Liquid Chromatographic Chiral Stationary Phase Research Articles

Synthesis of New Chiral Crown Ethers Incorporating Two Different Chiral Units and 'Matched/Mismatched' Effect of the Two Chiral Units on the Chiral Recognition

Chiral crown ethers have been widely applied as chiral selectors in differentiating the two enantiomers of racemic compounds containing a primary amino group. Especially chiral crown ethers have been quite useful as chiral selectors of liquid chromatographic chiral stationary phases (CSPs) for the recognition of the two enantiomers of racemic primary amino compounds. Among a large number of chiral crown ethers, two types have been most successfully utilized as liquid chromatographic CSPs. The first type is chiral crown ethers incorporating chiral 1,1'-binaphthyl unit, which were first developed by Cram. Especially, chiral crown ethers such as bis-(1,1'-binaphthyl)-22-crown-6 immobilized on silica gel or polystyrene were used as CSPs in separating the enantiomers of racemic α-amino acids and their derivatives in the late 1970s by Cram and coworkers. Afterward, chiral crown ethers related to (3,3'-diphenyl-1,1'binaphthyl)-20-crown-6 (1, Fig. 1) dynamically coated on octadecyl silica gel or covalently bonded to silica gel were developed as liquid chromatographic CSPs and successfully utilized in the resolution of racemic α-amino acids and other racemic compounds containing a primary amino group. The second type of chiral crown ether successfully utilized as a chiral selector of CSPs is the one incorporating two tartaric acid units, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2, Fig. 1), which was first synthesized by Lehn and coworkers. Initially, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2) was utilized as a chiral selector for the chiral separation by capillary electrophoresis. The use of (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2) was then extended as a chiral selector of liquid chromatographic CSPs. Liquid chromatographic CSPs based on (+)-(18crown-6)-2,3,11,12-tetracarboxylic acid (2) were very successful in the resolution of various racemic primary amino compounds or racemic non-primary amino compounds. In addition, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2) was utilized as an NMR chiral solvating agent. However, chiral crown ethers incorporating both chiral 3,3'-diphenyl-1,1'-binaphthyl and tartaric acid unit have never been known to the best of our knowledge. Chiral crown ethers incorporating both chiral 3,3'-diphenyl-1,1'binaphthyl and tartaric acid unit are expected to show characteristics somewhat different from chiral crown ether 1 or 2 in the chiral recognition. In this study, we wish to report the synthesis of novel chiral crown ethers incorporating both chiral 3,3'-diphenyl-1,1'-binaphthyl and tartaric acid unit and their applications as chiral selectors of liquid chromatographic CSPs. We prepared actually two diastereomeric chiral crown ethers, (R,R,R)-3 and (R,S,S)-3 (Fig. 2), incorporating both (R)-3,3'-diphenyl-1,1'-binaphthyl and (R,R)or (S,S)-tartaric acid unit as chiral barriers. Combination of the two different chiral units as chiral barriers is expected to endow the two diastereomeric chiral crown ethers with characteristic chiral recognition ability according to the stereochemistry of the two chiral units. Synthesis of the two diastereomeric chiral crown ethers was successfully performed as shown in Scheme 1. Diols (R,R)-4 and (S,S)-4 prepared from commercially available (R,R)and (S,S)-N,N,N',N'-tetramethyltartaramide respectively via the known procedure were converted into their tosyl derivatives, (R,R)-5 and (S,S)-5. Tosylation of (R,R)-4 or (S,S)-4 was most successful when a small amount of 4(dimethylamino)pyridine (DMAP) was added to the reaction mixture. The cyclization of tosyl derivative (R,R)-5 or (S,S)-

Preparation of two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units and their applications

Two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units such as optically active 3,3′-diphenyl-1,1′-binaphthyl and tartaric acid unit were prepared. Between the two CSPs, one was much superior to the other especially in the resolution of tocainide and its analogues (for example, in the resolution of tocainide the separation factor, α, was 4.26 vs. 1.00 on the two CSPs). From these results, the two chiral units composing the two diastereomeric chiral crown ether moieties of the stationary phases were expected to show “matched” or “mismatched” effect on the chiral recognition according to their stereochemistry. The different chiral recognition abilities of the two CSPs were rationalized by the different three-dimensional structures of the two diastereomeric chiral crown ethers.

An NMR Chiral Solvating Agent for the Chiral Recognition of the Two Enantiomers of N-(3,5-Dinitrobenzoyl)-α-amino Acids

Two enantiomers of chiral drugs often show different pharmaceutical activities. Consequently, the study on the stereochemical dependence of the pharmacokinetics and the pharmaceutical activities of chiral drugs is important. Determination of the enantiomeric composition of chiral drugs is, therefore, essential. Enantiomeric composition of chiral compounds has been determined most widely by highperformance liquid chromatographic chiral separations on chiral stationary phases (CSPs). Another important method of determining the enantiomeric composition of chiral compounds is the NMR techniques utilizing chiral solvating agent (CSA). The two transient diastereomeric adducts formed between a CSA and each of the two enantiomers of chiral analytes might induce anisochronous NMR resonances and consequently the enantiomeric composition of chiral analytes can be easily assessed from the NMR spectrum. Most CSAs, which have been developed and successfully utilized in the determination of the enantiomeric composition of chiral compounds by NMR spectroscopy, contain aromatic functional group(s) to induce magnetic anisotropic influence and to invoke π-π interaction with analytes. Derivatives of optically active trans-1,2-diaminocyclohexane has also been utilized as NMR CSAs and these CSAs have also been designed to contain aromatic functional group(s) in order to induce magnetic anisotropic influence and to invoke π-π interaction with analytes. Consequently, CSAs derived from optically active trans-1,2diaminocyclohexane have been utilized for the determination of enantiomeric composition of chiral compounds containing aromatic functional group(s). For example, CSA 1 and CSA 2 derived from (1R,2R)-1,2-diaminocyclohexane shown in Figure 1 have been successfully applied to the determination of enantiomeric composition of chiral carboxylic acids by NMR spectroscopy. Recently, we found that a liquid chromatographic chiral stationary phase (CSP 3, Figure 1) derived from optically active (1S,2S)-1,2-diaminocyclohexane is quite successful for the resolution of N-(3,5-dinitrobenzoyl)-α-amino acids even though the CSP does not contain any aromatic functional group, but contains only a simple primary amino group and a ureide tethering group. From these results, we inferred that CSA 4, which has the same structure as the chiral selector of CSP 3, might show nonequivalences for the two enantiomers of N-(3,5-dinitrobenzoyl)-α-amino acids by NMR spectroscopy. CSA 4 was prepared simply by treating (1S,2S)-1,2diaminocyclohexane with ethyl isocyanate. CSA 4 thus prepared was used to see the H NMR chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine. As shown in Figure 2, the chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine was observed at the NMR peaks corresponding to the protons on the 3,5-dinitrophenyl ring (denoted by a in Figure 1). In addition the chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine was observed to be greatest when the ratio of CSA 4 and N-(3,5-dinitrobenzoyl)phenylglycine is 2:1 at ambient temperature (22 ± 0.5 C). The H NMR experimental results for the chemical shift nonequivalences of the two enantiomers of five N-(3,5dinitrobenzoyl)-α-amino acids in the presence of two equivalents of CSA 4 are summarized in Table 1. As shown

Liquid chromatographic resolution of secondary amino alcohols on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid: Dependence of temperature effect on analyte structure

Two types of secondary amino alcohols were successfully resolved on a liquid chromatographic chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. The secondary amino alcohols containing a secondary amino group connected directly to the stereogenic center were resolved much better than those containing a hydroxyl group attached directly to the stereogenic center. In addition, the dependence of the separation factor ( α) on column temperature was exactly opposite for the two different types of secondary amino alcohols. From the thermodynamic calculations based on Van’t Hoff plots, we conclude that the enantioselectivity for the resolution of the secondary amino alcohols containing a secondary amino group connected directly to the stereogenic center is enthalpically controlled while that for the resolution of the secondary amino alcohols containing a hydroxyl group attached directly to the stereogenic center is entropically controlled.

Liquid Chromatographic Chiral Separations by Crown Ether‐Based Chiral Stationary Phases

Liquid chromatographic chiral stationary phases (CSPs) based on chiral crown ethers have been successfully utilized in the separation of the enantiomers of various racemic compounds containing a primary amino group. Especially, CSPs based on chiral crown ethers incorporating chiral binaphthyl unit or tartaric acid unit have been most successful. In this review, the development of CSPs based on chiral crown ethers incorporating a chiral binaphthyl unit or a tartaric acid unit, their applications to the resolution of various primary and non‐primary amino compounds with the variation of the type and the content of organic, acidic, and inorganic modifiers in an aqueous mobile phase or with a non‐aqueous mobile phase and the efforts to improve the chiral recognition efficiency or the stability of the CSPs have been discussed.

Preparation of a New Chiral Stationary Phase Based on (2S,3S)-O,O'-Bis-(10-undecenoyl)-N,N'-bis-(3,5-dinitrobenzoyl)-2,3-diamino-1,4-butandiol and Its Application

A new liquid chromatographic chiral stationary phase based on (2S,3S)-O,O'-bis-(10-undecenoyl)-N,N'-bis-(3,5-dinitrobenzoyl)-2,3-diamino-1,4-butandiol was prepared starting from (2R,3R)-1,4-bis(benzyloxy)-2,3-butanediol. The new chiral stationary phase was applied to the resolution of racemic anilide derivatives of N-acetyl-a-amino acids, 1,1'-bi-2-naphthol and 3,3'-diaryl-1,1'-bi-2-naphthols. The CSP was also applied to the resolution of some chiral drugs including a diuretic, bendroflumethiazide, and non-steroidal anti-inflammatory agents such naproxen and alminoprofen. In every case, the chiral recognition efficiency of the new CSP was quite excellent.ࡺ?⨀ЀĀ衺?⨀ЀĀऐ돀졽?⨀堘�⨀ꢐ?⨀돐좐?⨀��⨀킐?⨀뤏덐?⨀렏頏࢑?⨀਀࢑?⨀ᢑ?⨀؀ကᢑ?⨀?幺Ѐက䂑?⨀䂑?⨀

Resolution of β-Amino Acids on a Chiral Stationary Phase Based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxilic Acid without Extra Free Aminopropyl Groups on Silica Surface

A liquid chromatographic chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxilic acid without extra free aminopropyl groups on silica surface has been demonstrated to be quite effective for the resolution of various /?-amino acids. The retention factors (k 1 ) for the resolution ofy β-amino acids on the CSP were quite large and the large retention factors might be quite attractive along with the reasonable separation factors (a) for preparative scale enantioselective chromatography. The large retention factors on the CSP were found to be reduced effectively by adding ammonium ion to mobile phase without sacrificing the chiral recognition efficiency of the CSP. Consequently, the CSP is also quite applicable for use in analytical enantioselective chromatography.

Effect of the residual silanol group protection on the liquid chromatographic resolution of racemic primary amino compounds on a chiral stationary phase based on optically active (3,3′-diphenyl-1,1′-binaphthyl)-20-crown-6

A liquid chromatographic chiral stationary phase (CSP) based on (3,3′-diphenyl-1,1′-binaphthyl)-20-crown-6, which has been utilized in the resolution of α-amino acids, amines and amino alcohols, was treated with excess of n-octyltriethoxysilane to prepare a new improved CSP. The residual silanol groups of the original CSP were protected by n-octyl groups in the new CSP. The chiral recognition ability of the new CSP was superior to that of the original CSP in the resolution of α-amino acids, amines and amino alcohols. Retention factors ( k 1) for the resolution of α-amino acids were lower on the new CSP than on the original CSP while those for the resolution of amines and amino alcohols were higher on the new CSP than on the original CSP. The improved chiral recognition ability of the new CSP and the retention behaviors of the two enantiomers on the new CSP have been rationalized to stem from the removal of the non-enantioselective interactions between the analytes and the residual silanol groups of the original CSP and the improved lipophilicity of the CSP.

A (4R)‐Hydroxy‐L‐proline‐derived chiral scaffold and its oligomers as chiral selectors in liquid chromatography chiral stationary phases for enantioseparation

The chromatographic behaviour of a poly-L-proline-derived chiral stationary phase (CSP) is compared to the corresponding single proline-derived CSP. Structurally diverse racemic test compounds and mobile phases, including normal- and RP conditions, were used. Although the application domain of the poly-L-proline-derived CSP (CSP-3) was considerably restricted, this CSP showed a higher retention and a slightly broader application domain than the monomeric analogue (CSP-1) when heptane/2-PrOH was used as mobile phase. The presence of an alcohol in the mobile phase was essential for enantioseparation in the poly-L-proline-derived CSP when normal-phase conditions were applied.

Preparation and application of HPLC chiral stationary phases based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Preparation of liquid chromatographic chiral stationary phases (CSPs) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid and their application are reviewed. The various methods of connecting (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid to silica gel covalently or dynamically are demonstrated. The CSPs based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid have been very successful for the resolution of various primary amino compounds with the use of an aqueous mobile phase containing organic and acidic modifiers. In addition, the resolution of secondary amino compounds including beta-blockers and N-(3,5-dinitrobenzoyl)-alpha-amino acids has been demonstrated on a CSP based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid with a non-aqueous mobile phase.

Liquid chromatographic enantioseparation of aryl?-amino ketones on a crown ether-based chiral stationary phase

A liquid chromatographic chiral stationary phase based on (3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 covalently bonded to silica gel was applied in the resolution of aryl alpha-amino ketones including cathinone, the main psychoactive alkaloid found in the leaves of the khat plant. The resolution was excellent, the separation factors ranging between 1.72 and 8.58 and the resolution factors (R(S)) ranging between 2.60 and 11.10. The chromatographic resolution behaviour was dependent on the type and the content of organic and acidic modifiers and the ammonium acetate concentration in aqueous mobile phase and the column temperature.

New hybrid polymeric liquid chromatography chiral stationary phase prepared by surface-initiated polymerization

A new hybrid organic/inorganic HPLC chiral stationary phase (CSP1) has been synthesized by the grafting from (g-from) radical polymerization of an enantiopure diacryloyl derivative of trans-1,2-diaminocyclohexane in the presence of mesoporous, azo-activated silica particles. The new chiral stationary phase has been fully characterized by elemental analysis, differential scanning calorimetry, diffuse reflectance infrared spectroscopy, scanning electron microscopy, inverse size exclusion chromatography and Van Deemter analysis. CSP1 shows improved chromatographic performances compared to its analog CSP2 synthesized by the alternative grafting to (g-to) approach in which the azo initiator is kept in solution. CSP1 can successfully resolve several chemically diverse chiral compounds, using both organic and water-based eluents (normal phase, polar organic, etc.).

Handbook of basic tables for chemical analysis

Handbook of basic tables for chemical analysis

Chiral separations by capillary electrophoresis using proteins as chiral selectors.

AbstractHigh-performance liquid chromatography (HPLC) chiral stationary phases based on a protein are of special interest because of their unique properties of stereoselectivity and because they are suited for separating a wide range of enantiomeric mixtures. These come from the multiple binding sites in a protein, and/or the multiple interactions between a solute and protein. Similarly, capillary electrophoresis (CE) methods using proteins as the immobilized or adsorbed ligands or running buffer additives have been developed for the separation of enantiomeric mixtures (1–4). Proteins used so far as chiral selectors have included albumins, such as bovine serum albumin (BSA), human serum albumin (HSA), and serum albumins from other species; glycoproteins such as α1-acid glycoprotein (AGP), ovomucoid from chicken egg whites (OMCHI), ovoglycoprotein from chicken egg whites (OGCHI), avidin, and riboflavin-binding protein (or flavoprotein); enzymes, such as fungal cellulase from fungus Aspergillus niger, cellobiohydrolase I, pepsin, and lysozyme; and other proteins such as ovotransferrin (or conalbumin), β-lactoglobulin, casein, and human serum transferrin.KeywordsHuman Serum AlbuminCapillary ElectrophoresisChiral SelectorFuse Silica CapillaryChiral SeparationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Effect of analyte lipophilicity on the resolution of α‐ and β‐amino acids on liquid chromatographic ligand exchange chiral stationary phases

AbstractSeparation of the two enantiomers of racemic α‐ and β‐amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N‐((S)‐1‐hydroxymethy‐3‐methylbutyl)‐N‐undecylaminoacetate or sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.

Quantitative determination of the enantiomers of methadone and its metabolite (EDDP) in human saliva by enantioselective liquid chromatography with mass spectrometric detection

A sensitive enantioselective liquid chromatographic assay with mass spectrometric detection (LC–MS) has been developed and validated for the simultaneous determination of saliva concentrations of ( R)- and ( S)-methadone (Met) and ( R)- and ( S)-2-ethylidene-1,5-dimethyl-3,3-diphenyl-pyrrolidine (EDDP, a primary metabolite of Met). Saliva specimens were collected using Salivette devices (Sarsedt), and centrifuged; collected saliva was then spiked with deuterated internal standards, D3-Met and D3-EDDP, and directly injected into the LC–MS. Enantioselective separations were achieved on a liquid chromatographic chiral stationary phase (CSP) based upon immobilized α 1-acid glycoprotein (AGP) using a mobile phase composed of acetonitrile: ammonium acetate buffer (10 mM, pH 7.0) in a ratio of 18:82 (v/v), a flow rate of 0.9 ml/min and a temperature of 25 °C. Under these conditions, enantioselective separations were observed for methadone ( α=1.30) and EDDP ( α=1.17) within 15 min. Met, EDDP, D3-Met and D3-EDDP were detected using selected ion monitoring at m/ z 310.20, 278.20, 313.20 and 281.20, respectively. Linear relationships between peak height ratio and drug-enantiomer concentrations were obtained for methadone in the range of 5.0–600.0 ng/ml, and for EDDP from 0.5 to 15.0 ng/ml per enantiomer with correlation coefficients better than 0.9994, where lower limit of quantification (LLOQ) for Met was 5 ng/ml and for EDDP 0.5 ng/ml. Acceptable intra- and inter-day precision of the method (CVs<4.0%) and accuracy (CVs<4.0%) were obtained. These findings demonstrate the accuracy and precision of the method used to successfully analyze saliva obtained from patients enrolled in a methadone-maintenance program.

Resolution of Tocainide and Its Analogues on Liquid Chromatographic Chiral Stationary Phases Based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic Acid

Two liquid chromatographic chiral stationary phases (CSPs) based on (+)-(18-crown-6)-2,3, 11,12-tetracarboxylic acid were successfully applied in the resolution of racemic tocainide and its analogues. In the resolution oftocainide, especially, the CSP containing N-CH 3 amide tethering groups was quite effective, showing clear baseline resolution (R s : 2.66) with reasonable enantioselectivity (a: 1.25). Consequently, the CSP containing N-CH 3 amide tethering groups is expected to be useful to monitor the enantiomeric composition of tocainide in clinical samples. In addition, the chromatographic behaviors for the resolution of tocainide and its analogues on the two CSPs were found controllable by varying the content and the type of organic and acidic modifiers in aqueous mobile phase.

Determination of the enantiomers of ketamine and norketamine in human plasma by enantioselective liquid chromatography–mass spectrometry

A sensitive enantioselective liquid chromatographic assay with mass spectrometric detection has been developed and validated for the simultaneous determination of plasma concentrations of ( R)- and ( S)-ketamine, and ( R)- and ( S)-norketamine. The compounds were extracted from human plasma using solid-phase extraction and then directly injected into the LC–MS system for detection and quantification. Enantioselective separations were achieved on a liquid chromatographic chiral stationary phase based upon immobilized α 1-acid glycoprotein (the Chiral AGP column). The separations were achieved using a mobile phase composed of 2-propanol–ammonium acetate buffer (10 m M, pH 7.6) (6:94, v/v), a flow-rate of 0.5 ml/min and a temperature of 25 °C. Under these conditions, the analysis time was 20 min. Detection of the ketamine, norketamine and bromoketamine (internal standard) enantiomers was achieved using selected ion monitoring at m/ z 238.1, 224.1 and 284.0, respectively. Extracted calibration curves were linear from 1 to 125 ng/ml per enantiomer for each analyte with correlation coefficients better than 0.9993 and intra- and inter-day RSDs of less than 8.0%. The method was applied to samples from a clinical study of ketamine in pain management.

Comparison of Chirasil-DEX CB as gas chromatographic and ULMO as liquid chromatographic chiral stationary phase for enantioseparation of aryl- and heteroarylcarbinols

For a broad spectrum of simple chiral alcohols, incorporating a (substituted) (het)aryl building block, enantiomer separation characteristics are reported for both gas chromatography on a Chirasil-DEX phase, and liquid chromatography on an ( S, S)-ULMO phase. On this chiral Pirkle-type phase, homochiral enantiomers (mostly R) are eluted first without exception. The elution order R before S appears conserved as a rule also for gas chromatographic separations on Chirasil-DEX, though with some remarkable exceptions indicating a change in the dominant discriminative mechanism. This was shown in the homologous series 1-phenylethanol to 1-phenylhexanol having the point of reversal at C4, while the o-methoxy analogues elute from C1 to C4 already in the reversed order.

Preparation and application of a new modified liquid chromatographic chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

As an effort to improve the chiral recognition efficiency of a previously reported chiral stationary phase (CSP) based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic acid, a new CSP was prepared by simply replacing the amide N–H hydrogens of the tethering groups of the old CSP with methyl groups. The new CSP was superior to the old one in the resolution of racemic primary amines. However, in the resolution of α-amino acids and amino alcohols, the new and the old CSPs were complementary with each other. The elution orders on the new CSP were sometimes opposite to those on the old one. Consequently, the chiral recognition mechanism on the new CSP was presumed to be different from that on the old one. The chiral recognition behavior of the new CSP were investigated with four selected analytes and found to be dependent to some extent on the content of organic and acidic modifiers in aqueous mobile phase and the column temperature.