Boc-L-aspartic acid 4-tert-butyl 1-hydroxy succinimide ester is a protected amino acid derivative. It features a Boc (tert-butoxycarbonyl) group protecting the alpha-amino group and a tert-butyl ester protecting the side-chain beta-carboxylic acid of aspartic acid. The activated 1-hydroxy succinimide (HOSu) ester is attached to the alpha-carboxylic acid, making it a highly reactive acylating agent. This compound is a crucial building block in solid-phase peptide synthesis (SPPS) and solution-phase peptide chemistry. The HOSu ester is a common active ester used for amide bond formation, offering advantages such as reduced racemization during coupling compared to some other methods. The tert-butyl protection on the aspartic acid side chain is orthogonal to the Boc group on the N-terminus and is stable under acidic conditions used for Boc deprotection but can be cleaved under strong acidic conditions (e.g., trifluoroacetic acid). Its primary application is in the synthesis of peptides containing aspartic acid residues, particularly for research and development of peptide-based pharmaceuticals, diagnostic agents, and biochemical tools. Proper handling is required as it is moisture-sensitive and should be stored under inert conditions.