Also known as dioxopiperazines, piperazine-2,5-diones or DKPs. Diketopiperazines may occur as by-products during peptide synthesis or during the degradation of peptides. These cyclic dipeptides have been detected as taste-modulating compounds in food, they often show biological activity. DKPs are valuable chiral synthons, employed e.g. in Schöllkopf's versatile bislactim ether approach. They also have found use as catalysts for enantioselective synthesis, e.g. in the asymmetric Strecker reaction. See also the TRH metabolite cyclo(-His-Pro), G-1745, and cyclo(-Asp-Phe), G-1695, the major degradation product of aspartame.

Cyclo(His-Gly) inhibited the growth of MCF-7 cancer cells in vitro and showed antithrombotic effects. Cyclo-(His-Gly), which is related to anorexigenic peptide, was shown to inhibit release of catecholamine and norepinephrine in vitro significantly. The diketopiperazine could play a role in the hypothalamic modulation of appetite suppressant circuitry.