Boc-Asp(OtBu)-OH, also known as N-(tert-Butoxycarbonyl)-L-aspartic acid 4-tert-butyl ester, is a protected derivative of the natural amino acid L-aspartic acid. It features two orthogonal protecting groups: the acid-labile tert-butoxycarbonyl (Boc) group on the α-amino function and the base-labile tert-butyl (OtBu) ester on the side-chain β-carboxylic acid. This white to off-white crystalline powder is a fundamental building block in solid-phase peptide synthesis (SPPS), particularly in Boc-based strategies. Its primary and most significant application is in the pharmaceutical and life sciences research for the synthesis of peptides and peptidomimetics. The protected form prevents unwanted side reactions during the stepwise assembly of peptide chains. After incorporation into the growing peptide, the Boc group can be selectively removed with acids like trifluoroacetic acid (TFA), while the OtBu ester remains stable, allowing for selective deprotection later in the synthesis sequence. This compound is crucial for producing peptides containing aspartic acid residues for drug discovery, biochemical studies, and the development of therapeutic agents. As a high-purity, specialty chemical, it is a staple reagent in bioconjugation and the preparation of complex biomolecules. It is handled as a non-hazardous substance under standard conditions but requires storage in a cool, dry place.