Use of derivatized cyclodextrins as chiral selectors for the separation of enantiomers by gas chromatography.

The state-of-the-art of enantioseparations of various chiral compounds on derivatized cyclodextrins, employed as chiral stationary phases (CSPs) by gas chromatography, is reviewed. Heptakis(2,3,6-O-trimethyl)-beta-cyclodextrin and octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma-cyclodextrin (Lipodex E), either dissolved in a semipolar polysiloxane or bonded to poly(dimethylsiloxane) (Chirasil-Dex), proved to be the most popular chiral selectors. Special applications refer to the use of heptakis (2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin and heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin. Enantioselectivity is generally low with enantioseparation factors in the range of 1.02

A mixed stationary phase containing two versatile cyclodextrin-based selectors for the simultaneous gas chromatographic enantioseparation of racemic alkanes and racemic alpha-amino acid derivatives.

In an effort to simultaneously enantioseparate racemic unfunctionalized alkanes and racemic alpha-amino acid derivatives by gas chromatography (GC) in forthcoming experiments related to the search for extraterrestrial homochirality, the two versatile modified cyclodextrin (CD) selectors octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) and heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin were dissolved in a polysiloxane and the mixed binary chiral selector system was coated onto a 50m x 0.25 mm i.d. fused silica capillary column. Whereas the former CD selector enantioseparates racemic unfunctionalized alkanes the latter CD selector preferentially resolves N-(O,S)-trifluoroacetyl-alpha-amino acid alkyl esters. With both CD selectors employed as mixed binary chiral selector system present in one chiral stationary phase (CSP), the simultaneous gas chromatographic enantioseparation of racemic alkanes and of racemic derivatized alpha-amino acids is achieved in a single temperature-programmed run. Also for other classes of racemic compounds, the scope of enantioseparation could be extended as compared to the conventional use of the single CD selectors in GC.

The control of the nitrogen inversion in alkyl-substituted diaziridines.

For the first time the nitrogen inversion barriers in 3,3-unsubstituted trans-diaziridines, such as 1,2-di-tert-butyldiaziridine (1) and 1,2-di-n-butyldiaziridine (2) were determined. Enantioselective stopped-flow multidimensional gas chromatography was used to investigate the enantiomerization barrier of 1 between 126.2 and 171.0 degrees C (DeltaG ++ gas (150.7 degrees C) = 135.8+/-0.2 kJ mol(-1), DeltaH++ gas = 116.1+/-2.5 kJ mol(-1), DeltaS ++ gas == -46+/-2 J K(-1) mol(-1)). The separation of the enantiomers has been achieved in presence of the chiral stationary phase (CSP) Chirasil-beta-Dex with a high separation factor (alpha = 1.44 at 80 degrees C). In a complementary approach, the enantiomerization barriers of 1,2-di-tert-butyldiaziridine (1), 1,2-di-n-butyldiaziridine (2), 1-n-butyl-3,3-dimethyldiaziridine (3), and 1,2,3,3-tetramethyldiaziridine (4) were determined for comparison by enantioselective dynamic chromatography (DGC) and computer simulation of the dynamic elution profiles. The enantiomerization barrier of 2 was shown to be the highest among the nonsterically hindered diaziridines studied so far, whereas 1 exhibited the highest value found for strained nitrogen-containing rings, that is, aziridines, diaziridines and oxaziridines.

Accumulation of S(+) enantiomer in human beings after general anaesthesia with isoflurane racemate.

BACKGROUND AND OBJECTIVE: Isoflurane is a chiral, volatile anaesthetic with low metabolic rate (0.17%) that is routinely administered in its racemic form. Knowledge about the distribution of the enantiomers in human beings may give some important information about the understanding of the mechanisms of volatile anaesthetics. METHODS: Blood samples were drawn immediately after tracheal extubation and daily up to 8 days postoperatively from patients undergoing general anaesthesia with isoflurane racemate. The enantiomer enrichment of isoflurane was determined by headspace gas chromatography-mass spectrometry. RESULTS: At all time points, there was a statistically significant accumulation of the S(+) enantiomer in blood, especially at days 2 (52.01%) and 7 (52.1%). Separate analysis of obese patients or in a small group of patients with co-existing lung disease did not show any difference to the total population. In addition, duration of anaesthesia did not influence the enantiomer concentrations. CONCLUSIONS: We suggest that a slower association and dissociation rate is responsible for the S(+) enrichment.

Absolute configuration and conformational analysis of a degradation product of inhalation anesthetic Sevoflurane: A vibrational circular dichroism study.

1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane, compound B, is a product obtained in the degradation of the anesthetic Sevoflurane. Enantiopure (+)-B was investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-B in CDCl(3) solution in the 2,000-900 cm(-1) region are compared with the ab initio predictions of absorption and VCD spectra obtained from density functional theory using B3LYP/6-31G* basis set for different conformers of (S)-1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane. This comparison indicates that (+)-B is of the (S)-configuration in CDCl(3) solution, in agreement with previous literature results. Our results also indicate that this compound adopts six predominant conformations in CDCl(3) solution.

Chiral discrimination of limonene by use of beta-cyclodextrin-coated quartz-crystal-microbalances (QCMs) and data evaluation by artificial neuronal networks.

The enantiomeric composition of the chiral flavoring agent limonene was analyzed by means of a quartz-crystal microbalance (QCM) sensor. As chiral selectors three different modified beta-cyclodextrins were investigated. The selector molecules were applied as mixtures in different polysiloxane matrices. The chiral separation factors alpha for limonene obtained at 30 degrees C by gas chromatography and by use of the QCM sensor were comparable. Evaluation of sensor data was performed by use of an artificial neuronal network (ANN); this enabled prediction of the enantiomeric composition of the gas mixtures.

Determination of the enantiomerization barrier of thalidomide by dynamic capillary electrokinetic chromatography.

Enantioselective chromatographic methods, representing the most commonly used techniques for the determination of enantiomeric ratios, can also be used for the evaluation of stereochemical integrity. In the present study, dynamic capillary electrokinetic chromatography (DEKC) was employed to determine the enantiomerization barrier of thalidomide. In the presence of the chiral mobile phase additive carboxymethyl-beta-cyclodextrin, the interconverting enantiomers of thalidomide produced characteristic elution profiles exhibiting plateaus and/or peak broadening between 25 and 55 degrees C at pH 8. To obtain the enantiomerization barrier of thalidomide from experimental data, the fast and efficient simulation program ChromWin was used to simulate the experimental interconversion profiles and to obtain the apparent rate constants k1app(T). Additionally, these values were compared with the novel approximation function for the direct calculation of enantiomerization barriers from chromatographic parameters of elution profiles. From the rate constants k1app(T) of temperature-dependent measurements the kinetic activation parameters deltaG(T)#,deltaH#, and deltaS# of the enantiomerization of thalidomide were obtained. At 25 degrees C, the enantiomerization barrier deltaG# was determined to be 102 +/- 1 kJ/mol at pH 8 in the dynamic electrokinetic chromatographic experiment.

Miniaturization of enantioselective gas chromatography.

In the context of the Titan Chiral Organics Explorer (TCOE) mission, the theoretical and practical aspects of the miniaturization of enantioselective gas chromatography are discussed. The use of short columns (0.5-10 m) with a conventional inside diameter of 250 microm is proposed. The inside diameter should be increased for vacuum-assisted GC-MS using a restrictor at the inlet of the capillary. The feasibility of cryogenic enantioselective gas chromatography is demonstrated. The hitherto elusive use of chiral mobile phase additives in enantioselective gas chromatography is proposed.

Search for extraterrestrial enantioenrichment by using chemical microsensors.

The use of enantioselective chemical microsensors is proposed for the search of extraterrestrial homochirality in space. The already established enantiomer-discrimination-capability of chemical sensors and the feasibility of quantitatively determining the enantiomeric composition of a target analyte are demonstrated. The benefits of applying modern microsensor technology are presented followed by some concepts and scenarios including how chemical microsensors could be used in space.

Pharmacokinetics of levetiracetam and its enantiomer (R)-alpha-ethyl-2-oxo-pyrrolidine acetamide in dogs.

PURPOSE: The new antiepileptic drug, levetiracetam (LEV, ucb LO59), is a chiral molecule with one asymmetric carbon atom whose anticonvulsant activity is highly enantioselective. The purpose of this study was to evaluate and compare the pharmacokinetics (PK) of LEV [(S)-alpha-ethyl-2-oxo-pyrrolidine acetamide] and its enantiomer (R)-alpha-ethyl-2-oxo-pyrrolidine acetamide (REV) after i.v. administration to dogs. This is the first time that the pharmacokinetics of both enantiomers has been evaluated. METHODS: Optically pure LEV and REV were synthesized, and 20 mg/kg of individual enantiomers was administered intravenously to six dogs. Plasma and urine samples were collected until 24 h, and the concentrations of LEV and REV were determined by an enantioselective assay. The levels of 2-pyrrolidone-N-butyric acid, an acid metabolite of LEV and REV, were determined by high-performance liquid chromatography (HPLC). The data were used for PK analysis of LEV and REV. RESULTS: LEV and REV had similar mean +/- SD values for clearance; 1.5 +/- 0.3 ml/min/kg and volume of distribution; 0.5 +/- 0.1 L/kg. The half-life (t1/2) and mean residence time (MRT) of REV (t1/2, 4.3 +/- 0.8 h, and MRT, 6.0 +/- 1.1 h) were, however, significantly longer than those of LEV (t1/2, 3.6 +/- 0.8 h, and MRT, 5.0 +/- 1.2 h). The renal clearance and fraction excreted unchanged for LEV and REV were significantly different. CONCLUSIONS: In addition to the enantioselective pharmacodynamics, alpha-ethyl-2-oxo-pyrrolidine acetamide has enantioselective PK. The enantioselectivity was observed in renal clearance. Because REV has more favorable PK in dogs than LEV, the higher antiepileptic potency of LEV is more likely due to intrinsic pharmacodynamic activity rather than to enantioselective PK.

Determination of the cis-trans isomerization barrier of several L-peptidyl-L-proline dipeptides by dynamic capillary electrophoresis and computer simulation.

Dynamic capillary electrophoresis (DCE) and computer simulation of the elution profiles with the theoretical plate and the stochastic model has been applied to determine the isomerization barriers of the three dipeptides L-alanyl-L-proline, L-leucyl-L-proline, and L-phenylalanyl-L-proline. The separation of the rotational cis-trans isomers has been performed in an aqueous 70 mM borate buffer at pH 9.5. Interconversion profiles featuring plateau formation and peak broadening were observed. To determine the rate constants k1 and k(-1) of the cis-trans isomerization in dynamic capillary electrophoresis, equations have been derived for the theoretical plate model and stochastic model. The electropherograms were simulated with the ChromWin software which uses the experimental data plateau height h(plateau), peak width at half height Wh, the total migration times of the cis-trans isomers tR and the electroosmotic break-through time t0 as well as the peak ratio [cis]/[trans]. From temperature-dependent measurements, the rate constants k1 and k(-1) and the kinetic activation parameters deltaG#, deltaH# and deltaS# of the cis-trans isomerization of the three dipeptides were obtained.

Enantiomer separation by nonaqueous and aqueous capillary electrochromatography on cyclodextrin stationary phases.

Native beta- and gamma-cyclodextrin bound to silica (ChiraDex-beta and ChiraDex-gamma) were packed into capillaries and used for enantiomer separation by capillary electrochromatography (CEC) under aqueous and nonaqueous conditions. Negatively charged analytes (dansyl-amino acids) were resolved into their enantiomers by nonaqueous CEC (NA-CEC). The addition of a small amount of water to the nonaqueous mobile phase enhanced the enantioselectivity but increased the elution time. The choice of the background electrolyte (BGE) determined the direction of the electroosmotic flow (EOF). With 2-(N-morpholino) ethanesulfonic acid (MES) or triethylammonium acetate (TEAA) as BGE an inverse EOF (anodic EOF) was observed while with phosphate a cathodic EOF was found. The apparent pH (pH*), the concentration of the BGE, and the nature of the mobile phase strongly influenced the elution time, the theoretical plate number and the chiral separation factor of racemic analytes.

Time-resolved cryogenic modulation reveals isomer interconversion profiles in dynamic chromatography.

The dynamic chromatographic study of interconversion of E and Z forms of oximes has been investigated by using a novel cryogenic modulation method in a two-dimensional gas chromatographic array. The primary column is a conventional capillary GC column on which the molecular interconversion proceeds. In this case, the molecular dynamical process leads to a peak profile describing the kinetics and thermodynamics of the interconverting molecules during its chromatographic elution. Thus an interconversion region intercedes the elution of the individual stereoisomers of the reaction. Since the molecules are isomers, classical molecular identification methods such as gas chromatography-mass spectrometry are unable to study the individual instantaneous amounts of each of the compounds. Hence the infinitesimal profiles of interconversion along the entire column have never been experimentally observed; rather the total profile is normally subjected to mathematical modelling studies in order to match experiment with theory, and to gain the kinetic parameters of the process. In the present study, an instantaneous ratio of the individual isomers can be found during the chromatographic elution by direct measurement. This is achieved by using a cryogenic zone focussing process, with rapid longitudinal modulation of a cold trap and continual pulsing of collected zones into a fast-analysis high-resolution capillary column on which isomer interconversion is minimized. The data can be displayed as a two-dimensional contour plot to demonstrate the individual isomer profiles. The two-dimensional analysis also allows easy measurement of the peak ratios of the two isomers which is an indicator of the extent of interconversion that has taken place. Two model systems, acetaldoxime and butyraldoxime, were chosen to illustrate the use of the cryogenic modulation procedure. It is anticipated that the procedure could be applied to other molecules which exhibit gas-phase isomerizations or reactions.

Determination of enantiomerization barriers by dynamic and stopped-flow chromatographic methods.

In recent years, dynamic chromatography and stopped-flow chromatographic techniques have become versatile tools for the determination of enantiomerization and isomerization barriers. Increasing demands for the stereochemical safety of chiral drugs contributed to the rapid development of new techniques. New computer-aided evaluation systems allow the on-line determination of interconversion barriers from the experimental chromatograms. Both dynamic chromatography and stopped-flow chromatography have been applied to the entire range of chromatographic methods (GC, SFC, HPLC, CE).

Separation of enantiomers by open capillary electrochromatography on polysiloxane-bonded permethyl-beta-cyclodextrin.

The separation of enantiomers by open capillary electrochromatography (o-CEC) using Chirasil-Dex as chiral stationary phase (CSP) is reviewed. In Chirasil-Dex, permethylated beta-cyclodextrin is linked via a single octamethylene spacer to polydimethylsiloxane. The CSP is coated and thermally immobilized onto the internal surface of a fused-silica column (i.d. 50 microm). Employing a single open-tubular column coated with Chirasil-Dex, a unified enantioselective approach can be realized using the four common chromatographic techniques: o-GC, o-SFC, o-LC and o-CEC. The chiral stationary phase Chirasil-Dex can be combined with a charged cyclodextrin derivative, which is added into the mobile phase. In the resulting dual chiral recognition system, enhancement of enantioselectivity (matched case) or compensation of enantioselectivity (mismatched case) are observed. The overall enantioselectivity is dependent on the sense of enantioselectivity of the selectors chosen and their influence on the electrophoretic and electroosmotic migration of the enantiomers of a selectand. The feasibility to couple chiral o-CEC and ESI/MS is demonstrated for trace analysis of enantiomeric drugs in body fluids.

Determination of interconversion barriers by dynamic gas chromatography: epimerization of chalcogran.

The four stereoisomers of chalcogran 1 ((2RS,SRS)-2-ethyl-1,6-di-oxaspiro[4.4]nonane), the principal component of the aggregation pheromone of the bark beetle pityogenes chalcographus, are prone to interconversion at the spiro center (C5). During diastereo- and enantioselective dynamic gas chromatography (DGC), epimerization of 1 gives rise to two independent interconversion peak profiles, each featuring a plateau between the peaks of the interconverting epimers. To determine the rate constants of epimerization by dynamic gas chromatography (DGC), equations to simulate the complex elution profiles were derived, using the theoretical plate model and the stochastic model of the chromatographic process. The Eyring activation parameters of the experimental interconversion profiles, between 70 and 120 C in the presence of the chiral stationary phase (CSP) Chirasil-beta-Dex, were then determined by computer-aided simulation with the aid of the new program Chrom-Win: (2R,5R)-1: deltaG(++) (298.15 K) = 108.0 +/-0.5 kJ mol(-1), deltaH(++) = 47.1+/-0.2 kJ mol(-1), deltaS(++) = -204+/-6 JK(-1) mol(-1): (2R,5S)-1: deltaG(++) (298.15 K) = 108.5+/-0.5 kJ mol(-1), deltaH(++) = 45.8+/-0.2 kJ mol(-1), deltaS(++) = -210 +/-6 J K mol(-1); (2S,5S)-1: deltaG(++) (298.15 K)= 108.1+/-0.5 kJ mol(-1), deltaH(++) = 49.3+/-0.3 kJ mol(-1), deltaS(++) = -197+/-8 J K(-1) mol(-1); (2S,5R)-1: deltaG(++) (298.15 K)=108.6+/-0.5 kJ mol(-1), deltaH(++) = 48.0+/-0.3 kJ mol(-1), deltaS(++) = -203+/-8 J K(-1) mol(-1). The thermodynamic Gibbs free energy of the E/Z equilibrium of the epimers was determined by the stopped-flow multidimensional gas chromatographic technique: deltaG(E/Z) (298.15 K)= -0.5 kJ mol(-1), deltaH(E/Z) = 1.4 kJ mol(-1) and deltaS(E/Z) = 6.3 J K(-1) mol(-1). An interconversion pathway proceeding through ring-opening and formation of a zwitterion and an enol ether/alcohol intermediate of 1 is proposed.

Approximation function for the direct calculation of rate constants and Gibbs activation energies of enantiomerization of racemic mixtures from chromatographic parameters in dynamic chromatography.

An approximation function for enantioselective dynamic chromatography of racemic mixtures of interconverting enantiomers has been derived that allows the direct calculation of enantiomerization rate constants (k1 and k(-1)) and Gibbs activation energies of enantiomerization, deltaG++ , from chromatographic parameters, i.e., retention times of the enantiomers A and B ((t(A)R and t(B)R), peak widths at half height (WA and wB) and the relative plateau height (hplateau), without computer simulation. The reaction rate constants of enantiomerization, k(-1), obtained with this approximation function, have been validated by comparison with a simulated dataset of 15,625 chromatograms. The mean, standard deviation and confidence interval show a high correlation between the approximated and simulated rate constants. The average deviation from the Gibbs activation enthalpy of enantiomerization, deltaG++, has been estimated to be as small as about +/- 0.11 RT.

ChromWin--a computer program for the determination of enantiomerization barriers in dynamic chromatography.

The software program ChromWin simulates interconversion profiles in dynamic chromatography (rearrangements, isomerizations, epimerizations, diastereomerizations and, notably, enanatiomerizations) on a personal computer in a user-friendly environment. ChromWin is especially suited for systems involving large plate numbers, e.g. gas chromatography (GC) and capillary electrophoresis (CE, CEC, MEKC), and first or pseudo-first order reactions. ChromWin (1) simulates the elution profiles of interconverting enantiomers by different models and yields the rate constant, (2) allows automation of the 'find enantiomerization barrier' function, (3) helps to optimise separation parameters by visualization of the separation process and (4) provides other useful tools for chromatography. In addition to the theoretical plate and the stochastic model a modified and improved stochastic model has been developed and implemented in the program.

Separation of enantiomers by gas chromatography.

The separation of enantiomers by gas chromatography is performed on chiral stationary phases (CSPs) via hydrogen bonding, coordination and inclusion. Thus, typical chiral selectors are amino acid derivatives, terpene-derived metal coordination compounds and modified cyclodextrins. In Chirasil-type stationary phases the chiral selector is anchored to a polysiloxane backbone improving gas chromatographic performance. The present review article describes the state-of-the-art, scope and limitations, applications and mechanistic considerations at the advent of the millennium incorporating 16 figures and 168 references.