Z-Phe-OSU (N-Benzyloxycarbonyl-L-phenylalanine N-hydroxysuccinimide ester) is a white to off-white crystalline powder with the molecular formula C21H20N2O6 and a molecular weight of 396.40 g/mol. It is a key derivative of the amino acid L-phenylalanine, featuring a benzyloxycarbonyl (Z or Cbz) protecting group on the amine and an activated N-hydroxysuccinimide (OSu or NHS) ester on the carboxyl group. This compound is primarily used as a versatile reagent in peptide synthesis. The NHS ester moiety is highly reactive towards primary amines, enabling efficient and mild amide bond (peptide bond) formation under aqueous or organic conditions with minimal racemization. The Z-protecting group on the α-amino group is stable under the coupling conditions but can be readily removed by catalytic hydrogenation or acidolysis (e.g., using HBr in acetic acid). Therefore, Z-Phe-OSU serves as a crucial building block for introducing the phenylalanine residue into peptide chains during both solution-phase and solid-phase peptide synthesis (SPPS) strategies. Its application extends to the preparation of peptide-based pharmaceuticals, biochemical probes, and antigenic peptides for immunological studies. As an activated ester, it is also employed in bioconjugation chemistry for labeling proteins or other biomolecules. Proper handling is required as it may cause skin and eye irritation.