Simple Chiral Liquid Chromatographic Enantioseparation of Some Racemic Antiepileptic Drugs

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Our research cooperation with Professor Yoshio Okamoto

This article summarizes our cooperation with the research group of Prof. Yoshio Okamoto at Nagoya University during the period of time between 1992 and 2005. Although the text deals entirely with enantioseparations in high-performance liquid chromatography, capillary electrophoresis, and capillary electrochromatography, this is not a detailed review in any of these areas. The text highlights selected aspects of these techniques, which have been the subject of our joint research and in part their reflection in follow-up research by our and other research groups. Together with more systematically studied topics, aspects such as ultrafast separation of enantiomers, uncommonly high separation factor of enantiomers and other related issues are also addressed.

Enantioresolution of basic pharmaceuticals using cellulose tris(4-chloro-3-methylphenylcarbamate) as chiral stationary phase and polar organic mobile phases

A polysaccharide-based chiral stationary phase (Sepapak-4), with cellulose tris(4-chloro-3-methylphenylcarbamate) as chiral selector, has been investigated in liquid chromatography (LC). Its enantioresolution power was evaluated towards 13 basic amino-drugs with widely different structures and polarities, using polar organic mobile phases. After preliminary experiments, acetonitrile was selected as the main mobile phase component, to which a low concentration of diethylamine (0.1%) was systematically added in order to obtain efficient and symmetrical peaks. Different organic solvents were first added in small proportions (5-10%) to acetonitrile to modulate analyte retention. Polar organic modifiers were found to decrease retention and enantioresolution while hexane had the opposite effect, indicating normal-phase behaviour under these conditions. The addition of an organic acid (formic, acetic or trifluoroacetic acid) was found to strongly influence the retention of the basic amino drugs in these nonaqueous systems. The nature and proportion of the acidic additive in the mobile phase had also deep impact on enantioresolution. Therefore, the studied compounds could be subdivided in three groups in respect to the acidic additive used. All analytes could be enantioseparated in relatively short analysis times (10-20 min) using these LC conditions.

Comparative enantioseparation of selected chiral drugs on four different polysaccharide-type chiral stationary phases using polar organic mobile phases

The enantiomers of randomly selected chiral drugs and drug analogs of various structural and pharmacological groups were resolved on four different polysaccharide-type chiral stationary phases (CSP) using pure methanol and acetonitrile as mobile phases. Polysaccharide phenylester type CSP, Chiralcel-OJ although resolving the enantiomers of some chiral drugs was less universal in the combination with methanol and acetonitrile as mobile phases. Among polysaccharide phenylcarabamates amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak-AD) was superior over the corresponding cellulose derivative, cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel-OD). However, another derivative of cellulose, namely, cellulose tris(3,5-dichlorophenylcarbamate) (CDCPC) exhibited higher chiral recognition ability compared to Chiralpak-AD material. This study confirms previous findings about the applicability of polysaccharide type CSPs in so called polar organic mode as well as shows high potential of CDCPC as a practically useful CSP for High performance liquid chromatography (HPLC) enantioseparations.

Enantioseparations by capillary electrochromatography

The review summarizes recent developments in enantioseparations by capillary electrochromatography (CEC). Selected fundamental aspects of CEC are discussed in order to stress those features which may allow the success of this technique in the competitive field of enantioseparations. In addition, the comparative characteristics of the different modes of chiral CEC and the stationary phases are presented. The effects of the characteristics of the stationary and liquid phases and operational conditions on the separation results are discussed. Finally, some future trends are briefly addressed.

Enantioseparations using nonaqueous capillary electrochromatography on cellulose and amylose tris(3,5-dimethylphenylcarbamates) coated on silica gels of various pore and particle size

Enantioseparations of chiral compounds were studied in nonaqueous capillary electrochromatography (NAQ CEC) with cellulose and amylose tris(3,5-dimethylphenylcarbamates) (Chiralcel OD and Chiralpak AD, respectively) coated on the silica gels of various pore and particle size. Increasing intraparticle perfusive transport with increasing pore size of silica favorably affected peak efficiency and resolution of enantiomers, although some decrease of separation factor was observed in the pore size range 60-200 A. Further improvement of peak efficiency was observed when the particle size of silica was reduced from 5 to 3 microm. The effects of a separation medium and temperature are also reported and the data obtained in the same capillaries in CEC and capillary liquid chromatography (LC) mode are compared.

Reversed-phase liquid chromatographic separation of enantiomers on polysaccharide type chiral stationary phases

The direct chiral separation by chiral stationary phases (CSPs) is one of the most important techniques to analyze enantiomeric purity as well as to get enantiomerically pure material quickly. Among various types of CSPs, polysaccharide type CSPs are well known by their versatility and durability. They are not only effective under normal-phase conditions, but also under reversed-phase conditions. In order to get a good separation under the reversed-phase conditions, it is the key to choose an appropriate mobile phase. For example, a simple mixture of water/acetonitrile or water/methanol are sufficient for a neutral analyte, while it is necessary to use an acidic solution instead of water for an acidic analyte and a solution of a chaotropic salt (or a basic solution) for a basic analyte, respectively. The paper also presents lists of more than 350 separation examples that include 22 validated methods for drug analyses from serum, plasma, or urine samples on polysaccharide type CSPs under reversed-phase conditions.

Enantioseparations using capillary electromigration techniques in nonaqueous buffers

This review summarizes recent developments in the field of enantioseparations in capillary electromigration techniques using nonaqueous background electrolytes. The more established and rather intensively reviewed field of nonaqueous chiral capillary electrophoresis (NAQ-CE) is covered in less detail whereas more attention is paid to the relatively new field of nonaqueous capillary electrochromatography (NAQ-CEC).

Simultaneous determination of methylphenobarbital enantiomers and phenobarbital in human plasma by on-line coupling of an achiral precolumn to a chiral liquid chromatographic column

A fully automated liquid chromatographic (LC) method for the simultaneous determination of methylphenobarbital enantiomers and phenobarbital in human plasma has been developed. The method is based on the use of a precolumn packed with an internal-surface reversed-phase packing material (LiChrospher ADS) for sample clean-up coupled to LC analysis on a cellulose tris(4-methylbenzoate) based chiral stationary phase (Chiralcel OJ-R). A 100-microliter plasma sample was injected directly on the precolumn packed with LiChrospher RP-18 ADS using a mixture of pH 5.0 phosphate buffer-methanol (97:3, v/v) as washing liquid. The analytes were then eluted in the back-flush mode with the LC mobile phase. The enantiomeric separation of methylphenobarbital was achieved on Chiralcel OJ-R). The retention times were modelled using a D-optimal design with ten experimental points in order to optimise the LC mobile phase for the separation of phenobarbital from the enantiomers of mephobarbital. The factors selected were the acetonitrile content, the pH and the sodium perchlorate concentration in the mobile phase. A Derringer's desirability function was used to find an optimal and robust solution within the experimental domain. The mobile phase selected consisted of a mixture of pH 7.0 phosphate buffer-acetonitrile (60:40, v/v). The elution profiles of phenobarbital, methylphenobarbital and blank plasma samples on the precolumn and the time needed for analyte transfer from the precolumn to the analytical column were then determined. Finally, the method developed was validated.

Recent applications of chromatographic resolution of enantiomers in pharmaceutical analysis

The principles and applications of chromatographic separation of enantiomers in pharmaceutical analysis have been reviewed. Several of recently reported enantioselective analysis of various racemic drugs using both the 'indirect' and 'direct' methods have been presented.

High performance liquid chromatography enantioseparation of chiral pharmaceuticals using tris(chloro-methylphenylcarbamate)s of cellulose

Chiral recognition abilities of a recently developed new type of cellulose phenylcarbamates were studied. These chiral stationary phases (CSPs) simultaneously contain both electron-withdrawing (Cl) and electron-donating (CH3) substituents on the phenyl moiety. Chiral pharmaceuticals which belong to the various pharmacological groups (sedatives, hypnotics, anticonvulsants, Ca2+ channel blockers, beta-blockers, antitusives, antihystaminics, choleretics, diuretics, antimycotics, etc) were resolved to enantiomers. These new CSPs sometimes exhibit alternative chiral recognition ability to that most successful commercially available cellulosic CSP Chiralcel-OD and can be used as a good complement to it in analytical and preparative scale enantioseparations.