Practical Synthesis of N-Lactoyl-Phenylalanine and Its Isotopically Deuterated Variant Lac-Phe-d5

Abstract: N-Lactoyl-phenylalanine (Lac-Phe) is an emerging endogenous metabolite whose physiological importance continues to grow, particularly due to its anorexigenic activity. Its quantification in biological samples and the exploration of its structure-activity relationships will greatly benefit from the availability of an isotopically labeled standard. Reliable and reproducible access to both Lac-Phe and its deuterated form would be useful to support analytical studies and mechanistic research involving this metabolite. To prepare a deuterated variant, a practical synthesis of Lac-Phe was optimized and successfully applied to obtain the isotopically deuterated form Lac-Phe-d5. In parallel, a complementary synthesis of Lac-Phe inspired by solid-phase peptide synthesis (SPPS) was developed to enable flexible access to structural analogs of Lac-Phe. The developed Fmoc-SPPS strategy used a Wang resin preloaded with phenylalanine and lactic acid, whose hydroxyl group is protected by a THP group. The optimized method presented herein provides a straightforward and reproducible route to Lac-Phe-d₅, which is fully characterized, including measurement of its optical rotation. The SPPSinspired method enabled the efficient production of Lac-Phe with excellent yield and high purity, demonstrating its effectiveness for the preparation of other Lac-Phe analogues. Access to Lac-Phe-d₅ will facilitate quantitative analysis of Lac-Phe in biological matrices and support future analytical, metabolomic, and mechanistic investigations.