Amide‐Forming Ligation Reactions

One of the long‐standing pursuits of synthetic organic chemists is the development of chemical methods for the synthesis of peptides and proteins as tools for understanding biological processes and providing therapeutic solutions to human diseases. The advent of chemoselective ligation reactions for the chemical synthesis of peptides and proteins has enabled synthetic chemists to realize this long‐held dream. In an amide‐forming ligation reaction, two uniquely placed functional groups within the peptide allow peptide or protein segments to be joined in a chemoselective manner with an amide bond. In this chapter, some of the early methods based on principles of the capture/rearrangement strategy for ligation of peptides are catalogued, followed by some of the most versatile and well‐established contemporary methods for the preparation of linear/cyclic peptides and proteins such as the native chemical ligation (NCL) and the α‐ketoacid–hydroxylamine (KAHA) ligation. The chapter concludes with some of the most promising new generation ligation methods such as the potassium acyltrifluoroborate–hydroxylamine (KAT) ligation. The tremendous progress in the past two decades in the development of several ligation methods that rely on a pair of unique functional groups is set to expand the horizons of fully functional proteins and multi‐protein assemblies that are purely synthetically prepared. Although contemporary ligation reactions do not match the speed and efficiency at which proteins are assembled in biological systems, the repertoire of chemoselective amide‐forming ligation methods has already enabled synthetic protein chemistry to surpass biology in terms of chemical and structural diversity.