Unconventional Synthetic Approaches to Unusual Peptide Derivatives.

Peptides that contain unusual motifs, such as non-proteinogenic amino acids (AAs) and/or macrocyclic substructures, have recently attracted great attention as a new modality in medium-sized-molecule drug discovery. Therefore, it is highly important to develop methods for the chemical synthesis of a wide variety of such unusual peptide derivatives, which are often difficult to prepare via conventional synthetic approaches. In this review, the development of unconventional approaches for the synthesis of unusual peptide derivatives is discussed. Specifically, a novel external-oxidant-mediated decarboxylative condensation of α-ketoacids that can be applied to the synthesis of a wide variety of unusual peptide derivatives is reported. Moreover, an organocatalytic asymmetric Mannich-type addition is discussed that provides chiral β-amino-α-ketoacids, which are required as starting materials for the decarboxylative condensation. In this reaction, the adducts corresponding to various unusual AA side chains are obtained in high yield and excellent stereoselectivity. Furthermore, the "N-chloropeptide strategy" is proposed as a new method for the chemical modification of peptides without the need for a reactive AA residue.