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.

Cain et al. obtained an array of small molecule peptide mimetics targeting the melanocortin receptors from this diketopiperazine. Brauns et al. demonstrated that cyclo(Phe-Pro) inhibits the growth of HT-29, MCF-7 and HeLa cells and induces apoptosis in HT-29 colon cancer cells.