Bifunctional 2,5-Diketopiperazines as Rigid Three-Dimensional Scaffolds in Receptors and Peptidomimetics

2,5-Diketopiperazines (DKPs) are heterocyclic scaffolds easily accessible from alpha-amino acids and characterized by rather flat six-membered-ring cores that can be functionalized at four positions (N1, N4, C3, C6), two of which are stereogenic centers (C3, C6). In this microreview, the syntheses and the solution conformations of several DKP scaffolds are discussed with regard to their application as two-armed receptors for the selective recognition of small peptides and anions and as peptidomimetics mimicking topologically relevant elements of protein secondary structure (e.g., beta-turns, beta-hairpins, and alpha-helices). In many cases the properties of these compounds can be interpreted in terms of their conformationally defined three-dimensional structures, which can be assessed by spectroscopic and computational methods.