Z-D-Leu-onp is a protected and activated derivative of D-leucine, featuring a benzyloxycarbonyl (Z) group on the amino function and a p-nitrophenyl (ONp) ester on the carboxyl group. It serves as a key building block in peptide synthesis, particularly for incorporating D-leucine residues. The active ester facilitates efficient coupling reactions, while the Z-protection prevents unwanted side reactions during peptide bond formation. It is commonly used in solid-phase peptide synthesis (SPPS) to enable selective deprotection and high-yield coupling, ensuring the production of complex peptides with high purity and stereochemical specificity.