Emerging methods in amide- and peptide-bond formation.

Amide bonds are the key chemical connections in proteins and peptides and have a long-established presence in small-molecule therapeutics. Despite their recognized importance, amide bond synthesis relies almost solely on variants of a single reaction: the dehydrative coupling of carboxylic acids and amines. While phenomenally successful in many applications, this process requires superstoichiometric quantities of expensive reagents and precludes chemoselective ligation reactions. This review summarizes emerging chemical alternatives to amide-bond formation with particular emphasis on the development of ligation reactions that permit the coupling of unprotected peptide fragments.