Fmoc-α-methyl-D-4-fluorothienylalanine, with CAS number 1217777-84-8, is a non-natural, fluorinated amino acid derivative. Its structure features a thiophene ring substituted with a fluorine atom at the 4-position, an α-methyl group (creating a quaternary center), and an Fmoc (9-fluorenylmethoxycarbonyl) protecting group on the amine functionality. This specific stereochemistry (D-configuration) and the α-methyl substitution make it a conformationally constrained building block. This compound is primarily employed in solid-phase peptide synthesis (SPPS) as a specialized amino acid building block. The incorporation of fluorinated, non-proteinogenic amino acids like this one is a key strategy in medicinal chemistry and peptide drug discovery. It is used to enhance the metabolic stability, modulate the lipophilicity, and improve the binding affinity of therapeutic peptides by introducing structural rigidity and altering electronic properties. The Fmoc group is standard for temporary N-terminal protection in Fmoc-based SPPS protocols. The fluorinated thiophene side chain contributes unique physicochemical properties, making it valuable for creating peptide analogs with tailored biological activities, such as protease inhibitors or receptor ligands. Its use is almost exclusively within research and development settings for novel peptide-based pharmaceuticals and chemical biology probes.