Used in solid-phase peptide synthesis as a protected amino acid building block, particularly for incorporating a sterically hindered aspartic acid derivative. The Fmoc group allows for base-labile protection compatible with standard Fmoc-strategy, while the tert-butyl ester (OtBu) protects the side-chain carboxylic acid, enabling selective deprotection under mild acidic conditions. The (S)-configuration ensures stereochemical control in the synthesis of complex peptides. Commonly applied in the preparation of bioactive peptides and peptidomimetics where precise side-chain functionality and orthogonal protection are required.