N-Fmoc-O-formyl-D-aspartic acid is a protected derivative of the non-proteinogenic amino acid D-aspartic acid. Its structure features an N-terminal 9-fluorenylmethoxycarbonyl (Fmoc) protecting group and an O-formyl (formate ester) protecting group on the side-chain carboxylic acid. This specific protection scheme renders the side-chain carboxylate inert during standard Fmoc-based solid-phase peptide synthesis (SPPS) conditions, while the α-carboxyl group remains free for coupling. This compound is a specialized, high-purity building block primarily used in the synthesis of complex peptides and peptidomimetics. The O-formyl protecting group on the aspartic acid side chain is stable under the basic conditions used for Fmoc deprotection (e.g., piperidine) but can be selectively removed under mild acidic conditions or via hydrazinolysis. This orthogonal protection strategy is crucial for constructing peptides containing aspartic acid where selective side-chain modification or to prevent aspartimide formation is required. As a chiral, protected D-amino acid derivative, it is a key reagent in medicinal chemistry and chemical biology research for introducing D-aspartate residues into peptide sequences. These sequences are investigated for their potential in developing novel therapeutics, enzyme inhibitors, and probes with enhanced metabolic stability or altered biological activity compared to their L-amino acid counterparts.