N-terminal end or at a lysine side chain into model peptides in solution and on solid support. Due to the Eei protecting group, these new reagents prevent the N-overacylation side reaction in comparison with Boc-Aoa derivatives. Subsequent deprotection under mild acidic conditions gave the corresponding pure aminooxylated peptides.