(S)-3-(4-Acetylphenyl)-2-aminopropanoic acid hydrochloride, also known as 4-Acetyl-L-phenylalanine hydrochloride, is a chiral amino acid derivative. It is a white to off-white crystalline powder with the molecular formula C11H13NO3·HCl. The compound features an L-configuration at the α-carbon and contains a 4-acetylphenyl group attached to the β-position of the alanine backbone. This compound serves as a crucial chiral building block in pharmaceutical synthesis. Its primary application is as an intermediate in the production of peptide-based drugs and other bioactive molecules where the incorporation of a modified phenylalanine residue is required. The acetyl group on the phenyl ring provides a handle for further chemical modifications, such as reduction to an alcohol or conversion to other functional groups, enabling the synthesis of diverse drug candidates. It is particularly valuable in medicinal chemistry for structure-activity relationship (SAR) studies and the development of protease inhibitors or receptor ligands. As a protected and functionalized derivative of L-phenylalanine, it is used in solid-phase peptide synthesis (SPPS) and other bioconjugation strategies. The hydrochloride salt form enhances its stability and solubility in various reaction solvents. Handling requires standard laboratory precautions for amino acid derivatives and hydrochloride salts.