Last ten years (2008-2018) of chiral ligand-exchange chromatography in HPLC: An updated review.

Chiral ligand-exchange chromatography is one of the elective strategies for the direct enantioresolution of small chelating compounds: amino acids, diamines, amino alcohols, diols, small peptides, etc. Unlike other methods, the interaction between chiral selector and analyte enantiomers is mediated by a cation, thus producing diastereomeric ternary complexes. Two main approaches are conventionally applied in chiral ligand-exchange chromatography. The first relies upon chiral stationary phases where the chiral selector is either covalently immobilized or physically adsorbed onto suitable packing materials (coated phases). In the second approach, chiral molecules are added to the eluent, thus generating chiral eluent systems. Among the advantages of chiral ligand-exchange chromatography, the generation of UV/vis-active metal complexes, and the use of commercially available or easy-to-synthesize chiral selectors, in combination to rather inexpensive achiral columns for coated phases and chiral eluents, are noteworthy. Besides amino acids and amino alcohols, other species have proven suitable for chiral ligand-exchange chromatography applications. Recently, the use of either chiral ionic liquids or micellar liquid chromatography systems as well as the successful off-column formation of diastereomeric complexes have expanded the selectivity profiles and application fields. All of these issues are touched in the review, shedding light to the contributions appeared in the last decade.

Improved chromatographic diastereoresolution of cyclopropyl dafachronic acid derivatives using chiral anion exchangers.

In the present paper we describe the optimization and the application of a chromatographic method suitable to get all four diastereoisomers of C24-C25 cyclopropyl dafachronic acid derivatives in sufficient amount for their biological appraisal towards the nuclear hormone receptor transcription factor DAF-12. A preliminary column screening of six anion exchange type Cinchona alkaloid-based chiral stationary phases (CSPs) allowed to identify the one with a quinine scaffold and carrying a O-9-(3,5-bis(trifluoromethyl)phenyl) moiety at C9 position as the best CSP. Few modifications of the experimental conditions revealed that a content of 18mM acetic acid used as counterion and displacer in acetonitrile and a column temperature fixed at 35°C were optimal for the simultaneous discrimination of all four diastereoisomers with a 1.0mL/min flow rate. With such conditions, transoid (S,S) and (R,R) diastereoisomers were resolved with RS>1.4. With non chiral reversed-phase columns, neither the cisoid nor the transoid diastereoisomers could be resolved. This way, ca. 1.0mg of each stereoisomer was isolated with a diastereomeric purity >98%, suitable for the following biological tests. The indirect stereochemical assignments of the four diastereoisomers, and hence the corresponding chromatographic elution order (24R,25R)<(24S,25S)<(24R,25S)<(24S,25R) were made in an analogy manner on the basis of the resolution of fully assigned and structurally very similar ursodeoxycholic acid derivatives. As support of this indirect way of assigning the absolute configuration of the C24 and C25 chiral centre a molecular modeling procedure based on dynamic simulation was successfully applied.

(S)-(-)-alpha,alpha-Di(2-naphthyl)-2-pyrrolidinemethanol, a useful tool to study the recognition mechanism in chiral ligand-exchange chromatography.

A dynamic coating of the RP-18 carbon chain layers with the new chiral selector (S)-(-)-alpha,alpha-di(2-naphthyl)-2-pyrrolidinemethanol allowed the formation of a mixed chiral stationary phase that has been used in the separation of a selected set of amino acid racemates. Both a representative model and classification structure-property relationship studies have been performed in order to study the contribution of hydrophobic, bulky and electron-donating groups in the side chain of the chiral selector to the mechanism of chiral recognition.