Boc-Leu-OSu is an activated N-Boc-protected L-leucine derivative, featuring a N-hydroxysuccinimide (OSu) ester on the carboxylic acid group. It is widely used in peptide synthesis for the efficient and selective coupling of the Boc-Leu residue to the amino terminus of another amino acid or growing peptide chain. This active ester enables amide bond formation under mild conditions with high yield and minimal side reactions, making it particularly valuable in solid-phase peptide synthesis (SPPS). It plays a crucial role in the synthesis of complex peptides and peptide-based therapeutics, supporting the development of pharmaceuticals with precise sequences for research and medical applications. Additionally, it finds use in organic synthesis for constructing biologically active molecules.