Enhanced Stereochemical Analysis of β‑Diastereomeric Amino Acids with Variants of Marfey's Reagent

The analysis of mixtures of amino acid stereoisomers is a classic challenge in bioorganic chemistry and organic synthesis. Chemical derivatization with Marfey's reagent, 1-fluoro-2,4-dinitrophenyl-5-l-alanine amide, is frequently employed as chromatographic separation of the resulting diastereomers enables convenient assignment of d/l-configuration and determination of enantiomeric ratio. However, it is often challenging to resolve Cβ-epimeric diastereomers of noncanonical amino acids after treating with Marfey's reagent. This report describes the effectiveness of alternative chiral derivatization reagents. We demonstrate that the Cβ epimers of l-phenylserine and other β-hydroxy noncanonical l-amino acids are more easily separated when the traditional Marfey's reagent is substituted by a proline analogue or the d-enantiomers of several other Marfey's reagents. Additionally, in many cases achiral Sanger's reagent, 1-fluoro-2,4-dinitrobenzene, was found to produce the largest separation of β-diastereomers. The "mixed Marfey's reaction" protocol is established, which allows for screening multiple Marfey's reagents within a single reaction. This method was applied to the challenging case of resolving the methyl-ethyl Cβ stereocenter of l-isoleucine (Ile), providing accurate quantification of mixtures of l-Ile and l-allo-Ile. This strategy is applicable to a range of noncanonical amino acids and suggests that commercially available d-Marfey's analogues or achiral Sanger's reagent will provide enhanced separation without synthesis of more complex derivatizing agents.