Acetyl-L-leucine methyl ester (methyl (2S)-2-acetamido-4-methylpentanoate) is an N-acetylated, C-terminally esterified derivative of the branched-chain amino acid L-leucine, with a molecular formula of C9H17NO3 and a molecular weight of 187.24. The structure incorporates an acetamide-protected α-amino group and a methyl esterified carboxyl group, yielding a neutral, non-zwitterionic amino acid derivative under physiological pH. Its defined stereochemistry at the α-carbon (2S configuration) preserves the chiral center characteristic of proteinogenic L-leucine, making it suitable for stereospecific studies. The combination of moderate polarity (TPSA 55.4) and ester functionality supports solubility and compatibility in common organic and mixed aqueous-organic systems used in analytical workflows.

This compound functions as a useful building block and model substrate in peptide chemistry, amino acid modification studies, and enzymatic reaction profiling. The N-acetyl and methyl ester protecting groups provide controlled reactivity, enabling selective transformations at the side chain or backbone under tailored conditions. Its compact structure and well-defined functional groups make it a convenient reference in chromatographic method development for protected amino acid derivatives. Researchers can also employ acetyl-L-leucine methyl ester in structure–activity investigations and in optimizing conditions for deprotection, coupling, and derivatization strategies in synthetic and analytical laboratories.