Amino acids derived from ornithine

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 produce a peptide library consisting of unnatural amino acids, three amino acids were prepared by acylation/sulfonation of the delta-amino function of ornithine and they were incorporated in a random sequence. Depending upon the acidity of the remaining proton on the delta-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(R) column and their identity was confirmed by mass spectrometry.