Boc-D-Ala-OSu is a protected amino acid active ester widely used in peptide synthesis. It features the N-tert-butoxycarbonyl (Boc) protecting group on D-alanine, with the carboxylic acid activated as an N-hydroxysuccinimide (OSu) ester. This activation enables efficient and selective coupling to the amino group of another amino acid or peptide, forming amide bonds crucial for chain elongation. The Boc group can be orthogonally deprotected under acidic conditions without affecting the newly formed peptide bond. This reagent is particularly valuable in solid-phase peptide synthesis (SPPS) and solution-phase methods, especially for incorporating D-amino acids into peptides. It plays a key role in pharmaceutical research for developing peptide-based drugs and in biochemical applications for studying peptide structure, function, and modifications.