N-[(9H-Fluoren-9-ylmethoxy)carbonyl]-4-[[[(4S)-hexahydro-2,6-dioxo-4-pyrimidinyl]carbonyl]amino]-L-phenylalanine, commonly referred to as Fmoc-Aph(Hor)-OH, is a non-proteinogenic, conformationally constrained amino acid derivative. Its structure features an Fmoc (9-fluorenylmethoxycarbonyl) protecting group on the amine, a central L-phenylalanine backbone, and a side chain modified with a (S)-hexahydro-2,6-dioxopyrimidine-4-carbonyl group (often termed a "hydroorotic acid" or Hor moiety). This Hor group introduces a rigid, cyclic urea-like structure. This compound is primarily used in solid-phase peptide synthesis (SPPS) as a specialized building block. The incorporation of the Hor-modified phenylalanine (Aph) into peptide sequences is designed to impose specific conformational restraints, which can enhance peptide stability against enzymatic degradation, improve receptor binding affinity, and modulate biological activity. It is a key intermediate in the research and development of peptide-based therapeutics, particularly for targeting protein-protein interactions where rigid structural motifs are advantageous. As a protected amino acid derivative, it is supplied for research use in peptide chemistry and medicinal chemistry. Handling requires standard precautions for laboratory chemicals, including the use of personal protective equipment. Its application is highly specialized within the field of peptide drug discovery and structural biology studies.