High-performance liquid chromatographic enantioseparation of 1-(phenylethylamino)- or 1-(naphthylethylamino)methyl-2-naphthol analogs and a temperature-induced inversion of the elution sequence on polysaccharide-based chiral stationary phases

High-performance liquid chromatographic enantioseparation of azole analogs of monoterpene lactones and amides focusing on the separation characteristics of polysaccharide-based chiral stationary phases

High-performance liquid chromatography-based enantioseparation of newly prepared azole analogs of monoterpene lactones and amides was studied. Effects of additives and mobile phase composition were evaluated both in normal and polar organic modes. Applying amylose tris-(3,5-dimethylphenylcarbamate) selector in normal and polar organic modes acid and base additives were found to affect the peak profiles, without significantly influencing the enantiorecognition ability of the studied selector. In most cases, differences observed in retention times and enantioselectivities were lower than 10 and 20 % under normal phase and polar organic conditions, respectively. Under normal phase conditions decreased retention was observed for all the studied analytes with increased eluent polarity. Interestingly, enantioselectivity was only slightly (<10 %) influenced by the variation in the n-hexane/2-propanol ratio between 80/20 and 20/80 v/v. In polar organic mode, five different neat solvents (acetonitrile, methanol, ethanol, 1-propanol, and 2-propanol) were tested, and the best results were obtained with acetonitrile and ethanol in the case of Lux Amylose-1 column with enantioresolutions most often above 2. Based on results obtained with amylose and cellulose-based columns the amylose tris-(3,5-dimethylphenylcarbamate) selector is found to offer a superior performance both in normal and polar organic modes. When evaluating the possible effects of the selector immobilization, no striking differences were found in the normal phase. Usually, enantioselectivities and resolutions were higher (10-20 %), while retention factors of the first peaks were lower (20-30 %), on the coated-type column. In contrast, in polar organic mode, the retention characteristics and enantiorecognition ability of the coated and immobilized selectors were heavily affected by the nature of the polar solvent. Special attention has been paid to the history-dependent behavior of polysaccharide-based selectors. A confidence interval-based evaluation is suggested to help comparison of the histereticity observed in different systems. Several examples are shown to confirm that the recently discovered hysteresis is a common characteristic of polysaccharide-based selectors.

The Applicability of Chromatographic Retention Modeling on Chiral Stationary Phases in Reverse-Phase Mode: A Case Study for Ezetimibe and Its Impurities

Mechanistic modeling is useful for predicting and modulating selectivity even in early chromatographic method development. This approach is also in accordance with current analytical quality using design principles and is highly welcomed by the authorities. The aim of this study was to investigate the separation behavior of two different types of chiral stationary phases (CSPs) for the separation of ezetimibe and its related substances using the mechanistic retention modeling approach offered by the Drylab software (version 4.5) package. Based on the obtained results, both CSPs presented with chemoselectivity towards the impurities of ezetimibe. The cyclodextrin-based CSP displayed a higher separation capacity and was able to separate seven related substances from the active pharmaceutical ingredient, while the cellulose-based column enabled the baseline resolution of six impurities from ezetimibe. Generally, the accuracy of predicted retention times was lower for the polysaccharide CSP, which could indicate the presence of additional secondary interactions between the analytes and the CSP. It was also demonstrated that the combination of mechanistic modeling and an experimental design approach can be applied to method development on CSPs in reverse-phase mode. The applicability of the methods was tested on spiked artificial placebo samples, while intraday and long-term (2 years) method repeatability was also challenged through comparing the obtained retention times and resolution values. The results indicated the excellent robustness of the selected setpoints. Overall, our findings indicate that the chiral columns could offer orthogonal selectivity to traditional reverse-phase columns for the separation of structurally similar compounds.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

High-performance liquid chromatographic enantioseparation of isopulegol-based ß-amino lactone and ß-amino amide analogs on polysaccharide-based chiral stationary phases focusing on the change of the enantiomer elution order

The enantioselective separation of newly prepared, pharmacologically significant isopulegol-based ß-amino lactones and ß-amino amides has been studied by carrying out high-performance liquid chromatography on diverse amylose and cellulose tris-(phenylcarbamate)-based chiral stationary phases (CSPs) in n-hexane/alcohol/diethylamine or n-heptane/alcohol/ diethylamine mobile phase systems. For the elucidation of mechanistic details of the chiral recognition, seven polysaccharide-based CSPs were employed under normal-phase conditions. The effect of the nature of selector backbone (amylose or cellulose) and the position of substituents of the tris-(phenylcarbamate) moiety was evaluated. Due to the complex structure and solvation state of polysaccharide-based selectors and the resulting enantioselective interaction sites, the chromatographic conditions (e.g., the nature and content of alcohol modifier) were found to exert a strong influence on the chiral recognition process, resulting in a particular elution order of the resolved enantiomers. Since no prediction can be made for the observed enantiomeric resolution, special attention has been paid to the identification of the elution sequences. The comparison between the effectiveness of covalently immobilized and coated polysaccharide phases allows the conclusion that, in several cases, the application of coated phases can be more advantageous. However, in general, the immobilized phases may be preferred due to their increased robustness. Thermodynamic parameters derived from the temperature-dependence of the selectivity revealed enthalpically-driven separations in most cases, but unusual temperature behavior was also observed.

Chromatographic and thermodynamic comparison of amylose tris(3-chloro-5-methylphenylcarbamate) coated or covalently immobilized on silica in high-performance liquid chromatographic separation of the enantiomers of select chiral weak acids

The separation of enantiomers of some chiral weak acids was studied in HPLC with chiral HPLC columns prepared by coating or covalent immobilization of the same chiral selector, namely amylose tris(3-chloro-5-methylphenylcarbamate) onto silica. After screening some representatives of arylpropionic acid derivatives, coumarins and barbiturates in hydrocarbon-alcohol type mobile phases, we studied the temperature dependence of separation parameters for ketoprofen and naproxen. Instances of reversal of the enantiomer elution order were observed function of column temperature, nature of polar modifier and its content in the mobile phase, as well as between the coated and covalently immobilized versions of the columns made with more-or-less the same chiral selector. Thermodynamic parameters such as Gibb's free energy, the standard molar entropy and the standard molar enthalpy of analyte transfer from the mobile to the stationary phase were calculated in some cases in order to explain the differences observed in the enantiomer separation ability and pattern of coated and covalently immobilized columns.

Temperature and eluent composition effects on enantiomer separation of carvedilol by high-performance liquid chromatography on immobilized amylose-based chiral stationary phases

Carvedilol is a chiral drug with potent antihypertensive and antianginal activities. Although it is clinically used as a racemic mixture, its enantiomers show different pharmacokinetic and pharmacodynamic profiles. Here, the direct chiral separation of racemic drug by high performance liquid chromatography using two immobilized-type amylose-based chiral stationary phases is presented. Some chromatographic parameters, such as retention and selectivity, were determined under multimodal eluent conditions and different temperatures. A temperature-dependent inversion of the elution order of enantiomers was observed in the operative temperature range of chiral chromatographic support. Finally, an effective direct enantioselective method was successfully applied to the separation of the enantiomers of carvedilol on a semipreparative scale.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.

Enantioseparation of aldols by high-performance liquid chromatography on polysaccharide-based chiral stationary phases that bear chlorinated substituents

Two families of aldols, obtained from the condensation of aromatic aldehydes with cyclohexanone or acetone (ten examples in each group), were analyzed by high-performance liquid chromatography in normal phase elution mode on three polysaccharide-based chiral stationary phases of the Lux series, namely, Lux Cellulose-2, Lux Cellulose-4 and Lux Amylose-2, which share the common feature of chlorinated substituents in the chiral selectors. Following simple optimization steps, the enantioseparation of all aldols derived from cyclohexanone was achieved and the highest values of separation factor (α, 1.32 < α < 2.20) and resolution (Rs , 4.5 < Rs <17.2) were observed on Lux Cellulose-2, with the only exception of the 4-nitro-substituted derivative that was better resolved on Lux Cellulose-4. On the contrary, Lux Amylose-2 was the best choice for aldols derived from acetone and only specific analytes in this group were resolved on the cellulose-based supports. A variable-temperature study of selected compounds allowed us to determine thermodynamic parameters of the enantioseparation process, which was enthalpy-controlled in all the cases except one.