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(-Pro-Val) and other Pro-containing diketopiperazines were detected in roasted coffee. The amount of these compounds increases with the intensity of roasting, they contribute to the bitterness of taste. The bitter-tasting cyclodipeptide has also been detected in other foodstuffs as cocoa, chocolate, beef, and cheese. Cyclo(-Pro-Val), which shows antifungal activity, has also been isolated from microorganisms and further natural sources.