(R)-4-Nitrophenyl 2-((tert-butoxycarbonyl)amino)-3-phenylpropanoate is a protected amino acid active ester derivative. It features a Boc-protected D-phenylalanine core coupled to a 4-nitrophenol leaving group. This compound is a white to off-white crystalline powder with a molecular formula of C20H22N2O6. This product is primarily used as a key building block in peptide synthesis, particularly for solid-phase peptide synthesis (SPPS) and solution-phase methods. The active ester (4-nitrophenyl ester) is highly reactive towards nucleophiles like amines, facilitating efficient amide bond formation to incorporate D-phenylalanine into peptide sequences. The tert-butoxycarbonyl (Boc) group serves as a standard amine protecting group that is stable under basic conditions and can be cleanly removed under acidic conditions, making it a crucial reagent for constructing peptides with specific stereochemistry and sequence. As a derivative of a non-natural D-amino acid, it is valuable for creating peptides with enhanced metabolic stability, altered biological activity, or specific structural properties for research in medicinal chemistry and biochemistry. It is a specialized reagent rather than a general-purpose synthesis block.