Diketopiperazines (DKP) are a class of cyclic organic compounds that result from peptide bonds between two amino acids to form a lactam. They are combinations of ketones and the compound piperazine. A diketopiperazine was the first peptide to have its complete three-dimensional structure described, in work undertaken at Caltech by Robert Corey in the 1930s. Corey studied the cyclic anhydride of the dipeptide glycyl glycine.

Some diketopiperazines are considered carcinogenic.

Nitrogen heterocycles