Urethane-protected N-carboxyanhydrides (UNCAs) as unique reactants for the study of intrinsic racemization tendencies...

We have established that urethane-protected N-carboxyanhydrides (UNCAs) are uniquely suited for the study of intrinsic racemization tendencies in peptide synthesis. The UNCA allows epimerization only by the direct enolization pathway (Proton abstraction from the alpha-carbon) and does not decompose upon epimerization. A protocol employing the quantitative separation and analysis of enantiomeric N-protected amino acid derivatives by chiral HPLC has been developed to measure the intrinsic rate of racemization of UNCAs under widely varying reaction conditions. The influence of the tertiary amine structure, UNCA side chain structure, and solvent were studied. The same protocol was employed to study the intrinsic rate of racemization of N-protected activated amino acid intermediates generated via 'onium-type' activating reagents. We have shown that the trends influencing the intrinsic rate of racemization of UNCAs are maintained under the conditions of in situ activations, and are consistent with the trends found in classical studies in the literature. The results are relevant to peptide synthesis both in solution and on solid phase. The intrinsic rate of racemization for any type of activation with any tertiary amine can be measured by this protocol.