The use of copper complexes in the protection of lysine and tyrosine side chains has been extensively described. Based on this work and in order to produre a peptide library consisting of unnatural amino acids, three amino acids were prepared by acylation/sulfonation of the δ-amino function of ornithine and they were incorporated in a random sequence. Depending upon the acidity of the remaining proton on the δ-nitrogen, an intramolecular cyclization competed with the amide formation using standard coupling conditions (DIC/HOBt/DIEA). This side reaction could be suppressed by simple removal of the base from the condensation mixture. The obtained peptides were purified on a PLRP-s® column and their identity was confirmed by mass spectrometry.