Brush-type Chiral Stationary Phases Research Articles

Separation of BINAPO enantiomers on brush-type chiral stationary phases

Brush-type chiral stationary phase CSP 12 was specifically designed for separation of BINAPO enantiomers, which can be useful as organocatalysts or as chiral ligands in organometallic catalysis. This chromatographic separation process can compete with other routes to enantiomerically pure BINAPO.

Development of novel brush-type chiral stationary phases based on terpenoid selectors: HPLC evaluation and theoretical investigation of enantioselective binding interactions

The terpenoid chiral selectors dehydroabietic acid, 12,14-dinitrodehydroabietic acid and friedelin have been covalently linked to silica gel yielding three chiral stationary phases CSP 1, CSP 2 and CSP 3, respectively. The enantiodiscriminating capability of each one of these phases was evaluated by HPLC with four families of chiral aromatic compounds composed of alcohols, amines, phenylalanine and tryptophan amino acid derivatives and β-lactams. The CSP 3 phase, containing a selector with a large friedelane backbone is particularly suitable for resolving free alcohols and their derivatives bearing fluorine substituents, while CSP 2 with a dehydroabietic architecture is the only phase that efficiently discriminates 1,1′-binaphthol atropisomers. CSP 3 also gives efficient resolution of the free amines. All three phases resolve well the racemates of N-trifluoracetyl and N-3,5-dinitrobenzoyl phenylalanine amino acid ester derivatives. Good enantioseparation of β-lactams and N-benzoyl tryptophan amino acid derivatives was achieved on CSP 1. In order to understand the structural factors that govern the chiral molecular recognition ability of these phases, molecular dynamics simulations were carried out in the gas phase with binary diastereomeric complexes formed by the selectors of CSP 1 and CSP 2 and several amino acid derivatives. Decomposition of molecular mechanics energies shows that van der Waals interactions dominate the formation of the diastereomeric transient complexes while the electrostatic binding interactions are primarily responsible for the enantioselective binding of the ( R)- and ( S)-analytes. Analysis of the hydrogen bonds shows that electrostatic interactions are mainly associated with the formation of N–H⋯O C enantioselective hydrogen bonds between the amide binding sites from the selectors and the carbonyl groups of the analytes. The role of mobile phase polarity, a mixture of n-hexane and propan-2-ol in different ratios, was also evaluated through molecular dynamics simulations in explicit solvent.

Influence of Mobile Phase and Temperature on Separation of 1,1′-Binaphthyl-2,2′-Diol Enantiomers with Brush Type Chiral Stationary Phases Derived from l-Leucine

1,1′-Binaphthyl-2,2′-diol (BINOL) was analysed on three different chiral stationary phases comprising 3,5-dinitrobenzoyl-l-leucine as chiral selector. Chromatographic data were related to NMR measurements performed for the mixture of the soluble chiral selector and individual enantiomers of BINOL. The results indicate that the most important interactions for complexation are H-bondings accomplished through the hydroxyl protons of BINOL with carbonyl groups of 3,5-dinitrobenzoyl-l-leucine. Interactions between 3,5-dinitrobenzoyl and naphthyl aromatic units are not observed and that could explain the large variations of NMR spectra and HPLC chromatograms recorded at different temperatures.

Quantitative structure–enantioselective retention relationships for chromatographic separation of arylalkylcarbinols on Pirkle type chiral stationary phases

Quantitative structure–retention (QSRR, retention factors log k 1 and log k 2 for the first and second eluted enantiomer) as well as enantioselective retention relationships (QSERR, separation factor log α) for a series of 42 chiral arylalkylcarbinols on four brush-type chiral stationary phases are derived by multiple linear regression analyses and artificial neuronal network calculations using 2D and 3D molecular descriptors including those obtained by quantum chemical calculations. Separation factors are in addition modeled by the 3D-QSAR method of comparative molecular field analysis (CoMFA). For the retention factors the LUMO energy turns out to be the most important descriptor, whereas for log α it is the hydrophobicity of the analytes. With CoMFA both the steric and electrostatic field are found to be of almost comparable significance.

Mechanism of chiral recognition in the enantioseparation of 2-aryloxypropionic acids on new brush-type chiral stationary phases.

New brush-type chiral stationary phases (CSP I-IV) comprising N-3,5,6-trichloro-2,4-dicyanophenyl-L-alpha-amino acids (1-4) were prepared by binding of chiral selectors 1-4 to gamma-aminopropyl silica gel. To check the role of excess free aminopropyl groups, CSP V was prepared by binding N-3,5,6-trichloro-2,4-dicyanophenyl-L-alanyl-(3-triethoxysilyl)propylamide to unmodified silica gel. The best separation of racemic 2-aryloxypropionic acids (TR-1-13) was obtained with CSP I; the -(-)-S enantiomer were regularly eluted first, as determined by a CD detector. The mechanism of chiral recognition implies a synergistic interaction of carboxylic acid analyte with the chiral selector and achiral free gamma-aminopropyl units on silica. In fact, CSP V, which is lacking an achiral aminopropyl spacer, shows a lower separation ability for 2-aryloxypropionic acids, but a similar enantioselective discrimination of esters TR-19-20, in comparison with CSP I. CSP I-IV retain unaltered separation ability after a few months of continuous work using a large number of various mobile phases.

On-column deracemization of an atropisomeric biphenyl by quinine-based stationary phase and determination of rotational energy barrier by enantioselective stopped-flow HPLC and CEC.

The reversible enantiomerization of axially chiral 2'-dodecyloxy-6-nitrobiphenyl-2-carboxylic acid was studied in the presence of a brush type chiral stationary phase based on O-(tert-butylcarbamoyl) quinine as chiral selector unit by stopped-flow high-performance liquid chromatography (sfHPLC) and capillary electrochromatography (sfCEC). After initial separation of the enantiomers in the first section of the column, the flow was stopped and the resolved species allowed to enantiomerize on-column. From this conversion, which could be determined from the enantiomeric ratios at different enantiomerization times, kinetic rate constants were calculated. By sfHPLC at a constant temperature of 15 degrees C, kinetic rate constants in the presence of the CSP were found to be 4.1 x 10(-5) s(-1) and 2.2 x 10(-5) s(-1) for the (-) and (+)-enantiomers, respectively, corresponding to half-lives of 279 and 530 min. Thus, apparent activation energies of enantiomerization were calculated to be 93.0 and 94.6 kJ mol(-1) for the (-) and (+)-enantiomers. On the macroscopic level, the apparent difference of rotational energy barriers and kinetic rate constants for both enantiomers is reflected as deracemization. For example, starting from a racemic mixture, an enantiomeric excess (ee) of 14% was seen in the stopped-flow HPLC experiment described after an enantiomerization time of 220 min at 15 degrees C, and a maximal ee of 17% can be approximated after infinite enantiomerization time. There is good agreement between HPLC and CEC results as well as their experimental errors, confirming that the new sfCEC technique may be a valuable and convenient tool to study interconversion processes.

Immobilization of difunctional building blocks on hydroxysuccinimide activated silica: versatile in situ preparation of chiral stationary phases.

Several brush-type chiral stationary phases (CSPs) based on undecanoyl- or butanoyl-bound (R,R)-1,2-diphenylethane-1,2-diamine (DPEDA) as chiral selector were prepared by an innovative, fast, and less expensive kind of preparation. The key to this method is the immobilization of the enantiomeric pure diamine with only one amino function in a simple substitution reaction on hydroxysuccinimide ester-activated silica. No excess chiral material is lost. Loading can be easily monitored analyzing the filtrate. The free second amino function can subsequently be acylated with different acyl halogenides. Examples with benzoyl- and 3,5-dinitrobenzoyl (DNB) amides show that, based on our new approach, a library of differently acylated Pirkle-type CSPs can easily be obtained. A benzoylated analog of the commercially available ULMO CSP is shown to be very effective in separating enantiomers of N-acyl amino acids.

PREPARATION AND EVALUATION OF THE CHIRAL STATIONARY PHASES BASED ON N-4-(2,5,6-TRICHLORO-1,3-DICYANO)-PHENYL DERIVATIVES OF L-α-AMINO ACIDS

Regioselective functionalization of 2,4,5,6-tetrachloro-1,3-dicyanobenzene (TCDCB) by nucleophilic substitution of the chlorine atom at C(4) for N-L-α-amino acid residue enables preparation of new chiral selectors (1-5). Their binding to aminopropyl silica gel afforded new brush-type chiral stationary phases (CSP-1–CSP-4). Chiral stationary phases CSP-5 and CSP-6 that comprise dipeptide units L-Ala-L-Pro and L-Ala-L-Ala, respectively, were obtained by the solid-state coupling of C(4)substituted derivatives of 2,5,6-trichloro-1,3-dicyanobenzenes 4 and 5 to γ-L-alanylaminopropyl silica gel. Best resolution for some of 23 test racemates was achieved with CSP-4 and CSP-6. This study reveals hydrogen bonding via a sterically exposed amide group and a π-π type interaction via an π-acid persubstituted benzene ring in these CSPs as the main contribution to chiral recognition.

Investigation of capillary electrochromatography with brush-type chiral stationary phases

Investigation of capillary electrochromatography with brush-type chiral stationary phases

Stereoselective determination of mepivacaine in human serum using a brush-type chiral stationary phase and solid-phase extraction.

A stereoselective high-performance liquid chromatography assay method was developed for the quantitation of R-(+)- and S-(-)-mepivacaine in human serum. The assay uses a Pirkle brush-type (S)-tert.-leucine, (R)-1-(alpha-naphthyl)ethylamine stationary phase (Sumichiral OA-4700, 250 x 4 mm I.D.) at ambient temperature with a mobile phase of hexane-ethylenedichloride-absolute methanol (85:10:5, v/v) for the separation of R-(+)- and S-(-)-mepivacaine. The eluents were monitored using UV detection at 220 nm. Isolation of the analytes from serum was performed using a 1-ml C18 solid-phase extraction cartridge with high recovery and selectivity. The detection limits were 100 ng/ml for each enantiomer and the limits of quantitation were 150 ng/ml for both enantiomers. Linear calibration curves in the 150-2400 ng/ml range showed good correlation coefficients (r > 0.9994, n = 3). Precision and accuracy of the method were within 2.1-5.3 and 2.0-3.6%, respectively, for R-(+)-mepivacaine and 2.7-5.7% and 1.7-4.2%, respectively, for S-(-)-mepivacaine.

Stereoselective determination of R(−)- and S(+)-prilocaine in human serum using a brush-type chiral stationary phase, solid-phase extraction and UV detection

A chiral HPLC method was developed for the quantitation of R(−)- and S(+)-prilocaine in human serum. The method involves sensitive and selective detection of R(−)- and S(+)-prilocaine using normal-phase chiral HPLC on a pirkle-type naphthyl ethylamine stationary phase (Sumichiral OA-4700, 250 mm × 4 mm i.d.) at ambient temperature with a flow rate of 0.8 ml min −1. A sample clean-up procedure was used for isolation of the analytes of interest from human serum using Bond-Elut C 18 columns with high recovery and selectivity. The detection limits were 4 ng ml − for R-prilocaine and 5 ng ml −1 for S-prilocaine. The limits of quantitation were 10 ng ml −1 for both enantiomers. Linear calibration curves in the 10–1000 ng ml −1 range showed good coefficients of determination >0.999 ( n − 3). Precision and accuracy of the method were within 4–5.8% and 1.5–4.8% respectively for R(−)-prilocaine, and 2.8–5.7%, and 3.2–5.2% respectively for S(+)-prilocaine.

Quinine and quinidine derivatives as chiral selectors I. Brush type chiral stationary phases for high-performance liquid chromatography based on cinchonan carbamates and their application as chiral anion exchangers

Quinine and quinidine carbamates, respectively, have been immobilized onto porous silica utilizing two different types of spacer and/or grafting modes, obtaining four brush type chiral stationary phases (CSPs). These CSPs have preferentially been operated with buffered aqueous mobile phases to resolve the enantiomers of acidic analytes involving ion pair mechanisms as the dominating binding and/or retention principle beside additional adsorption towards the modified silica surface. The spectrum of successfully resolved racemates contains chiral aryl-, aryloxy- and arylthiocarboxylic acids, as well as N-derivatized amino acids (DNB-, B-, Ac-, F-, DNP-, DBD-, Fmoc-, Boc-, Z-, DNS-amino acids, etc.) and many other chiral acids including also sulfonic, phosphonic and phosphoric acids. The influence of mobile phase parameters, as well as of structure variations of analytes on retention and enantioselectivity was evaluated. Thus, a chromatographically derived tentative chiral recognition model is proposed for DNB-amino acids which are resolved into enantiomers with exceptionally high α-values (e.g., DNB-Leu with an α-value of about 7).

Diphenylethanediamine derivatives as chiral selectors V 1. Efficient normal-phase high-performance liquid chromatographic enantioseparation of underivatized chiral arylalcohols on four differently linked 3,5-dinitrobenzoyldiphenylethanediamine-derived chiral stationary phases

Four brush-type chiral stationary phases (CSPs) have been prepared, via immobilization of ( R, R)-3,5-dinitrobenzoyl-1,2-diphenylethane-1,2-diamine (DNB-DPEDA) with structurally different anchoring groups. These phases separate at normal-phase HPLC conditions the enantiomers of numerous underivatized chiral aromatic alcohols, including ArCH(OH)R, ArCH 2CH(OH)R, Ar(CH) 2CH(OH)R, simple tertiary arylalkylcarbinols and trans-2-arylcyclohexanols. In mobile phases of low polarity separations are characterized by considerate levels of enantioselectivity ( α = 1.1–2.7), excellent band shapes and short elution times. The hydroxyl group of the analytes is shown to be essential for stereodiscrimination, while π-basicity and steric bulkiness determine the magnitude of enantioselectivity. For simple structured arylalkylcarbinols a correlation is found between elution order and absolute configuration of the analyte. A rationale for molecular recognition of carbinol type analytes on DNB-DPEDA-derived CSPs is advanced. Their general applicability is demonstrated for a number of chiral auxiliaries, solvating agents and drug intermediates of carbinol structure.

Enantioselective chromatography on brush-type chiral stationary phases containing totally synthetic selectors theoretical aspects and practical applications

A family of totally synthetic, brush-type chiral stationary phases (CSPs) for HPLC applications are available through a simple and flexible methodology based on the covalent attachment of chiral amines to oxirane-activated silica microparticles. Tailored CSPs for specific classes of compounds can be obtained by a proper choice of the optically active amine. Some typical features of the above phases are illustrated, with new results on the thermodynamics of CSP-solute interactions and on the analysis of stereolabile compounds.

Observations relevant to the differential intercalation of enantiomers between the strands of brush-type chiral stationary phases

Differential penetration of the enantiomers of certain analytes between the strands of “brush-type” liquid chromatographic chiral stationary phases (CSPs) has been invoked in several instances to explain the progressive change in separation factors as one proceeds, chromatographically, through a homologous series of these analytes. These “intercalative effects” are presumed to arise as a consequence of the semi-ordered, side-by-side bonding of selector strands in conventional brush-type CSPs. Thus, they are not expected to occur in the absence of the stationary phase. A recently developed proline-derived CSP affords large separation factors for the enantiomers of some esters, thioesters, and amides of N-acylated α-amino acids, the separation factor being dependent upon the length of the C-terminal substituent of the analyte. For example, the separation factors increase with the length of the linear thioalkyl substituent when N-(3,5-dinitrobenzoyl)leucine thioesters are chromatographed on the proline-derived CSP. However, the length of this substituent does not affect the enantiomeric purity attained when the enantiomers of these thioesters are stereochemically equilibrated in solution in the presence of the proline-derived selector employed in the CSP. These observations further support the concept of differential intercalation of enantiomers between the strands of brush-type CSPs and may have implications for asymmetric syntheses conducted with immobilized catalysts or reagents.

Enantiomeric Resolution with a New Chiral Stationary Phase of 7-Chloro-3-methyl-3,4-dihydro-2H1,2,4-benzothiadiazine S, S-Dioxide, a Cognition-Enhancing Benzothiadiazine Derivative

The direct analytical and semipreparative high-performance liquid chromatographic (HPLC) resolution of the enantiomers of IDRA 21 [1, 7-chloro-3-methyl-3,-4-dihydro-2H-1,2,4-benzothiadiazine S, S-dioxide] is reported. (±)-DRA 21 administered orally to rats subjected to a water the maze cognition test elicited a performance enhancing effect. Between the two enantiomers, (+)-IDRA 21 was identified as being pharmacologically active in the water maze performance test, whereas (-)-IDRA 21 was completely devoid of activity when given in doses comparable to those of the dextrorotatory (+)-enantiomer. The design and preparation of a new chiral stationary phase (CSP) employed for the liquid chromatographic resolution of the enantiomers of racemic IDRA 21 is presented. This brush-type CSP, which has not been described before, is a “mixed” (π-donor π-acceptor) type and is derived from (R)-N-(3,5-dinitrobenzoyl) allylglycine 2,6-dimethylanilide. It is easily prepared and possesses a relatively broad scope of applicability, as determined by its ability to resolve the enantiomers of both π-acidic and π-basic compounds.

Naproxen-derived segmented and sidechain-modified polysiloxanes as chiral stationary phases

The preparation of a series of polysiloxane-based chiral stationary phases (CSPs) derived from ( S)-naproxen diallyl amide is described. Two types of chiral polysiloxanes have been synthesized. Segmented polymers were obtained by double hydrosilation of the diallyl amide with difunctional oligomethylhydrosiloxanes. The latter are obtained by a linear homologation strategy, thus allowing systematic variation of the spacing of the chiral selectors along the polymer chain. This spacing clearly affects chromatographic behavior, for increased spacing decreases retention even when the total amount of chiral selector present is unaltered. Alternatively, chiral polysiloxanes were prepared by adding, through hydrosilylation, the diallyl amide as pendent sidechains to a polymethyl-hydrosiloxane. The resulting polymers were coated on silica gel, packed into HPLC columns and evaluated using normal and reversed-phase conditions. Comparison of the performance of both types of chiral polysiloxane-based CSPs shows them to afford less retention and comparable or higher enantioselectivity than does the corresponding brush-type CSP.

Behaviour of allyl aryl sulfoxides in high-performance liquid chromatography on a chiral stationary phase

Several racemic allyl aryl sulfoxides were resolved by HPLC on a brush-type chiral stationary phase based on the 3,5-dinitrobenzoyl derivative of ( R, R)-1,2-diaminocyclohexane. Peakdeformations due to exchange phenomena (on-column enantiomerization) were observed at high column temperatures and low eluent flow-rates; under these conditions the extent of interconversion during chromatography was dependent on the kind of substitution of the aromatic ring of the analytes. Kinetic data for the racemization process in solution were also obtained after preparative HPLC separation of the individual enantiomers.

Imprintable brush-type chiral stationary phase

A chiral stationary phase (CSP) based upon a previously studied naphthamide atropisomer is described. Although CSPs based upon imprinted polymers have been known for some time, this naphthamide-derived CSP is the first reported example of an imprintable brush-type CSP. Deracemization of the racemic CSP using a soluble enantiopure imprint molecule affords a CSP which resolves the enantiomers of the imprint molecule and related compounds. A time-dependent HPLC study of the reversion of the imprinted stationary phase to the racemic form was performed, with a decrease in enantioselectivity being noted over a period of two weeks at room temperature, after which time the stationary phase was no longer capable of resolving enantiomers. Speculations about the potential utility of such CSPs are offered.

Chromatographic and 1H NMR support for a proposed chiral recognition model

Liquid chromatography and 1H NMR spectroscopy were used in an investigation of a chiral recognition rationale which was previously advanced to account for the resolution of naproxen (NAP-COOH) enantiomers on brush-type chiral stationary phases, CSP-1 and CSP-2, identical except for tether length. CSP-1 has recently been commercialized as the Whelk-O 1. This chiral stationary phase (CSP), basically an immobilized enantiomer of N-(3,5-dinitrobenzoyl)-4-amino-1,2,3,4-tetrahydrophenanthrene, was designed to have a cleft in which one enantiomer is preferentially bound. The cleft consists of π-acidic and π-basic aromatic systems held more or less perpendicular to each other. Aromatic substituents in the analyte were expected to be held in this cleft by simultaneous face-to-face and face-to-edge π—π interactions. To ascertain whether analytes are truely bound in this cleft, NMR studies of mixtures of several naproxen-like analytes and chiral solvating agent 3, a soluble version of the selector used in CSP-1, were undertaken. Additional motivation for the study came from the observation that the enantiomers of NAP-COOH and NAP-COOMe elute in a different order than do the enantiomers of NAP-CONHMe, and NAP-CON(Me) 2. This difference in the elution order of the amide derivatives with respect to the esters and free acid was not totally unexpected, for the chiral recognition hypothesis used in the design of the chiral selector allowed for such an eventuality. It was known from reciprocal chromatographic studies that the enantiomers of soluble analogs of the Whelk-O 1 CSP show different elution orders on naproxen-derived amide CSPs than they do on naproxen-derived esters CSPs. These data aided in formulation of the initial chiral recognition rationale. Evidence for the occurance of the specific molecular interactions suggested by this rational is provided by the presently described 1H NMR study of the enantioselective complexation of the enantiomers of NAP-COOH, NAP-COOMe, NAP-CONHMe, and NAP-CON(Me) 2 by a single enantiomer of a soluble analog of the Whelk-O 1 CSP.