D-Azetidine-2-carboxylic acid (D-Aze) is a non-proteinogenic amino acid characterized by a four-membered azetidine ring. It is a structural analog of proline, where the five-membered pyrrolidine ring is replaced by a smaller, more strained four-membered ring. This unique structure imparts distinct conformational and biochemical properties. This compound is primarily used as a proline mimetic in peptide and protein research. Its incorporation into peptides can significantly alter backbone conformation, stability, and biological activity, making it a valuable tool for studying protein folding, enzyme inhibition, and designing peptidomimetic drugs. It serves as a key building block in medicinal chemistry for the development of novel therapeutic agents, particularly in areas targeting proline-rich recognition motifs. D-Azetidine-2-carboxylic acid is also found in some plants (e.g., *Convallaria majalis*, Lily of the Valley) and can act as a natural toxin due to its misincorporation into proteins in place of proline, disrupting normal cellular function. In synthetic chemistry, it is an important chiral synthon for the construction of complex nitrogen-containing heterocycles and constrained peptides.