Fmoc-D-Phe(4-CN)-OH is primarily used in peptide synthesis, particularly in solid-phase peptide synthesis (SPPS). It serves as a protected amino acid building block, where the Fmoc (9-fluorenylmethoxycarbonyl) group acts as a temporary protecting group for the amine functionality during the synthesis process. The 4-cyano (4-CN) substitution on the phenyl ring introduces unique electronic and steric properties, which can be leveraged to modify peptide structure and function. This compound is valuable in the development of peptides with specific biological activities, such as enzyme inhibitors or receptor ligands, where the incorporation of the cyano group can enhance binding affinity or stability. It is also utilized in the synthesis of peptidomimetics and other bioactive molecules for pharmaceutical research.