Chiral Separations: Fundamental Review 2010 - Analytical Chemistry

May 24, 2010 - A monoclonal anti-d-hydroxy acid antibody was used as a CSP suitable for the enantioseparations of free α-hydroxy acids (160). A CSP w...
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Anal. Chem. 2010, 82, 4712–4722

Chiral Separations: Fundamental Review 2010 Timothy J. Ward* and Karen D. Ward Millsaps College, 1701 N. State Street, Box 150306, Jackson, Mississippi 39210 REVIEWS

Review Contents Reviews Various Technique Reviews Capillary Electrophoresis (CE) High Performance Liquid Chromatography (HPLC) Supercritical Fluid Chromatography (SFC) Gas Chromatography (GC) High Performance Liquid Chromatography (HPLC) Polysaccharide CSPs Cyclodextrin CSPs and Mobile Phase Additives Macrocyclic Antibiotic CSPs Crown Ether CSPs Protein CSPs Brush/Pirkle-Type CSPs Ligand-Exchange and Ion-Exchange CSPs Molecularly Imprinted Polymer (MIP) CSPs Miscellaneous CSPs Capillary Electrophoresis Cyclodextrins Ligand Exchange Capillary Zone Chromatography Micelles Miscellaneous Capillary Electrochromatography Gas Chromatography Supercritical Fluid Chromatography Thin Layer Chromatography Simulated Moving Bed Chromatography Countercurrent Chromatography Microtechnology Literature Cited

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This fundamental review article covers developments and applications in chiral separations from January 2008 to January 2010 and is restricted to the English language literature. Judging from the number of articles and reviews appearing in the literature worldwide, it is clear that the study and application of chiral separations is soundly entrenched and used across a broad array of disciplines. From the number of papers published over the past two years, high performance liquid chromatography (HPLC) and capillary electrophoresis (CE) continue to be two of the most heavily utilized techniques. Between these two techniques, HPLC is by far the dominantly used method, with a conservative estimate of over 1000 publications appearing using chiral HPLC in numerous journals and languages. With the tremendous number of publications in this field, a comprehensive review of all published papers is not feasible; thus, this fundamental review focuses on major developments and trends in the field of chiral separations as well as representative applications. * To whom correspondence should be addressed. Phone: (601) 974-1405. Fax: (601) 974-1401. E-mail: [email protected].

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This review period continued with a significant number of reviews having been published, reflecting the interest and activity in the field of chiral separations. This article first presents a number of very useful reviews covering the various applications and methods utilizing chiral separation techniques, followed by an examination of the specific applications utilized by each technique and chiral resolving agent. Various Technique Reviews. Preinerstorfer et al. published a review covering enantioselective separations by electromigration techniques such as capillary electrophoresis (CE), micellar electrokinetic chromatography (MEKC), microemulsion electrokinetic chromatography (MEEKC), capillary electrochromatography (CEC), and microchip CE from the literature from 2007 to mid-2008, with particular attention given to new chiral selectors and mechanism studies (1). The use of vancomycin degradation products as chiral stationary phases (CSPs) used in high performance liquid chromatography (HPLC) and CE and as chiral mobile phase additives was reviewed, with the chiral recognition mechanisms involved, including the role of dimerization, also presented (2). The chiral resolution capabilities of monosubstituted positively charged cyclodextrins in CE and chromatographic techniques such as HPLC and supercritical fluid chromatography (SFC) were investigated (3). Chiral stationary phases containing macrocylic antibiotics and their use in HPLC are discussed, as well as the structural properties of macrocyclic antibiotics and their additional application in SFC, thin layer chromatography (TLC), CE, and CEC (4). The use of polysaccharide CSPs in liquid chromatography was overviewed, including uses in HPLC, CEC, SFC, and TLC (5). The structure control of polysaccharide derivatives for CSPs for HPLC and SFC was presented (6). Generic screening approaches and separation strategies for chiral separations from 1998 to 2008 were reviewed, with overviews of the different techniques included (7). Progress in the development of enantioselective separation of D-amino acids in biological matrixes included techniques such as gas chromatography (GC), high performance liquid chromatography (HPLC), electrokinetic chromatography (EKC), and MEKC (8). A review concerning enantioselective chromatography to discriminate between biotic and abiotic transformation processes of chiral environmental pollutants was given, with HPLC, GC, CE, and supercritical fluid chromatography (SFC) methods discussed (9). The enantiomeric analysis of chiral pyrethroids using GC, HPLC, CE, subcritical fluid chromatography, and SFC was reviewed (10). Immobilized penicillin G acylase and its use as CSP for the LC separation of acidic enantiomers was presented, with the chiral behavior of penicillin G acylase in partial filling CE also discussed (11). 10.1021/ac1010926  2010 American Chemical Society Published on Web 05/24/2010

Capillary Electrophoresis (CE). Capillary electrophoresis remains a popular technique for the enantioseparation of biologically active compounds. An overview of current (2008) regulatory guidance and the validation parameters required for different analytical procedures including chiral separation by CE was discussed (12). Chiral separations by CE for performing high throughput drug discovery and development applications were reviewed (13). A review of advanced CE methods for the chiral analysis of drugs, metabolites, and biomarkers in biological samples was given (14). CE and column chromatography methods were reviewed for the chiral analysis of amino acids in biological fluids and tissues (15). The chiral resolution of environmental pollutants by CE was reviewed, with various aspects such as chiral selectors, optimization of conditions, detection, validation of the methods, and chiral recognition mechanisms discussed (16). Advances in the application of CE for food analysis were reviewed for June 2005-March 2007 (17), and advances during the time period of April 2007-March 2009 were also reviewed (18). The latest methodology and instrumental improvements for enhancing sensitivity in chiral separations in CE was reviewed from March 2007 to May 2009, including microchip technologies (19), and the time from January 2005 to March 2007 was also reviewed (20). Nonaqueous CE was reviewed from 2005 to the summer of 2008, with a section on chiral separations (21). The recent developments and applications of cyclodextrins in CE enantioseparations were reviewed between January 2006 and January 2008, with most applications in aspects of life sciences such as drug analysis, bioanalysis, environmental analysis, and food analysis (22), and another review also examined the role of cyclodextrins in chiral CE, with the most frequently used cyclodextrins and their derivatives characterized (23). A review covering reports from 2001 to December 2008 shows the significant contribution of ionic liquids in the improvement of CE and microchip electrophoresis, and separation of enantiomers was also discussed (24). Advances in microchips for chiral separations from 2003 to early 2009 are discussed, including fabrication, chemical and detection issues, and multifunctional microchips (25). An overview of chiral separation principles and their applications in capillary electromigration techniques was given by Guebitz and Schmid (26), with recent developments in capillary zone electrophoresis and capillary electrochromatography (CEC) included. The different types of monoliths used in CEC and pressurized CEC for enantioseparations and the analysis of pharmaceutically relevant molecules was overviewed, with the methods of monolith preparation and modification also discussed (27). High Performance Liquid Chromatography (HPLC). The strategies and chiral HPLC techniques developed for enantiomeric resolution and analysis of chiral drugs was the focus of a review by Alves et al. (28). Recent developments in the use of microscale chiral HPLC for the support of pharmaceutical process were reviewed, including topics on rapid multiparallel chromatography method development, online reaction monitoring, and green chemistry advantages (29). The use of simulated moving bed chromatography (SMB) for the separation of enantiomers was reviewed, with emphasis on the areas of design, improved process schemes, optimization, and robust control (30).

The retention mechanism of HPLC enantioseparation on macrocyclic glycopeptide-based CSPs was reviewed, and the physicochemical properties were characterized (31). A summary of the use of immobilized chiral packing material for HPLC based on polysaccharide derivatives to enhance the versatility of eluent selection for practical and economical HPLC enantiomeric separation methods was presented (32). Commercial CSPs based on polysaccharides, cellulose, and amylose and their derivatives were reviewed by Okamoto (33). The use of various CSPs in liquid chromatography-tandem mass spectrometry methods was presented, with a discussion of interface and matrix effects (34). Current improvements for direct enantiomeric separation of profens by HPLC including monolithic, combinatorial, bimodal, and polymeric CSPs was reviewed (35). Supercritical Fluid Chromatography (SFC). A brief historical review of SFC and current trends is presented with chiral separations also discussed (36). Chiral separation applications in sub- and supercritical fluid chromatography was reviewed from 2000 to 2008 (37). The use of SFC for chiral pharmaceutical analytical and preparative separations was reviewed (38), and commercial CSPs used in the enantioseparation of clinical racemic drugs by SFC was presented (39). Gas Chromatography (GC). The use of GC in the enantioseparation of unfunctionalized chiral alkanes was overviewed, with a discussion of the different modified cyclodextrins used as CSPs (40). HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) HPLC is by far the most popular method of choice for performing chiral separations, with applications using this techniques appearing nearly twice as often as all the other techniques combined. Of the CSPs used for chiral separations in HPLC, the polysaccharides are the most popular CSP employed followed by the macrocyclic antibiotic and cyclodextrin phases, respectively. The crown ether phases also remain in moderate use as do a number of other miscellaneous CSPs such as brush type and ligand exchange; however, the number of applications reported using protein-based CSPs have continued to decline in recent years. Polysaccharide CSPs. Polysaccharide-based CSPs are by far the most dominant and widely used CSP with over 500 publications appearing in English language journals alone during this two year review period. Common approaches for efficient method development with the polysaccharide CSPs Chiralpak IA, Chiralpak IB, and Chiralpak IC were overviewed showing enantiomeric resolutions with reasonable time frames and high success rates (41). A screening strategy for the use of the Chiralpak IA, IB, and IC columns was also published (42). The commercial Chiralpak AD CSP was used in a study of the perturbing effects of the CSP on enantiomerization second-order rate constants to give a practical tool to quantify the accessible acid and basic catalytic sites bonded to the stationary phase (43). The Chiralcel OJ-H CSP exhibited unusually high enantioselectivity in the HPLC resolution of N-thiocarbamoyl-3-(4′-prenyloxy)-phenyl-5-phenyl-4,5dihydro-(1H) pyrazole, and the method used the stopped flow technique to modulate the elution time of the longer retained enantiomer (44). Comparative studies between covalently immobilized CSPs (Chiralpak IA and IB) and coated-type CSPs Analytical Chemistry, Vol. 82, No. 12, June 15, 2010

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(Chiralpak AD and Chiralcel OD) were performed using Nprotected R-amino acids and their ester derivatives as analytes (45). The key interactions of norephedrine with three commercially important polysaccharide-based CSPs were studied in detail (46). The effects of the molecular structure for 13 structurally similar chiral solutes on HPLC retention and enantioresolution on a polysaccharide CSP were studied (47). Quantitative structure enantioselective retention relationship for HPLC chiral separation of 1-phenylethanol derivatives was obtained using a Chiralcel OB-H CSP (48). A validated enantioselective HPLC method for the quantification of (R)-(+)- and (S)-(-)-lansoprazole enantiomers and related impurities was developed and used a Chiralpak IA CSP (49). Chiralpak IA was again used in the enantioseparation of flavanone and 2′-hydroxychalcone under reversed phase conditions (50), and Chiralpak IA was also used in the chiral resolution of omeprazole and other proton pump inhibitors (51). The HPLC enantioseparation of seven binaphthyl compounds was achieved under normal phase condition on the Chiralpak IA CSP (52). The direct enantiselective HPLC monitoring of lipase-catalyzed kinetic resolution of tiaprofenic acid used a Chiralpak IB CSP with nonstandard organic solvents (53). The detection of pindolol enantiomers in plasma and pharmaceutical products by HPLC with fluorescence detection was accomplished using a Chiralpak IB column (54). The Chiralpak IC CSP was used in the HPLC enantioanalysis of tertalolol enantiomers (55) and also in the direct HPLC enantioseparation of terazosin (56). A Chiralpak AD-H column was used in the validated chiral LC method for the enantiomeric separation of duloxetine (57); Chiralpak AD-H was used in a HPLC method for the direct chiral resolution of the pesticide tetramethrin in soil (58), and the chiral separation of two enantiomers of T-3811ME also used a Chiralpak AD-H CSP (59). The enantiomers of benzoxazolinone derivatives were enantioresolved using Chiralpak AD and ChiralpakAS, with the best separation observed on Chiralpak AS and used for preparative HPLC (60). The enantioselective analysis of praziquantel and trans-4-hydroxypraziquantel in human plasma by chiral LC-MS/MS was accomplished using a Chiralpak AD column (61), and Chiralpak AD-H and Chiralcel OD-H columns were used in the enantioselective HPLC resolution of synthetic intermediates of armadafinil (62). The Chiralpak AD CSP was used in the preparative enantiomeric separation of the potent AMP-activated protein kinase activator (63). The HPLC chiral separation of 21 kinds of 2-aryl-1,3-dicarbonyl analogs was studied in normal phase mode on various polysaccharide CSPs, with Chiralpak AD-H giving the best enantioselectivity and enantioseparation (64). The enantioseparation of the enantiomers of novel phenylethanolamine derivatives was achieved on Chiralpak AS-H (65), and the chiral resolution of the enantiomers of eight substituted 4-oxo-1,4dihydroquinoline-3-carboxamide derivatives was accomplished on Chiralpak AD-H and AS (66). The HPLC chiral separation of ofloxacin using Chiracel OD-H was developed (67); the enantioresolution of the antidepressant reboxetine also used Chiralcel OD (68), and Chiracel OD-H and Chiralpak OT(+) were used in the HPLC enantioseparation of 29 chiral bridged polycyclic compounds (69). Enantiomeric separations of racemic 1-benzyl-N-methyltetrahydroisoquinolines were achieved using Chiralcel OD-H and OJ-H columns (70). The first 4714

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complete HPLC resolution of all five isomers of Brivanib Alaninate as carbobenzyloxy derivatives was achieved on a Chiralcel OJ-H (71), and the Chiralcel OJ-H CSP was also used in the quantification of ibuprofen enantiomers (72). Five polysaccharide-based CSPs were used for separation of enantiomers of fourteen heterocyclic oxiranes, including Chiralpak AD and AS and Chiralcel OD, OG, and OJ, and the Chiralpak AS was found to have the greatest chiral separation ability for these solutes (73). The enantiomers of linezolid were separated by HPLC on a Chiralcel OJ-RH CSP in the reversed phase (74), and Chiralcel OD and Chiralcell OJ were used in the enantioseparation of seven triazole fungicides (75). The enantioseparation of Nfluorenylmethoxycarbonyl R-amino acids was achieved on three polysaccharide-derived CSPs, namely Chiralcel OD, Chiralpak AD, and Chiralcel OJ (76). A CelluCoat CSP was used in the HPLC chiral separation of 10 β-adrenergic blockers, with data indicating that the chiral resolutions were governed by π-π interactions, hydrogen bonding, and steric effects (77). Cellulose tris(3,5dimethylphenylcarbamate)-based CSPs were found to be effective tools for the enantioselective HPLC separation of structurally different disubstituted binaphthyls (78), and semipreparative separation conditions were proposed. Sixteen β-adrenergic agonists and antagonists were enantioresolved on AmyCoat CSP (79), and the AmyCoat RP column was also used in the chiral separations of four racemic imidazole antifungal drugs (80). Cyclodextrin CSPs and Mobile Phase Additives. The commercially available derivatized cyclodextrin HPLC columns Cyclobond I 2000 DMP and Cyclobond DNP were used in the enantioseparation of unusual β-amino acids (81). The Cyclobond I 2000 DM CSP was used in the validated chiral separation of sertraline and its related enantiomers (82). A validated HPLC method for the enantioseparation of clopridogrel was developed on the ChiraDex column (83). A novel chiral cyclodextrin-based CSP, containing mono[6AN-1-(2-hydroxy)-phenylethylimino-6A-deoxy]-β-cyclodextrin, showed good enantioselectivity for several alkylaromatic alcohols and a variety of ferrocene derivatives (84). Another novel CSP based on mono(6A-azido-6A-deoxy)-per(p-chlorophenylcarbamoylated)β-cyclodextrin was used in the enantiomeric separation of 11 piperazine derivatives (85). Four cationic β-cyclodextrin derivatives were synthesized and used as CSPs in HPLC and SFC for the enantioseparation of 18 racemic aryl alcohols (86). Calix (4) arenecapped [3-2-O-β-cyclodextrin)-2-hydroxyproxy]propylsilyl-appended silica particles were synthesized and used as a CSP for the separation of several disubstituted benzenes and some chiral drug compounds under reversed phase conditions (87). Another cyclodextrin derivative, mono-6-(3-methylimidazolium)-6-deoxyperphenylcarbamoyl-β-cyclodextrin chloride, was synthesized for use as HPLC and SFC CSPs for enantioseparation of racemic aromatic alcohols (88). A click chemistry strategy was used in the preparation of novel native β-cyclodextrin CSP and evaluated by the enantioseparation of various racemic compounds including ketoprofen and benzoin (89). A click chemistry-derived native β-cyclodextrin CSP was used in the enantioseparation of various racemates, including dansylated amino acids, flavonoids, and some pharmaceutical compounds (90), and another click chemistry β-cyclodextrin CSP was tested with the enantioseparation of flavone glycosides (91).

Derivatized cyclodextrins were also used as chiral mobile phase additives in HPLC. A fast enantiomeric separation of a chiral aromatic amino acid was achieved using highly sulfated β-cyclodextrin as a chiral additive in the mobile phase in an ultra high pressure liquid chromatography method (92). The enantiomeric purity of S-(-)pantoprazole was determined using HPLC with sulfobutylether-β-cyclodextrin as the chiral additive (93). Carboxymethyl-β-cyclodextrin was used as a chiral selector in the enantioselective determination of chlorpheniramine in various commercial formulations (94). Macrocyclic Antibiotic CSPs. Macrocyclic antibiotics, primarily teicoplanin and also vancomycin and ristocetin A, are also commonly used for enantioseparations in HPLC. Chiral stationary phases containing teicoplanin (Astec Chirobiotic T and T2), teicoplanin aglycon (Chirobiotic TAG), or ristocetin A (Chirobiotic R) were used in the direct separation of the enantiomers of four 2-aminomono- or dihydroxycyclopentanecarboxylic acids and four 2-aminodihydroxycyclohexanecarboxylic acids, with Chirobiotic TAG and in some cases Chirobiotic T proving to be the most useful (95). The Chirobiotic T and T2 and TAG CSPs were used in the HPLC chiral separation of β-2-homoamino acids (96) and of β-2- and β-3-homoamino acids (97). The CSPs Chirobiotic T and T2, with different teicoplanin coverage and distinct linkage chemistry, were compared for the enantioseparation of structurally diverse compounds (98). Chiral tris-(1,10-phenanthroline) iron(II) complexes were resolved by HPLC on CSPs containing either teicoplanin or a cellulose derivative (99). Teicoplanin aglycone was again used in the enantioseparation of aryl-methyl sulfoxides, and a thermodynamic study of retention was reported (100). The Chirobiotic T2 CSP was found to be the most effective at the baseline enantioseparation of seven ruthenium(II) polypyridyl complexes (101). A teicoplanin-based CSP was prepared and used in the enantioseparation of some dipeptides and amino acids in a binding study using nonlinear liquid chromatography (102). The temperature effects on enantioseparation of chiral sulfoxides in rat serum was investigated using a methylated-teicoplanin aglycone CSP in polar organic mode (103). The Chirobiotic TAG CSP was used in a direct HPLC method for the resolution and quantification of the R-(-)- and S-(+)-enantiomers of vigabatrin in pharmaceutical products (104). CSPs containing teicoplanin and teicoplanin aglycon were used in the separation of triiodothyronine and thyroxine enantiomers (105). Molindone enantiomers in human plasma were determined using HPLC-tandem mass spectrometry using a Chirobiotic TAG CSP (106). The vancomycin-based Chirobiotic V2 CSP was used in a HPLC method for the determination of (R)- and (S)-warfarin (107). A validated HPLC method for the separation and determination of terbutaline enantiomers used a Chirobiotic V column (108). Vancomycin crystalline degradation products were used as CSPs for microcolumn LC methods for the enantioseparation of a variety of chiral compounds (109, 2, 110). Two new chiral and restricted-access materials containing glycopeptide antibiotics as chiral selectors used either teicoplanin or teicoplanin aglycon for the HPLC determination of chiral drugs in biological matrixes (111). A novel chiral selector, eremomycin, was first evaluated as a chiral selector in CE, then immobilized on silica, and evaluated for use as a CSP in HPLC (112).

Crown Ether CSPs. Two types of commercially available crown ethers stationary phases (Crownpak CR (+) and Chirosil RCA (+)) were used in the HPLC enantioseparation of amino alcohols (113). Two new HPLC CSPs based on diastereomeric chiral crown ethers incorporating two different chiral units such as optically active 3,3′-diphenyl-1,1′-binaphthyl and tartaric acid units were prepared and studied (114). A new crown ether-based CSP containing thioester linkages was prepared and found to have greater chiral recognition ability than the amide linkages in most cases (115). CSPs based on optically active (3,3′-diphenyl-1,1′binaphthyl)-20-crown-6 were used for the resolution of racemic β-amino acids by HPLC (116), aryl R-amino ketones (117), and fluoroquinolone antibacterials (118). HPLC CSPs based on (+)(18-crown-6)-2,3,11,12-tetracarboxylic acid were used in the separation of enantiomers of β-amino acids (119, 120) and β-3homoamino acids (121). Finally, synthetically complex crown ether CSPs were found to be applicable in the microscale format (122). Protein CSPs. Protein-based CSPs were not used in many publications during this time period, showing a steady drop off since 2008. The analytical and preparative enantioseparation of DL-penicillamine and DL-cysteine by HPLC was accomplished using R-glycoprotein (AGP) and β-cyclodextrin columns (123). Chiral HPLC analysis of formoterol stereoisomers was conducted on a Chiral-AGP column (124). An affinity silica monolith containing human serum albumin (HSA) was developed and evaluated for use in chiral separations, and it was concluded that the silica monolith can be a valuable alternative to silica particles when used with immobilized HSA as a CSP (125). Brush/Pirkle-Type CSPs. A molecular dynamics study of chiral recognition for the Whelk-O1 CSP was performed (126, 127). A simple method for the prediction of the separation of racemates with HPLC on Whelk-O1 CSP was developed (128). The enantiomeric resolution of racemic dispiro cyclotriphosphazene derivatives was accomplished on the (R,R)-Whelk-O1 (129). The WhelkO1 CSP was also used in the enantioseparations of nonbenzenoid and oligo-Troeger’s bases (130). The performance of Pirkle-type CSPs and polysaccharide-based CSPs for the resolution of the enantiomers of six phthalans was evaluated (131). A new Pirkletype CSP was obtained using activated Sepharose 4B as a matrix, L-tyrosine as a spacer arm, and an aromatic amine derivative of L-glutamic acid as a ligand, and this CSP was used in the enantiomeric resolution of (±)-β-methylphenylethylamine (132). A brush-type CSP was prepared in-column using click chemistry and a silica monolith to synthesize a proline-derived CSP and was used to enantioseparate four model analytes (133). Ligand-Exchange and Ion-Exchange CSPs. Three cysteinebased coated chiral selectors were used in the ligand-exchange separation of a selected set of natural and unnatural underivatized amino acids (134), and two of these selectors were used in a study of the molecular properties of the analyte affecting the enantiodiscrimination process (135). The effect of the copper(II) salt anion in the chiral ligand-exchange chromatography of amino acids when S-trityl-(R)-cysteine was used as the chiral selector was described (136). A porous graphitic carbon column coated with a new dinaphthyl derivative of neamine was prepared for the chiral ligandexchange chromatographic separation of amino acids (137). A Analytical Chemistry, Vol. 82, No. 12, June 15, 2010

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new class of ligand-exchange CSPs based on the copper complexes of lipophilic aminoglycoside derivatives was evaluated for amino acid enantioresolution (138). Zwitterionic CSPs for separations of chiral acids, bases, and amino acids by HPLC were prepared by fusing Cinchona alkaloid chiral weak anion exchangers and aminosulfonic acid-based strong cation exchangers (139), and systematic studies of the ionexchange-type mechanisms were made (140). Mobile phase contributions to enantioselective anion- and zwitterion-exchange modes on quinine-based zwitterionic CSPs were investigated (141). An in-line coupling of a reversed phase column to cope with the limited chemoselectivity of a quinine carbamate-based anion-exchange-type CSP was reported for the enantioseparation of the thyroid hormone thyroxine and its structural analog triiodothyronine (142). Molecularly Imprinted Polymer (MIP) CSPs. A noncovalent in situ molecular imprinting polymerization protocol was used to synthesize a S-ketoprofen molecularly imprinted polymer (MIP), which was then used in the enantioseparation of naproxen, ibuprofen, and fenoprofen by LC (143). A novel MIP prepared with bonded β-cyclodextrin and acrylamide on functionalized silica gel selective for tryptophan was found to have superior selectivity (144). One monomer MIPs and traditional MIPs were compared in terms of their multianalyte imprinting capability (145). Separation of phenylalanine racemates was accomplished using Dphenylalanine imprinted microbeads as HPLC CSPs (146). Miscellaneous CSPs. Unusual CSPs based on derivatized cyclofructans were developed and used in the enantioseparation of a broad range of racemic compounds, also demonstrating high loadability with good potential for preparative separations (147). A novel cyclic hexapeptide was immobilized to the surface of a monolithic support, and the enantioselectivity and performance of this CSP for the separation of dansyl amino and arylalkanoic acids was discussed (148). Chiral macrocycles with hydrogen bond donor/acceptor sites in the cavity were synthesized and covalently bonded to silica gel to give CSPs, which were used to enantioresolve various chiral compounds (149). Two new synthetic polymeric CSPs based on trans-(1S,2S)cyclohexanedicarboxylic acid bis-4-vinylphenylamide and transN,N′-(1R,2R)-cyclohexanediyl-bis-4-ethenylbenzamide monomers were prepared, evaluated by normal phase HPLC and SFC, and used to enantioseparate a variety of chiral compounds, with striking differences in the enantioselectivities of the two CSPs (150). Optically active copolymers of N-(oxazolinyl)phenylmaleimides with methyl methacrylate were coated on macroporous silica for use as a CSP for HPLC and used to resolve enantiomers of some compounds including amino and hydroxy-acids (151). A polymer-type CSP was derived from (1S,2R)-(+)-2-amino-1,2diphenylethanol and evaluated with chiral analytes (152). Different linkages to aminated silica gel were prepared with (1S,2R)-(+)2-amino-1,2-diphenylethanol, and the influence of the different linkages on enantioseparation was discussed (153). Triproline and tri-R-methylproline CSPs were evaluated with analytes having none, one, two, or three hydrogen-bond donors (154). Polyproline derivatives were used as CSPs, and chromatographic and conformational studies were discussed (155). Dendritic CSPs with surface-bonded L-phenylalanine were synthesized and evaluated for HPLC resolution of racemic compounds (156). 4716

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A novel CSP based on chiral aminoalcohol immobilized on mesoporous silica was used for the separation of different racemic compounds (157). Urea- and imide-bearing chitosan phenylcarbamate derivative CSPs were prepared and evaluated as CSPs for HPLC (158). A covalently bonded DNA aptamer CSP was prepared for the chromatographic resolution of adenosine and was found to be stable over time (159). A monoclonal anti-D-hydroxy acid antibody was used as a CSP suitable for the enantioseparations of free R-hydroxy acids (160). A CSP was prepared by bonding N-(3,5-dimethoxybenzoyl) cefaclor to spherical silica gel and was used for the resolution of racemic N-(3,5-dinitrobenzoyl)R-amino acid derivatives and other compounds (161). A tertbutylcarbamoylquinine-based CSP (Chiralpak QN-AX) was used in the enantioseparation of five underivatized chiral acidic amino acids, and baseline resolution of all five test solutes was achieved (162). A quinine tert-butyl-carbamate-type CSP was studied for a set of polar [1,5-b]-quinazoline-1,5-dione derivatives (163). A HPLC method for chiral separation of tenofovir enantiomers was developed using L-phenylalanine as a chiral mobile phase additive (164), and the method was validated. CAPILLARY ELECTROPHORESIS Cyclodextrins. Capillary electrophoresis (CE) continues to be used extensively with cyclodextrins remaining by far the most popular chiral selector used in CE. Their stability and wide array of selectivity offered through the various cyclodextrin derivatives available account for their widespread popularity in the field. An interesting study was published about the role of enantioselective selector-selectand interactions and the mobilities of the temporary diastereomeric associates in enantiomeric separations using cyclodextrins in CE, indicating that enantioselective binding of the analyte to the chiral selector may not be required for enantioseparation (165). NMR and molecular modeling were performed to study the chiral recognition process occurring in the CE enantioseparation between linezolid and anionic singleisomer cyclodextrin-heptakis-(2,3-diacetyl-6-sulfato)-β-cyclodextrin (166). CE with β-cyclodextrin as a chiral selector was used in the enantioseparation of racemic salsolinol, N-methylsalsolinol, and 1-benzyltetrahydroisoquinoline, and a computer modeling study of the inclusion complexes was reported (167). A separation selectivity model for chiral CE separations of dipeptides using heptakis-(2,6-di-O-methyl)-β-cyclodextrin as the chiral selector was described as a function of buffer pH and chiral selector concentration, and the model was used to rationalize the pH-dependent reversal of the migration order of selected dipeptides (168). The enantioseparation of the plant lignan matairesinol was accomplished by CE using a noncoated silica capillary with carboxymethyl-β-cyclodextrin as the chiral selector (169). The enantiomers of nateglinide and its geometry isomers were separated simultaneously by CE with a mixture of cyclodextrin derivatives as chiral selector (170). A chiral CE method was developed for the separation of imazethapyr enantiomers in soil using hydroxypropyl-β-cyclodextrin (171). The enantioseparation of aminoglutethimide in pharmaceutical formulations was achieved using CE with methylated-β-cyclodextrin as chiral selector, and computational calculations for the inclusion complexes were reported (172). A nonaqueous CE method was developed for the enantioseparation of flurbiprofen racemate in plasma samples using 6-monodeoxy-6-mono(3-hydroxy)propylamino-β-cyclodextrin

as chiral selector (173). Trihexyphenidyl enantiomers in human serum were enantioseparated and quantified using carboxylmethyl-β-cyclodextrin as chiral selector in CE (174). An analytical chiral CE method for the enantiomeric purity detection of fluvastatin enantiomers was developed using (2-hydroxypropyl)β-cyclodextrin as the running buffer (175). A method for the simultaneous determination of the impurities of escitalopram including the R-enantiomer and (S)-citadiol was validated, and the chiral selectors used were β-cyclodextrin and sulfated β-cyclodextrin (176). Ofloxacin enantiomers in pharmaceutical formulations were enantioseparated by CE using carboxymethyl-βcyclodextrin (177). Highly sulfated R-, β-, and γ-cyclodextrins were used in the enantioseparation of pharmaceutical compounds by multiplexed CE, and the system was evaluated for high-throughput screening of enantiomers of solutes of pharmaceutical interest (178). Sulfated and neutral cyclodextrins were used in a comparison of the chiral separation of basic drugs in CE and HPLC (179). A CE method for the separation of primaquine enantiomers used hydroxypropylγ-cyclodextrin as chiral selector and was successfully applied to pharmaceutical formulations (180). The analysis of the enantiomers of chiral pesticides and other small molecules in a variety of environmental matrixes used cyclodextrin chiral selectors and also the addition of a micelle-forming compound (181). Ligand Exchange. Chiral ligand-exchange CE for the enantioseparation of dansyl-amino acids with Zn(II) L-arginine complex as a chiral selecting system was used to resolve 17 pairs of amino acid enantiomers (182). Underivatized amino acids were enantioseparated by ligand-exchange CE in a counter-electroosmotic mode using copper(II) and a chiral selector of either L-proline or trans-4-hydroxy-L-proline (183). A Zn(II)-L-ornithine complex was used as a chiral selecting system for the enantioseparation of amino acids by chiral ligand-exchange CE (184). Amino acid ionic liquids were used as chiral ligands in ligand-exchange chiral CE and HPLC in the separation of four pairs of underivatized amino acid enantiomers (185). Capillary Zone Chromatography. A linear high-molecular weight homopolysaccharide, polygalacturonic acid, was used as a novel chiral selector in capillary zone electrophoresis (CZE) for the separation of enantiomers of basic drugs (186). Amlodipine enantiomers in urine samples were enantioseparated using online coupled isotachophoresis-CZE with 2-hydroxypropyl-β-cyclodextrin as chiral selector (187), and amlodipine enantiomers in pharmaceuticals were also separated using CZE and diode array detection (188). Cyclodextrin-modified CZE was used in a simple model for the enantioseparation of binaphthol and its monoderivatives (189). The chiral separation of three hydroxyflavanone aglycons was achieved in CZE using sulfated cyclodextrins as chiral selectors (190). A study of the reversal of order of migration of enantiomers as a function of cyclodextrin concentration and pH, using native and derivatized cyclodextrin-modified CZE was performed (191). A new CZE method for the determination of lansoprazole enantiomers using β-cyclodextrin was used to analyze three different pharmaceutical preparations, with recoveries of 91-102% of the label content (192). Carboxymethyl-β-cyclodextrin was used as chiral selector in the enantioseparation of cefotaxime enantiomers by CZE (193).

Micelles. A novel amphiphile, N-(2-hydroxydocecyl)-L-threonine was used in the enantiomeric separations of binaphthyl derivatives by micellar EKC (194). Dodecyl thioglycoside surfactants were used in MEKC for the enantiomeric separation of dansylated amino acids (195). Enantiomers of five Profen drugs were simultaneously separated by MEKC with the combined use of chiral ionic liquid and cyclodextrin (196), and binding constants were determined (197). An online stacking technique was used in the chiral separation of amino acids by MEKC using hydroxypropyl-β-cyclodextrin as chiral selector (198). A comparison between cyclodextrin-modified MEKC and cyclodextrin-modified microemulsion electrokinetic chromatography (MEEKC) for the enantiomeric separation of esbiothrin was studied (199). The chiral separation of raltiterxed by MEKC using carboxymethylβ-cyclodextrin was achieved, and the usefulness of the method was demonstrated in a purity test of a real synthetic drug sample (200). The enantioseparation of 1,1′-binaphthyl-2,2′-diamine was optimized by MEKC using a polymeric surfactant as a pseudostationary phase (201). A multivariate approach for the enantioselective analysis of binaphthyl derivatives was used in MEKCelectrospray ionization-mass spectrometry (202). Miscellaneous. A novel chiral selector, shinorhizobial linear octasaccharide, was used in the CE enantioseparation of some flavanones such as homoeriodicytol, hesperetin, naringenin, and isosakuranetin (203). A CE method using a high concentration of amylose solutions was used in the chiral separations of primaquine, trihexyphenidyl, sulconazole, and cetirizine (204). CAPILLARY ELECTROCHROMATOGRAPHY Capillary electrochromatography (CEC) has experienced moderate growth as indicated by the increased number of papers appearing in the literature compared to the previous review period. Two new negatively charged sulfate and sulfonated polysaccharide CSPs were introduced for capillary electrochromatography, with the sulfonated CSP providing fast throughput and excellent resolving power (205). The sulfonated CSP was also evaluated for CEC-MS enantioseparations, showing excellent durability and reproducibility. Phospholipid-protein coatings were developed for the chiral CEC separation of D- and L-tryptophan, with the immobilized bovine serum albumin showing better chiral selectivity than lysozyme, R-chymotrypsin, or avidin (206). A vancomycin CSP was used in the simultaneous analysis of beta-blockers with multiple chiral centers using CEC-MS (207), and this method has the potential for use as a screening method for the analysis of multiple chiral compounds using a single protocol using the same column and mobile phase conditions. The chiral separation of a set of racemic π-acidic and π-basic R-amino acid amides was achieved using (S)-N-(3,5-dinitrobenzoyl)leucine-N-propylamide-bonded silica as the CSP and electrolyteless acetonitrile-water eluents by CEC in the reversed phase and polar organic modes (208). Cellulose dimethylphenylcarbamate-immobilized zirconia was used as a CSP for the enantioresolution of a set of nine racemic compounds in reversed phase CEC, with the zirconia derivative showing better enantioselectivity and resolution than cellulose dimethylphenylcarbamate (209). Successive multiple ionic polymer layer coatings for capillaries were used for the CEC analysis of binaphthyl enantiomers, with a singlestranded DNA-containing coating found to provide the best chiral separation (210). A novel chiral separation of dansyl amino acids Analytical Chemistry, Vol. 82, No. 12, June 15, 2010

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by ligand exchange CEC in a low molecular weight organogel was demonstrated by the enantioseparation of five pairs of danslated amino acids by copper ligand exchange on a trans(1S,2S)-1,2-bis-(dodecylamido) cyclohexane (I) gel in methanol (211). Three novel derivatized β-cyclodextrins, pernaphthylcarbamoylated β-cyclodextrin, peracetylated β-cyclodextrin, and permethylated β-cyclodextrin, were used as CSPs for capillary liquid chromatography and pressure-assisted capillary electrochromatography (212). A novel chiral selector was prepared by the application of click chemistry-based (213) perphenylcarbamated β-cyclodextrin CSP in CEC for the enantioseparation of aryl alcohols (214). CSPs were prepared by the immobilization of β-cyclodextrin and three of its derivatives to the epoxy-activated poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith for the separation of eight amino acids and two chiral drugs with CEC (215). Another study described the polymethacrylate-based monolithic column prepared from a novel β-cyclodextrin derivative bearing 4-dimethylamino-1,8-naphthalimide and used for the CEC separation of chiral acidic enantiomers such as ibuprofen and naproxen (216). An open tubular capillary column with a monolithic layer of S-ketoprofen imprinted and 4-styrenesulfonic acid incorporated polymer was prepared and shown to be effective in the enantioseparation of ketoprofen by CEC (217). Long, open tubular, S-ketoprofen molecularly imprinted polymer (MIP) capillary columns were prepared for the chiral CEC separation of racemic ketoprofen (218). Silica-based monoliths modified with a novel aminosulfonic acid-derived strong cation exchanger were prepared for CEC for the enantioselective separations of various chiral bases (219). Molecularly imprinted hybrid monoliths were fabricated via a room temperature ionic liquid-mediated nonhydrolytic sol-gel route for the chiral separation of zolmitriptan by CEC (220). A novel CEC method using MIP nanoparticles showed monoclonal binding behavior in the enantioseparation of racemic propranolol, where (S)-propranolol was used as a template (221). GAS CHROMATOGRAPHY Gas chromatography remains a popular technique for chiral separations. Various CSPs for use in chiral GC were employed over this review period, with cyclodextrin-based stationary phases in particular continuing to see much use. Enantiomers of 19 racemic β-lactams were separated using GC on derivatized cyclodextrin-based CSPs (222), with Chiraldex G-TA exhibiting the broadest enantioselectivity and Chiraldex B-DM producing the fastest separations. Permethylated β-cyclodextrin in liquid poly(oxyethylene) was used to construct enantioselective capillary GC columns (223). Cyclopropane derivatives with multichiral centers were well separated on a new pyridine heterocyclic β-cyclodextrin CSP for capillary GC (224). Derivatized β-cyclodextrins were used in a study that employed conventional and narrow-bore short capillary GC columns to increase sample throughput and productivity (225). L- or D-valine tert-butylamide was grafted on permethylated β-cyclodextrin in the synthesis of two mixed binary chiral selectors (226). The enantiomers of the nerve agent VX were separated by the GC CSP Hydrodex-βTBDAc (β-cyclodextrin) (227). 4718

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Lipodex E (octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-γ-cyclodextrin) was used to coat a GC column for the chiral separation of compound B (1,1,3,3,3-pentafluoro-2-fluoromethoxy-1-methoxypropane), and structural and thermodynamic investigations were discussed (228). A new mono-6-O-benzyl methylated γ-cyclodextrin CSP was synthesized, and preliminary enantioselective GC applications were reported (229). An acyclic maltooctaose derivative was used as a CSP for enantioselective GC and was compared to its cyclic counterpart (230). Elution order was found to be reversed, and large changes in enantioselectivity were observed, in the enantioseparation of Zand E-2-ethyl-dioxaspiro[4,4]nonane, depending on whether the chiral selector was bonded (Chirasil-Nickel 1) or dissolved (ChiraNickel 2a) (231). A new CSP was prepared by the covalent attachment of a diproline chiral selector to a methylhydrosiloxanedimethyl siloxane copolymer and was used for the enantioresolution of racemic aromatic alcohol (232). SUPERCRITICAL FLUID CHROMATOGRAPHY Supercritical fluid chiral chromatography continued to be wellutilized for the separation of chiral compounds. The enantiomeric separations of six phospine-containing R-amino acid derivatives were studied using packed column SFC with two polysaccharidebased CSPs: Lux Cellulose-1 and -2 (233). The columns provided complementary selectivities, with the Lux Cellulose-1 column separating 5 of the 6 racemates and Lux Cellulose-2 resolving the sixth racemic mixture. An amylose-derived CSP, Diacel Chiralpak AD, was used in the enantioseparation of racemic precursors of trans-(-)-paroxetine by supercritical fluid chromatography (234). The enantiomers of flurbiprofen were separated by supercritical fluid chromatography on a Chiralpak AD-H CSP under both linear and nonlinear conditions (235). The chiral separation of proline derivatives using a polysaccharide-type CSP and supercritical fluid chromatography was compared to the HPLC separation, with a difference in retention interactions between the separation methods proposed (236). 2,2,2-Trifluorothanol was evaluated for use as an alternative modifier for the chiral SFC analysis of alcoholsensitive chiral compounds (237), with polysaccharide and Pirkletype CSPs used in the analyses of four alcohol-sensitive chiral compounds. A semipreparative chiral SFC separation of lansoprazole, pantoprazole, and rabeprazole was conducted, with different loads evaluated for high enantiomeric purities and production rates (238). A comprehensive approach to the development of a purification method for SFC chiral separation for an analyte that was found to be unusually difficult to scale-up was presented, using major factors such as the solubility in mobile phases, impurity profiles, and cycle time (239). Four cationic β-cyclodextrin derivatives were synthesized and coated onto porous silica gel to obtain novel CSPs (86), which were used in the enantioseparation of 18 aryl alcohols in HPLC and SFC. Another novel CSP, mono-6-(3-methylimidazolium)-6deoxyperphenylcarbamoyl-β-cyclodextrin chloride was synthesized and evaluated for use in both SFC and HPLC in the enantioseparation of racemic aromatic alcohols (88). THIN LAYER CHROMATOGRAPHY TLC continued to be applied to economical enantioseparations in this review period. A validated densitometric TLC method was

described for the analysis of (R)- and (S)-bupivacaine, using cyclodextrin derivatives as chiral selectors (240). Three different chiral selectors, L-tartaric acid, (R)-mandelic acid, and (-)erythromycin, were used in the direct resolution of racemic atenolol and propranolol by normal phase TLC on silica gel plates (241). A precoated chiral TLC plate was found suitable for the separation and quantification of lignans in Phyllanthus species (242). Chiral TLC was also employed in the study of in vitro chiral conversion in aged Profen solutions (243). SIMULATED MOVING BED CHROMATOGRAPHY The chiral separation of several racemates was achieved in this review period by simulated moving bed chromatography. An intermediate of the antidepressant Rolipram, n-boc-Rolipram, was enantioseparated on cellulose tris(3,5-demethylphenylcarbamate) CSP by SMB continuous chromatography, and the purity and enantiomeric excess of the extracted and raffinate streams were evaluated (244). The chiral separation of R,S-R-tetralol was achieved using the chiral adsorbent Chiralpak AD, and this study also presented a simulation package to predict the effect of the operating variables on the process performance (245). Microcrystalline cellulose triacetate columns were used in the chiral separation of ketamine enantiomers (246). An automated online enantiomeric analysis system was used to monitor the enantioseparation of Troeger’s base racemate using Chiralpak AD (247). The performances of different bed structures, conventional and monolith, was compared in the separation of chiral species by SMB technology (248). COUNTERCURRENT CHROMATOGRAPHY Countercurrent chromatography is a liquid-liquid chromatography technique which does not use a solid support. A cascade of six centrifugal contactor separations in a countercurrent liquid-liquid extraction mode and the use of cinchona alkaloid as a chiral hose were employed for the separation of one of the enantiomers of N-(3,5-dinitrobenzoyl)leucine in 55% yield and 98% enantiomeric excess (249). Multiple dual-mode countercurrent chromatography was applied to the chiral separation of two racemic mixtures of (+)-N-(3,4-cis-3-decyl-1,2,3,4-tetrahydrophenanthren-4-yl)-3,5-dinitrobenzamide and N-(3,5-dinitrobenzoyl)-(+)leucine, using (S)-naproxen N,N-diethylamide as chiral selector, and improved resolution factors were successfully obtained (250). MICROTECHNOLOGY Once again, chiral separations go small, with several examples in this review period of microseparations. Poly(ethylene glycol)functionalized polymeric microchips were synthesized, fabricated, and evaluated for the electrophoretic chiral separation of 10 different D,L-amino acid pairs with the addition of β-cyclodextrin as chiral selector in the running buffer (251). A high-speed microchip electrophoresis method for the enantiomeric separation of (R,S)-naproxen using methyl-β-cyclodextrin was compared with the capillary electrophoresis separation with regard to speed, efficiency, separation platform, and precision (252). The in situ molecular imprinting on the microchannel wall formed the imprinted polymer for fast enantioseparation of chiral compounds with amperometric detection (253). Model enantiomers Boc-D-Trp and Boc-L-Trp were baseline separated within 75 s, thus showing that the molecularly imprinted polymer-

microchip electrophoresis provided a powerful protocol for enantioseparations. Another microtechnique was employed for the rapid screening of commercial CSPs using a quartz crystal microbalance (254). In this study, the quartz crystal microbalance sensors were coated with stereospecific coatings developed for use in commercial HPLC columns and were exposed to individual enantiomers. Unique responses resulted from this exposure, and this technique seems to offer a venue to improve the efficiency of column method development. ACKNOWLEDGMENT Thanks to my secretary Ms. Dain Hayes for her invaluable assistance in preparing and proofing this manuscript. Timothy J. Ward, professor of chemistry and associate dean of sciences at Millsaps College (Jackson, MS 39210), received his B.S. degree from the University of Florida and his Ph.D. from Texas Tech University. Dr. Ward has served as chair of the International Symposium on Chirality. His research interests include chiral separations and the characterization of enantiomeric resolution, the development of analytical LC and CE methods, and their application to pharmaceutical and archaeological analysis. Karen D. Ward received her B.S. from Texas A&M University and her M.S. from Texas Tech University. Mrs. Ward has worked previously in the pharmaceutical industry at the Analytical Environmental Research Division at Syntex Pharmaceuticals.

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