Cyclo(-Phe-Ser) is a cyclic dipeptide, specifically a diketopiperazine (DKP), composed of phenylalanine (Phe) and serine (Ser) residues. Its IUPAC name is (3S,6S)-3-benzyl-6-(hydroxymethyl)piperazine-2,5-dione. This compound is a chiral molecule with a defined stereochemistry. This cyclic dipeptide serves as a key building block in peptide and medicinal chemistry. It is primarily used as a constrained scaffold or a privileged structure in the design and synthesis of novel bioactive compounds, including peptidomimetics and drug candidates. The DKP core is a common motif found in natural products and pharmaceuticals, contributing to enhanced metabolic stability and specific conformational properties. Its applications are centered in life science research and pharmaceutical development. As a specialized heterocyclic scaffold with a defined biological application context, it is categorized under life science reagents and pharmaceutical intermediates rather than as a general-purpose organic synthesis building block.