Review backbone N-modified peptides: How to meet the challenge of secondary amine acylation

Backbone N-substitution of peptides (N-Me and N-alkyl) has become of special interest as a chemical tool for peptide lead modification, either to improve biological activity or to optimize key pharmacokinetic characteristics. For the synthesis of backbone N-methylated peptides, many protocols have been developed already, yet some effort often has to be made to find appropriate conditions for the acylation of N-Me residues. Fewer examples are reported of peptides with other backbone N-substituents different than N-Me, and their synthesis is frequently reported as difficult. The synthesis of such peptides becomes more difficult as the size of the N-substituent increases. Coupling methods that work for the synthesis of N-methylated peptides were often found to fail when applied to peptides with larger N-substituents. This review addresses the challenges of the synthesis of backbone N-modified peptides, focusing on N-substituents larger than the N-Me group.