(2R,4S)-4-((tert-Butoxycarbonyl)amino)pyrrolidine-2-carboxylic acid, also known as Boc-D-Pro(4-NH2)-OH, is a non-proteinogenic amino acid derivative. It features a pyrrolidine ring structure with both a carboxylic acid and a Boc-protected amine group at specific stereocenters (2R,4S). This compound is a chiral building block of high purity, commonly used in research and development. Its primary application is as a key intermediate in the synthesis of complex peptides and peptidomimetics, particularly in pharmaceutical research. The presence of the constrained pyrrolidine ring and the additional amino functionality makes it valuable for introducing specific conformational constraints and enhancing metabolic stability in drug candidates. It is frequently employed in the solid-phase peptide synthesis (SPPS) of bioactive peptides and as a component in the development of protease inhibitors and other therapeutic agents targeting various diseases. This building block is commercially available from fine chemical suppliers specializing in amino acids and peptide synthesis reagents. Proper handling requires standard laboratory safety precautions for chemical substances.