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.

As its linear analog, N-1070, cyclo(-Gly-Gln) is a biologically active peptide. The diketopiperazine is capable of reversing the cardiorespiratory depression produced by β-endorphin or morphine.