Use of a naphthylethylcarbamoylated-β-cyclodextrin chiral stationary phase for the separation of drug enantiomers and related compounds by sub- and supercritical fluid chromatography

Enantiomeric separation of a variety of drugs and related compounds was achieved on an (S)-naphthylethylcarbamoylated-β-cyclodextrin (S-NEC-CD) chiral stationary phase (CSP) using sub- and supercritical fluid chromatography (SFC). Compounds previously resolved on native or derivatized cyclodextrin CSPs in liquid chromatography (LC) using reversed phase or polar organic mobile phase modes could be resolved in SFC using a simple carbon dioxide/methanol eluent. Resolution of cromakalim, which is not possible on the S-NEC-CD column in LC, was readily accomplished in SFC. The importance of modifier, temperature, and pressure was assessed in relation to retention, selectivity, and resolution. The nature of the modifier and the modifier concentration were found to be crucial parameters. © 1996 Wiley-Liss, Inc. Contribution of the National Institute of Standards and Technology. Not subject to copyright.     

Use of a new pirkle-type chiral stationary phase in analytical and preparative subcritical fluid chromatography of pharmaceutical compounds

Good results have been obtained with use of the new bonded chiral stationary phase Whelk-O 1 in analytical and preparative subcritical fluid chromatography. A wide variety of enantiomeric pairs of compounds with different functional groups that are of pharmaceutical and biological interest have been resolved. This Pirkle-concept CSP appears to be more rugged than cellulosic phases (e.g., Chiralcel) with regards to solvents and pressure. In comparing the usefulness of the column for SFC versus HPLC chiral analysis, we have observed a clear superiority of SFC in terms of higher speed and efficiency of analysis, and faster method development. This is consistent with our experience with Chiralcel CSPs. With the Whelk-O 1 we have shown that the effects of temperature and modifier on SFC separations are similar to what has been reported for most other CSPs. We also observed a unique selectivity advantage of SFC for verapamil. We had good success with using a 1-in. diameter column packed with Whelk-O 1 to perform preparative SFC separations of a number of enantiomeric mixtures. The advantages of preparative SFC over preparative HPLC will be discussed. The feasibility of preparative SFC is dependent on how well we meet the practical challenges such as sample introduction issues, special hardware requirements due to the high pressure, and fraction collection issues. © 1994 Wiley-Liss, Inc.     

Simulated moving bed chromatography with supercritical fluids for the resolution of bi-naphthol enantiomers and phytol isomers

The combination of the simulated moving bed (SMB) technique with supercritical fluid chromatography (SFC) leads to a process with unique features. Besides the known advantages of the SMB process, the use of supercritical carbon dioxide as the mobile phase offers the advantages of reduction in organic solvents and an easy eluent/solute separation. Because of the low viscosity and high diffusion coefficients of supercritical fluids, a high efficiency is possible. The steps of process development for SMB SFC are presented using the separations of the bi-naphthol enantiomers and phytol isomers as examples. The development of a packed column SFC method at an analytical scale is shown for the separation of the bi-naphthol enantiomers on a chiral stationary phase and CO(2) with a modifier as the mobile phase. The influence of the modifier, modifier content, and column configuration on productivity of the SMB SFC process was investigated by simulation. The first set of experiments was performed in the SMB separation of phytol isomers at low concentration to test the feasibility of the SMB SFC high purity separation of the binary mixtures. In the second set of experiments, the productivity of the process was increased by increasing the feed concentration up to 54 grams feed per liter stationary phase (SP) and hour (g(feed)/l(SP) h).     

Chiral separation of neonicotinoid insecticides by polysaccharide‐type stationary phases using high‐performance liquid chromatography and supercritical fluid chromatography

The enantiomeric separations of three neonicotinoid insecticides (identified as compounds 1 , 2 , and 3 ) were performed on three polysaccharide‐type chiral columns, that is, Chiralcel OD‐H, Chiralpak AD‐H, and Chiralpak IB, by high‐performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). Effects of the modifier percentage and column temperature on chiral recognitions of chiral stationary phases were also studied. Both 1 and 2 could be resolved on all three columns selected, with the highest R_s values obtained on Chiralpak AD‐H and Chiralcel OD‐H, respectively. However, satisfactory separation of the four stereoisomers of 3 was only achieved on Chiralcel OD‐H. Considering the effects of ethanol on the values of k, α, and R_s, we concluded that hydrogen bonding, π–π, and/or dipole–dipole interactions might be all responsible for the chiral separation. In comparison to HPLC, a shorter run time was achieved for 1 and 2 by SFC. However, 3 could not be stereoselectively resolved using SFC. On the basis of the calculated thermodynamic parameters, we found that the separation processes of enantiomers of 1 and 2 were entropy controlled and enthalpy controlled, respectively. Chirality, 2011. © 2010 Wiley‐Liss, Inc.     

Application of chiralcel OJ in supercritical fluid chromatography for the resolution of different groups of frequently used drug racemates

Supercritical fluid chromatography was applied to evaluate the direct chiral discriminative properties of Chiralcel OJ, a tris(4-methylbenzoate) cellulose column towards frequently administered drugs. Two groups of acidic drugs, profen and barbiturate derivatives, and a few basic drugs of the benzodiazepine type were analysed. The effect on enantioselectivity of carbon dioxide when using methanol are acetonitrile as primary modifier was studied. Acetonitrile proved to be a good alternative for methanol, especially for the profen compounds that were not well resolved using methanol. The results were compared to normal phase chromatographic applications on the same column. SFC was not necessarily superior to high-performance liquid chromatography. Chirality 9:126–132, 1997. © 1997 Wiley-Liss, Inc.     

A comparison of LC and SFC for cellulose- and amylose-derived chiral stationary phases

This paper presents a systematic comparison of liquid chromatography (LC) and supercritical fluid chromatography (SFC) for Chiralcel OD and Chiralpak AD chiral stationary phases (CSPs), performed using various chiral compounds having a known or potential pharmaceutical activity. The chiral recognition mechanisms involved in LC and SFC for the enantiomeric separation of β-blockers have been studied more particularly. As a general rule, it appears that the presence of polar functions, like primary or secondary hydroxyl or amine functions, may result in marked discrepancies in selectivity between LC and SFC. This result is peculiar to cellulose- and amylose-derived CSPs, for which the interactions involved in chiral recognition mechanism are not always well balanced, contrary to what happens for independent CSPs. In the case of chiral resolution of polar solutes or polymer-type CSPs, the analyst should try both the LC and SFC techniques to be able to choose the more stereoselective one. © 1995 Wiley-Liss, Inc.     

Strategic use of preparative chiral chromatography for the synthesis of a preclinical pharmaceutical candidate

The modern use of preparative chromatography in pharmaceutical development is illustrated by the case of a recent preclinical candidate from these laboratories. The synthesis of the candidate employed a coupling of two enantiopure intermediates, each of which could be resolved using preparative chiral chromatography. SFC screening was employed to identify the enantioselective stationary phases, and semipreparative SFC methods derived from this screening were used to produce gram amounts of enantiopure intermediate for initial studies. However, initial larger scale resolution required the translation of the SFC methods to HPLC conditions. Preparative chiral HPLC on a 30-cm i.d. column was then used to produce enantiopure intermediates which were coupled to give 170 g of the preclinical candidate. Subsequent preparation of the candidate at larger scale for later-stage clinical evaluation employed an improved synthesis in which one component was constructed by asymmetric synthesis. Resolution of the other component, now a more advanced intermediate, was carried out using newly obtained large-scale SFC equipment. Some discussion is presented on the varying strategies whereby preparative chiral chromatography can be used to support either short-term or long-term synthetic goals in preclinical pharmaceutical development.     

Supercritical fluid chromatography comparison of the poly(trans-1,2-cyclohexanediyl-bis acrylamide) (P-CAP) column with several derivatized polysaccharide-based stationary phases

The poly(trans-1,2-cyclohexanediyl-bis acrylamide) (P-CAP) column has so far been primarily used with normal phase and polar organic mobile phase chromatography. Its use in supercritical fluid chromatography (SFC) was investigated via the analysis of 40 commercial and 100 proprietary compounds using a 12-min gradient with methanol as a modifier. Results were then compared against those obtained from the popular derivatized polysaccharide-based chiral stationary phases (CSPs) such as Chiralpak AD-H and Chiralpak AS-H as well as Chiralcel OD-H and Chiralcel OJ-H columns. P-CAP demonstrated separation of 25% of the 140 total compounds, while each of the derivatized polysaccharide-based CSPs separated at least 46%. A study that compared the loading of 1,1'-bi-2-naphthol with P-CAP and Chiralpak AS columns indicated a similar trend in resolution vs. amount injected, though AS appeared capable of allowing a greater loading of material. The P-CAP column was found to be beneficial in the separation of a complex mixture of enantiomers and achiral impurities, where the derivatized polysaccharide-based columns did not show as desirable of a separation. A key advantage of this type of chiral stationary phase is the fact that it is available in both enantiomeric forms, allowing manipulation of elution order of enantiomers, which is especially helpful for preparative applications. P-CAP also demonstrated that it could resolve an achiral impurity from the desired compound in a different mixture, while the same impurity co-eluted on the Chiralpak AD-H column. Overall, the synthetic polymer-based P-CAP showed less chiral discrimination power compared to the derivatized polysaccharide-based CSPs under the conditions explored in this study.     

Separation of albendazole sulfoxide enantiomers by chiral supercritical-fluid chromatography

The enantioseparation of albendazole sulfoxide (ABZSO) by chiral supercritical-fluid chromatography (SFC) on two columns, based on the polysaccharide derivatives Chiralpak AD and Chiralcel OD, was studied. The effect of different modifiers, methanol, ethanol, 2-propanol, and acetonitrile, was examined. The results showed that ABZSO can be separated on both columns, using an alcohol-type modifier. Using the Chiralpak AD column, the best results were obtained with 2-propanol and, in the case of the Chiralcel OD, with methanol.     

Enabling chiral separations in discovery chemistry with open‐access chiral supercritical fluid chromatography

Work from this paper details a novel walk‐up open‐access (OA) approach to enable chiral analytical method development and preparative separation of enantiomers in early discovery chemistry using supercritical fluid chromatography (SFC). We have demonstrated the success of this OA approach using immobilized chiral stationary phases (CSPs). After screening a diverse set of racemic drug candidates, we have concluded that a simplified OA chiral SFC platform can successfully purify approximately 60% of the analysed racemates. This streamlined OA workflow enables medicinal chemists with limited expertise in chiral method development to successfully and rapidly purify enantiomers for their projects using Waters UPC² and Prep100‐SFC instrumentation.     

Effects of Hexane in Supercritical Fluid Chromatography for the Separation of Enantiomers

Supercritical fluid chromatography (SFC), operated in conventional mode, is normally recognized as normal phase chromatography, and uses a solvent combination of supercritical CO₂ and alcohols to separate compounds. Hexane, a commonly used solvent in normal phase liquid chromatography (NP‐LC), is rarely used in SFC and, in some cases, is added to the organic modifiers to increase liquid content in order to achieve better efficiency in preparative SFC for poorly retained compounds. Although hexane is believed to have similar solvent strength to that of supercritical CO₂, its effects on the enantioseparation in SFC is largely unknown. To understand the chromatographic effects of an apolar solvent, such as hexane in SFC, we compared the chromatographic behaviors of 35 chiral compounds using a parallel SFC method under traditional SFC mode of only “pure” alcohol‐CO₂ to that of hexane‐assisted SFC (HA‐SFC), which uses mixtures of alcohol and hexane (as cosolvents) and CO₂. We observed that, in some cases, hexane behaves just like supercritical CO₂, where replacement of a portion of CO₂ with hexane does not significantly change retention times or resolution of the peaks. In many cases, however, addition of hexane in mobile phases does affect chromatographic behavior of one or both enantiomers. Such effects might provide opportunities for separation of some enantiomers. Chirality 28:192–198, 2016. © 2016 Wiley Periodicals, Inc.     

Enantioseparation of chiral perfluorooctane sulfonate (PFOS) by supercritical fluid chromatography (SFC): Effects of the chromatographic conditions and separation mechanism

Perfluorooctane sulfonate (PFOS) is one of the most frequently detected perfluoroalkyl substances in environmental and human samples. Previous studies have shown that nonracemic PFOS in biological samples can be used as a marker of PFOS exposure sources. In recent years, supercritical fluid chromatography (SFC) has emerged as a powerful method to separate chiral compounds. In this study, a method of perfluoro‐1‐methylheptane sulfonate (1 m‐PFOS) enantioseparation by SFC was established. The optimal separation was obtained using a Chiralpak QN‐AX column with CO₂/2‐propanol (70/30, v/v) as the mobile phase with a flow rate of 1 mL/min, column temperature was 32°C, and BPR pressure was 1800 psi. The resolution (Rs) and retention time were 0.88 and 130 minutes, respectively. This method is more economic and greener than HPLC. Modifier pH and column temperature were determined to be significant factors of SFC chiral separation. Modifier pH is negatively correlated with the retention factors and Rs. Adsorption thermodynamics were used to explain the influence of temperature change, and it was concluded that the transfer of two enantiomers from the mobile phase to the stationary phase is enthalpy‐driven. Enantioseparation of 1 m‐PFOS by SFC follows the same rules of ion exchange as those for the chiral separation by HPLC.     

Enantioselective separation of (±)‐β‐hydroxy‐1,2,3‐triazoles by supercritical fluid chromatography and high‐performance liquid chromatography

This paper reports the enantioseparation of β‐hydroxy‐1,2,3‐triazole derivatives, which present a broad range of biological properties, by supercritical fluid chromatography (SFC) and high‐performance liquid chromatography techniques (HPLC). Polysaccharide‐based chiral columns (cellulose and amylose) were used to evaluate the separation in SFC and HPLC. Time of analyses, consumption of solvent, and parameter optimization were reduced using SFC technique. The columns based on cellulose chiral stationary phase using 2‐propanol and ethanol as modifiers showed the best results for the enantioresolution of the (±)‐β‐hydroxy‐1,2,3‐triazoles by SFC analyses. These techniques were applied to evaluate the selectivity of biocatalytic reduction of β‐keto‐1,2,3‐triazoles by marine‐derived fungus Penicillium citrinum CBMAI 1186 to obtain the (±)‐β‐hydroxy‐1,2,3‐triazoles.     

Supercritical fluid chromatography as basis for identification and quantitative determination of indol-3-ylmethyl oligomers and ascorbigens

Indol-3-ylmethylglucosinolate (glucobrassicin) occurs in most plants of the Brassicaceae family together with hydroxy and methoxy derivatives of glucobrassicin. These compounds and products produced therefrom have been the subject of considerable research interest due to their potential anticarcinogenic effects, and thereby a need for techniques to work with the individual compounds. A method using normal-phase supercritical fluid chromatography (SFC) with methanol as modifier has been developed for determination and quantification of the various indol-3-ylmethyl derivatives including ascorbigens formed from the glucobrassicin degradation product, indol-3-ylmethanol, under acidic conditions (pH 2-6) with and without the presence of ascorbic acid. The SFC method had detection limits in the 10-100-pmol range. In the absence of ascorbic acid a range of oligomers were formed, whereas the presence of ascorbic acid favoured the formation of ascorbigen and products thereof. Quantitatively important indol-3-ylmethyl oligomers consisting of up to five indol rings have been purified with preparative SFC and identified from MS and 1D and 2D NMR experiments with complete assignment of chemical shifts to all of the atoms. Investigation of the autolysis products of white cabbage showed that ascorbigens were the quantitatively dominating degradation products of indol-3-ylmethylglucosinolates.     

Liquid Chromatography Enantioseparations of Halogenated Compounds on Polysaccharide‐Based Chiral Stationary Phases: Role of Halogen Substituents in Molecular Recognition

Halogenated chiral molecules have become important in several fields of science, industry, and society as drugs, natural compounds, agrochemicals, environmental pollutants, synthetic products, and chiral supports. Meanwhile, the perception of the halogen moiety in organic compounds and its role in recognition processes changed. Indeed, the recognition of the halogen bond as an intermolecular interaction occurring when the halogen acts as a Lewis acid had a strong impact, particularly in crystal engineering and medicinal chemistry. Due to this renewed interest in the potentialities of chiral organohalogens, here we focus on selected recent applications dealing with enantioseparations of halogenated compounds on polysaccharide‐based chiral stationary phases (CSPs), widely used in liquid chromatography (LC). In particular, recently the first case of halogen bonding‐driven high‐performance LC (HPLC) enantioseparation was reported on a cellulose‐based CSP. Along with enantioseparations performed under conventional HPLC, representative applications using supercritical fluid chromatography (SFC) are reported. Chirality 27:667–684, 2015. © 2015 Wiley Periodicals, Inc.     

Enantiomeric separation of imidazolinone herbicides using chiral high-performance liquid chromatography

Chiral high-performance liquid chromatography (HPLC) is one of the most powerful tools to prepare enantiopure standards of chiral compounds. In this study, the enantiomeric separation of imidazolinone herbicides, i.e., imazethapyr, imazapyr, and imazaquin, was investigated using chiral HPLC. The enantioselectivity of Chiralpak AS, Chiralpak AD, Chiralcel OD, and Chiralcel OJ columns for the three analytes was compared under similar chromatographic conditions. Chiralcel OJ column showed the best chiral resolving capacity among the test columns. The resolved enantiomers were distinguished by their signs of circular dichroism detected at 275 nm and their structures confirmed with LC-mass spectrometric analysis. Factors affecting the chiral separation of imidazolinones on Chiralcel OJ column were characterized. Ethanol acted as a better polar modifier than the other alcohols including 2-propanol, 1-butanol, and 1-pentanol. Although the acidic modifier in the mobile phase did not influence chiral recognition, it was necessary for reducing the retention time of enantiomers and suppressing their peak tailing. Thermodynamic evaluation suggests that enantiomeric separation of imidazolinones on Chiralcel OJ column is an enthalpy-driven process from 10 to 40 degrees C. This study also shows that small amounts of pure enantiomers of imidazolinones may be obtained by using the analytical chiral HPLC approach.     

Challenges and frustrations in the separation and analysis of chiral agrochemicals

The development of chiral HPLC methods and isolation techniques within Zeneca Agrochemicals (formerly ICI Agrochemicals) is reviewed. The use of low temperature to improve chiral separations has been successfully applied to production analysis, but although useful for some compounds it is regrettably not a universal panacea for all poor separations. The need to isolate small quantities of individual enantiomers from new compounds for research evaluation has led us to devise a more universal and cheap chiral stationary phase (CSP) for Preparative-LC. Joint academic research produced a CSP based on tartaric acid which was made commercially available and it was gratifying to find it was the only phase able to resolve a novel insecticide. However, as new CSPs emerged almost every month, our attention turned to using a universal chiral detector for analysis, rather than via separation of individual enantiomers. Diode laser-based polarimeters offered the opportunity of cheap, sensitive chiroptical detectors for HPLC and the ability to move away from chiral columns in both research and production analysis. Jointly sponsored research with a university has successfully explored the versatility of chiroptical detectors in agrochemical and food analysis. Comparison of chiral SFC with chiral HPLC and an extensive evaluation of established and research agrochemicals on a wide range of commercial CSPs have led to a revised method development strategy. Current work with high load displacement chiral chromatography will be described as a potential means of isolating pure enantiomers from racemates. © 1994 Wiley-Liss, Inc.     

Tuning of mobile and stationary phase polarity for the separation of polar compounds by SFC

The separation of polar compounds by supercritical-fluid chromatography (SFC) is reviewed. New developments in mobile and stationary phase tuning are reviewed for packed and packed capillary SFC. In terms of mobile phase polarity adjustment, new pure and multiple component fluids are presented. The approach of tuning the polarity of the stationary phase as a way of increasing the range of polar compounds analyzed by SFC using pure CO(2) is discussed using either silica-based or new materials as stationary phase. Chiral, liquid crystal and polymer-based stationary phases coated on particles are widely covered in this review as an interesting approach to separate polar compounds avoiding the major drawbacks associated to the use of modifiers in SFC.     

Direct high-performance liquid chromatographic separation of the enantiomers of venlafaxine and 11 analogs using amylose-derived chiral stationary phases

A direct liquid chromatographic enantioselective separation of venlafaxine and 11 analogs was obtained in the normal phase mode using Chiralpak AD. For some compounds, a comparison between the enantioseparation using coated and immobilized amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phases (Chiralpak AD and Chiralpak IA, respectively) was made. The best separations were achieved on Chiralpak AD with ethanol as alcoholic modifier in a mobile phase made basic by DEA addition: separation factor ranges between 2.08 and 1.15 and resolution factor between 7.0 and 1.0. Using the same CSP and 2-propanol doped with TFA as acidic modifier, 10 compounds were enantioseparated with separation factor ranging between 1.40 and 1.04 and resolution factor between 3.1 and 0.3. The use of ethanol as alcoholic modifier also has the advantage of better solubility of the compounds in the mobile phase. The nature of the substituent (electron donating or withdrawing) affects in general the separation factor. A memory effect that involves a long equilibration time of the CSP is present when switching from an acidic mobile phase to a basic one.     

Benzoylcellulose derivatives as chiral stationary phases for open tubular column chromatography

The application of cellulose-based stationary phases for chiral separations has been extended to open tubular column chromatography. Efficient columns were obtained by coating the capillaries with mixtures of chiral cellulose materials and conventional achiral stationary phases for gas chromatography. In this study, various siloxane and polyethylene glycol polymers were used as achiral components and mixed with different substituted benzoylcellulose derivatives as chiral components. Systematic investigations were carried out to determine the optimal ratio for the components of the stationary phase. Depending on the chromatographic mode—gas chromatography (GC) or supercritical fluid chromatography (SFC)—the stationary phases were found to behave differently. The applicability of the technique was demonstrated by the resolution of various racemic compounds. © 1993 Wiley-Liss, Inc.