Separation of enantiomers in thin-layer chromatography

Abstract

Up to now in thin-layer chromatography a series of retention mechanisms for the determination of enantiomeric compositions have been applied. Subject of this paper is a corresponding survey with a focal point on ligand exchange chromatography. In connection with this retention mechanism most successfully used in TLC for the separation of optical antipodes, some additional findings of the authors are included. Retention mechanisms in question are: - Formation of inclusion complexes within the cavities of cyclodextrins. Substance classes separated by this system are e.g. dansyl amino acids or metallocenes. - Interaction with so-called “brush type” phases. In this case, chiral groups with π-acceptor of π-donor properties are chemically or physically bonded to an aminomodified silica gel matrix. Chiral compounds, like aromatic alcohols, were separated on these stationary phases. - Differences in retention of diastereomers formed previously or during the chromatographic process on an achiral stationary phase. Some examples for the manifold applicability of this principle are the separation of chiral amines from biological matrices, hydroxy-fatty acids, and amino acids. - Selective interactions with the helical structures of cellulose or cellulose triacetate. Referring to this technique, separations of several amino acids and of the analgesic agent Oxidanac in the R-and S-species are described. - Enantioselective ligand exchange, based on reversed phase silica gels impregnated with copper salts and optically active amino acid derivatives. In thin-layer chromatography this is the most important principle used mainly for the separation of racemic mixtures of free amino acids and amino acid derivatives. The recently developed HPTLC pre-coated plate CHIR with concentrating zone is also based on this retention mechanism. Aside from the applications mentioned above, dipeptides, hydroxy-carboxylic acids, α-halogenated carboxylic acids, amino lactones, and thiazolidine derivatives have been separated on this plate up to now. Furthermore, this new pre-coated layer is particularly suitable for an optical in situ evaluation and quantitative determinations of optical antipodes.