A diketopiperazine (DKP), also known as a dioxopiperazine or piperazinedione, is a class of organic molecules in which the two nitrogen atoms of a piperazine 6-membered ring are part of amide linkages. There are three regioisomers possible, differing in the locations of the two carbonyl groups around the ring. While these isomers share a piperazine core, optimal strategies for their synthesis differ. The 2,3-diketopiperazines may be viewed as oxamides obtained from ethylenediamine. 2,5-Diketopiperazines are cyclodipeptides often obtainable via condensation of two α-amino acids. 2,6-Diketopiperazines may be viewed as cyclized imide derivatives derived from iminodiacetic acids. Of these three isomeric diketopiperazines, the 2,5-derivatives have attracted the greatest interest.