Recent Progress in Chromatographic ...

Chem.Anal. (Warsaw), 42,139 (1997)

REVIEW

Recent Progress in Chromatographic Enantioseparations * by Jacek Bojarski Department of Organic Chemistry, College ofMedicine, J agiellonian University, Medyczna 9, 30-688Krak6w, Poland

Key words: chromatographic enantioseparations, derivatizing reagents, chiral stationary phases, capillary electrophoresis, review ------ ----------Ihe-reyieW-$U1U1IlarJzes-impO-rtanL-de¥elopments-ill--chw-rnato.g-raphi_e-separltuon_oL - - - - - - - - enantiomers in the last years. Both indirect and direct separation modes are discussed. New derivatizing reagents, chiral stationary phases and applications are presented. Special attention is devoted to enantioseparations of drugs and other biologically active substances and electrophoretic resolutions of enantiomers. W przegl science [33,34] and pharmaceutical analysis [35] were decribed and the studies on the mechanism of HPLC separation of diastereomeric isoindole compounds formed in this reaction were reported [36]. Application of this reaction for pharmaceutical analysis of N-acetylcysteine by CE [37,38] and resolution of enantiomers of a-dialkylamino acids by HPLC [39] were also reported. Jegorov et at. used 1-thio-~-D-ga­ lactose and o-phthalaldehyde to derivatize amino acid enantiomers prior to their HPLC separation [40], while derivatization with o-phthalaldehyde/2,3,4,6-tetra-Oacetyl-l-thio-~-D-glucopyranose (OPNTATG) reagent was donefof subsequent separations of amino acid diastereomers by MEKC [41]. Post-column derivatization of amino acids from biological samples with o-phthalaldehyde and 2-mercaptoethanol was used for fluorescence detection after HPLC on a ligand-exchange and crown ether CSP dual column system [42]. D- and L-serine was analyzed in rat brain after esterification with isobutanol and acylation with (S)-(-)-N-(heptafluorobutyryl)prolyl chloride by GC-MS technique [43]. Different urethane-protected L-amino acid N-carboxyanhydrides (UNCAs) were used for derivatization of D- and L- amino acids and the resultingN-protected diastereomeric dipeptides were resolved on a CIS stationary phase [44]. Symmetrical anhydrides of N-ethoxycarbonylvaline and phe.;. nylalanine proved useful for derivatization of some amino acids prior to RP-HPLC separations [45]. Marfey's reagent [Na-(2,4-dinitro-5-fluorophenyl)-L-alaninamide] was used for derivatization of some amino acids with tetrahydroisoquinoline, tetraline and cy----------duVenTIrn~rin_g-s1ructure-~46=4~;-uthe-l~rmtmr1rcids;-f(Jr-peptirre-chinrl-purit}' - -' . - .-

determination [50), for peptide synthesis [51] and racemization [52], for HPLC assay of enantiomers of phosphoserine in rat brain [53], for absolute stereochemistry determinations of cryptophycins [54], and determination of diaminopimelic acid in rumen bacteria [55]. Derivatizations with 2,3,4,6-tetra-O-acetyl-~-D-glucopyranosylisothiocyanate (TAGIT or GITC) were also employed in amino acids analysis [46-49,56] and enantioseparations of tetrahydrocarbazole derivatives by MEKC [57]. Other chiral isothiocyanates such as (S)-1-phenylethyl- (SAMBI) and (S)-1-(1-naphthyl)ethyl(SNEIT) were also used for derivatization of selected amino acid enaritiomers resolved by free-solution CE [58]. Applications of fluorescent chiral derivatization reagents have been recently reviewed [59]. Enantiomers of several aldohexoses and aldotetroses were derivatized with (S)-(-)-1-phenylethylamine and separated by CZE [60]. (S)-( + )-2-tert-butyl-2methyl-l,3-benzodioxole-4-carboxylic acid proved useful for derivatizations of amino deoxy sugars followed by HPLC separations of the resulting diastereoisomers [61]. Racemic amino acids [62] and some coumarin-derived drugs [63] were derivatized with (+)-O,O'-dibenzoyl-L-tartaric anhydride (DBTA) prior to CZE enantioseparation. 0,0' -(R,R)-diacylated tartaric acid anhydrides were also used for derivatization of enantiomers of ~-blocker - atenolol, in the studies on chemical and thermal stability of its acetamido group by HPLC and GC-MS [64]. Different methods of amphetamine and methamphetamine derivatizations for their determinations in biological samples by HPLC are reviewed in [65].

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142

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Enantioseparations by direct mode In this mode chiral discrimination of enantiomers is due to chiral selectors of stationary phases or those added to the mobile phase. Gas chromatography Enantiomer separation by gas chromatography on chiral stationary phases was thoroughly rewieved by Schurig [66]. Among new developments a well known CSP for GC chiral separations - Chirasil-Val, has been modified in its polysiloxane backbone yielding new phases with better properties [67-69]. L-Phenylalanine tetraamides were used as chiral selectors in new CSPs for resolutions of enantiomers of amino acids by capillary GC [70]. Applications of Chirasil-nickel {nickel(II) bis[3-heptafluorobutanoyl)-(1R)-10-methylene camphorate} for enantioseparations by capillary GC and SFC methods were investigated [71], while chiral organic chelates of europium as CSPs yielded enantioseparations of some alcohols and ketones [72]. New chiral polysiloxanes were synthesized and capillary columns coated with them were tested for GC enantioseparations of several drugs [73,74]. Various derivatives of cyc10dextrins (for review see [75]) were recently used as GC-CSPs for separation of enantiomers of 2,3-isopropylidene-1,2,3-cyc1ohexanetri01 derivatives [76], cyc1oalkanols [77], constituents of natural volatile oils [78-81], some volatile anesthetics (isoflurane, enflurane-preparative-scale resolutions) [82-84], ------------nfeTnyTa:.-clirOiopfoj'jI6-Iiafe-T85],irilils..:sobfe-for;hex-ooaro-iIar-~,-n(f-meIllYTjas~-···----·---

monate [86]. tert-Butyldimethylsilylated cyc10dextrins were recently introduced and successfully applied for separations of enantiomers of different types of compounds [87-95]. Other new or modifications of old CSPs based on cyclodextrins were obtained and tested in gas and supercritical fluid chromatography [96-101]. Glycerol-based a-, f3and y-cyc1odextrin phases were checked as CSPs for gas-liquid chromatography and their advantages in comparison with those formamide-based, discussed [102]. New copolymeric CSPs made from 6A,6B-bis-O-[p-(allyloxy)phenyl]per-O-methyl-f3-cyc10dextrin and hexasiloxane separated some chiral hydrocarbons and alcohols [103]. A very valuable collection of enantiomer separation factors by capillary GC on various modified cyc10dextrin CSPs was published in several issues of Journal of High Resolution Chromatography [104]. Determination of Rohrschneider-McReynolds constants served for estimations of polarity of 22 CSPs, mainly derivatized cyc1odextrins, used in capillary open tubular GC. It was stated that retention parameters of these CSPs were related to the McReynolds constants, while their enantiomeric resolution capabilities were not [105]. Supercritical fluid chromatography While a review of Perrut [106] discusses advances in SFC processes in general, with minor part only devoted to enantioseparations, a review of Petersson and Markides presents the state of the art of this chromatographic technique used for chiral separations with special attention paid to various CSPs used in open-tubular

Progress in chromatographic enantioseparations

143

and packed columns [107]. Several papers discuss the advantages and disadvantages of this technique compared with chiral HPLC for the same analytes [108, 109]. Carbon dioxide based eluents were used in enantioseparations of stereolabile compouns (hindered naphthyl ketones, fullerenes) on the (R,R)-DACH-DNB CSP [110]. The results of SFC on Chiralpak AD and Chiralcel aD. for several compounds of pharmaceutical interest were compared with those obtained by liquid chromatography. It was found that the presenceof polar functions may entail strong discrepancies in selectivity between these two techniques [111]. An interesting case of temperature dependence of chiral discrimination inSFC and HPLC was described by Smithet al. [112]. SFC enantioseparations of l-octenyl 3-acetate (on a preparative scale) [113], carboranylalanine [114] and racemic analogues of glibenclamide (potassium channel blocker) [115] have been reported recently. Liquid chromatography A-new monograph on chiral separations by liquidchromatography has been added to the chiral chroluatography library [116]. Brush type CSPs William H. Pirkle is the most active designer of brush-type chiral stationary phases. An excellent review desribes contributions in this field from his laboratory [117] and other publications report a research on new CSPS1118-1241anOTneCllan- - - - _.. isms of chiral recognition [125-133]. In many ofthem chiral selectors are: naproxen - a nonsteroidal antiinflammatory drug - and 3,5-dinitrobenzamidederivative of 1,2,3,4-tetrahydrophenantrene - present in a CSP WHELK-O 1, recently introduced to the market. (S)-naproxen-based CSP was also described by Hyun eta!' [134]. An improved enantiorecognition model for chiral resolutions of N-acyl-a-(I-naphthyl)ethyl amines on Pirkle-type N-(3,5-dinitrobenzoyl- (R)-phenylglycine and (S)leucine CSPs was proposed and applied to explain chromatographic resolutions of similar compounds on similar CSP~ [135]. Other CSPs of brush-type are also constructed and evalualed with such immobilized on silica gel chiral selectors as (S)-1,1'-binaphthyl-2,2'-dicarboxylic acid [136], enantiomers of phenylalanine and phenylglycine [137], 3,5-disubstituted N-benzoyl-phenylalanine [138], ergot alkaloid - terguride [139-141], 2-(2,4,5,7-tetranitrofluoren-9-ylideneaminooxy)propionic acid [142], monoamide derivatives of (R,R)-tartaric acid derived from (R)-I-(a-naphthyl)ethylamine, (S)-valine-(S)-I-(anaphthyl)ethylamide and its (R,R) enantiomer [143], 6,6'-dinitrobiphenyl-2,2'-dicarboxylic acid [144], 1,2'-bianthracene-2,2'-dicarboxylic acid [145], chiral diamides [146], N-alkylcarbamoyl derivatives of amino acids [147], chiral amide and urea derivatives of amino acids [148], (S)-1-(6,7-dimethyl-l-naphthyl)isobutylamine [149], urea derivatives derived from (S)- and (R)-l-( a-naphthyl)ethylamine with (S)-valine, (S)-tert-Ieucine, (S)-proline and (S)-indoline-2-carboxylic acid [150], N-(3,5-dinitrobenzoyl)tyrosine [151], N-(3,5-dinitrobenzoyl)allylglycine 2,6-dimethylanilide [152], O-gluconolactone [153], tripeptide derivatives [154] and diaminocyclohexane derivatives [155]. Stlldyilig the· effect of lllohile phase ··oli . the

144

J. Bojarski

enantioselectivity of chromatographic separation of rac. 2,2,2-trifluoro-1-(9-anthryl)ethanol on a quinine-bonded CSP it was found that the presence of polar constituents in the mobile phase may significantly change this enantioselectivity. The most significant in enantiorecognition was a hydrogen bond between the chiral analyte and the chiral selector of the CSP [156]. The enantiomers of this compound used as CMPAs showed competition what was indicated by the chromatographic behavior of their system peaks [157]. It was demonstrated that the method of immobilization of chiral selectors on silica support effected the separability of enantiomers on various brush-type CSPs [158]. Derivatives of cyanuric chloride differently substituted with chiral selectors were used for preparation of several new CSPs for chiral separations of dansyl amino acids, methyl esters of N-(3,5-dinitrobenzoyl)-amino acids and N-(3,5-dinitrobenzoyl)amino alcohols [21, 159-163]. Armstrong et al. described for the first time the use of macrocyclic antibiotics as chiral selectors for liquid chromatography. Thus va ncomycin, its 3,5-dimethylphenyl derivative, rifamycin B, thiostrepton [164] and teicoplanin [165] covalently linked to silica gel were used for HPLC enantioseparations of many bioactive compounds, while vancomycin, teicoplanin and ~-cyclodextrin CSPs served for resolution of enantiomers of substituted 2-methoxy-6-oxo-1,4,5,6-tetrahydropyridine-3-carbonitriles [166]. Totally synthetic macrocyclic C3 symmetric chiral selector containing benzene

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