Heterocycles: Versatile control elements in bioactive macrocycles.

The potential of macrocyclic peptides as therapeutics has garnered much attention over the last several years. Unlike their linear counterparts, macrocycles have higher resistance to enzymatic degradation and often display improved bioavailability. However, macrocycles are typically not lipophilic enough for cellular membrane penetration, which prevents them from interacting with intracellular targets. Methods to increase cellular permeability have involved the incorporation of bicyclic scaffolds, d-amino acids and N-methylation of amides. These modifications exert their effect through conformational control of macrocycles and have been well studied in the literature. In contrast, the structural consequences of heterocycle incorporation into macrocyclic rings has not been as exhaustively investigated. In this mini-review we discuss key examples in which heterocycles influence the conformational stability and other properties of macrocycles.