Preface

The enchantment of reading about the first developments on chiral selectors for enantioseparation persists up to now. The papers from Hesse and Hagel [1], Pirkle [2, 3], Allenmark [4, 5], Okamoto [6], Armstrong [7] back in the 1970s-1980s, to cite a few, marked the vertiginous development of chiral stationary phases (CSPs).

Pirkle's paper [8] about ionically bonded CSPs is a milestone paper. It pointed to the need of stablishing a relationship between absolute configuration (AC) and elution order providing a rationale for chiral recognition. This publication came alongside with the 3,5-dinitrobenzoyl phenylglycine CSP, the first commercial chiral column for liquid chromatography (LC) [9]. Ariens' crusade [10] to raise awareness about the implication of stereochemistry in pharmacokinetics studies and, specially, in drug development programs and in clinical practice gave the tone for the development of appropriate chiral drug analysis procedures. The chiral analysis of drugs in biological fluids was, then, frenetically pursued in the 1980s and resulted in the FDA's regulations on pharmaceutical development of single enantiomers and racemates [11]. The approval of a new chiral drug by regulatory agencies requires a complete dossier of the pharmacology and pharmacokinetic profiles of the single enantiomers and their mixtures. Methods to monitor the biological effects of a chiral drug are required since pre-clinical level with a few milligrams of racemates and single enantiomers moving to kilogram quantities at clinical trials. At each stage, it is important to know which is the most appropriate procedure either to the analysis or production of enantiomers. Such knowledge is thus considered a very important asset [12].

The advent of direct enantioseparation by chromatography and related techniques has just completed half a century, and it is nowadays used as routine in research and industry laboratories around the world. This book covers the main features of chiral separation by LC, gas chromatography (GC), capillary electrophoresis (CE), and supercritical fluid chromatography (SFC). Chiral separation at semi- and preparative scales are also included.

With the five fundamental chapters (1-5) covering LC, GC, CE, SFC, and preparative chromatography, the book encompasses the main advances of each technique and discusses the application of chiral separation in different areas of science, such as enantioselective synthesis, chiral drug designing and development, bio, forensic, and environmental markers, chiral materials, quality control in pharmaceuticals, food, and fragrances. Chiral separations in metabolomics and lipidomics to disclose the enantiomeric signature in various biological processes is another topic that has been positively impacted by the advance in chiral selector technologies alongside with the appropriate analytical platforms. Application examples can be found throughout the book. The role of 2D-LC in chiral separation method development and applications is discussed in Chapter 1 as well as in other chapters of the book.

The quality of the separation in CE, GC, LC, and SFC is dictated by the chiral selector, and with so many interconnected interactions playing a role in chiral discrimination, it is expected that there will never be either a universal chiral selector or CSP.

The advances in chiral selectors and their use for all these separation technologies are thoroughly discussed from Chapters 6 to 11, including new CSP for LC and/or SFC with high-throughput and ultra-high-efficiency capabilities. These chapters portray the most used chiral selectors including their designing, mechanism, and applications for a variety of fields with practical examples. The impact on the chiral recognition mechanism caused by the elution mode in LC is also reviewed as well as the best chiral selectors either for SFC or for preparative separation.

Following chiral separation, the next challenge is the determination of molecular stereochemistry. Methods for stereochemical elucidation are widely used across biochemistry, chemistry, biology, and physics, but there is a clear need for streamlining their application for characterization of small chiral organic molecules.

The 3D structure characterization of a given chiral molecule is of prime importance, and, although not trivial, it is routinely required in many research areas, especially in drug discovery programs. For chiral molecules, prior to setting up several biological experiments, it is indispensable to unambiguously determine not only their stereoisomeric composition but also their AC [13, 14].

This approach ensures that a correct structure-activity relationship is established. The AC assignment of a given molecule is usually carried out by different methods in which X-ray crystallography is considered the gold standard approach. Nuclear magnetic resonance (NMR) is routinely used for establishing relative configurations, while the AC can be determined either by diastereomeric derivatizations or by a combination of anisotropic NMR and chiroptical spectroscopy. Chiroptical methods, mainly associated with quantum-mechanical calculations, have proved to be an excellent choice to assign AC especially for compounds in which a well-defined single crystal is not available.

In this regard, Chapters 12-14 cover these three main stereochemical elucidation methods. Information on their theoretical backgrounds, advantages, and limitations, as well as examples of application are provided. With that, we do hope many research projects dealing with small organic molecules will benefit from a streamlined approach to both enantiomeric separation and AC determination.

By

Quezia Bezerra Cass, Maria Elizabeth Tiritan, João Marcos Batista Junior, and Juliana Cristina Barreiro, São Carlos, August 2022

References

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2. 2 Pirkle, W.H. and Sikkenga, D.L. (1976). Resolution of optical isomers by liquid chromatography. J. Chromatogr. A 123: 400-404. https://doi.org/10.1016/S0021-9673(00)82210-4.
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