Racemates, resolution using cyclodextrins

TABLE 1 Chiral Resolution of Some Racemic Compounds Using Cyclodextrins as the Mobile Phase Additives... 

In 1978, Harada et al. [17] used polymerized CD with gel support for the chiral resolution of mandelic acid and its derivatives. Later Zsadon et al. [18-21] used cyclodextrin-based CSPs for the chiral resolution of indole alkaloids, with aqueous buffers as the mobile phases. Today CD-based CSPs have a good reputation. In separate studies, Fujimura [22] and Kawaguchi [23] and their colleagues resolved the enantiomers of aromatic compounds in the reversed-phase mode. Armstrong et al. [29,30,33,34,41,44 46,48,54-63] carried out extensive and remarkable work on the chiral resolution of various racemic compounds using CD-based CSPs. 

Because the steric effect contributes to the complex formation between guest and host, the chiral resolution on these CSPs is affected by the structures of the analytes. Amino acids, amino alcohols, and derivatives of amines are the best classes for studying the effect of analyte structures on the chiral resolution. The effect of analyte structures on the chiral resolution may be obtained from the work of Hyun et al. [47,48]. The authors studied the chiral resolution of amino alcohols, amides, amino esters, and amino carbonyls. The effects of the substituents on the chiral resolution of some racemic compounds are shown in Table 6. A perusal of this table indicates the dominant effect of steric interactions on chiral resolution. Furthermore, an improved resolution of the racemic compounds, having phenyl moieties as the substituents, may be observed from this Table 6. ft may be the result of the presence of n—n interactions between the CCE and racemates. Generally, the resolution decreases with the addition of bulky groups, which may be caused by the steric effects. In addition, some anions have been used as the mobile phase additives for the improvement of the chiral resolution of amino acids [76]. Recently, Machida et al. [69] reported the use of some mobile phase additives for the improvement of chiral resolution. They observed an improvement in the chiral resolution of some hydrophobic amino compound using cyclodextrins and cations as mobile phase additives. 

The optical enrichment of halothan, CFjCHBrCl 41), using a-cyclodextrin (26) is the only known strategy hitherto for racemate resolution of this substance. This is a good example to demonstrate the independency of the inclusion method on functional groups, even if the optical yield is only low as yet. These are indispensable when classical methods of resolution, e.g. use of diastereomeric salts, are applied consequently their application field is very restricted. 

Figure 10.1 Analysis of racemic 2,5-dimethyl-4-hydroxy-3[2H]-furanone (1) obtained from a strawbeny tea, flavoured with the synthetic racemate of 1 (natural component), using an MDGC procedure (a) dichloromethane extract of the flavoured strawbeny tea, analysed on a Carbowax 20M pre-column (60 m, 0.32 mm i.d., 0.25 p.m film thickness earner gas H2, 1.95 bar 170 °C isothermal) (b) chirospecific analysis of (1) from the sti awbeny tea exti act, ti ansfened foi stereoanalysis by using a pemiethylated /3-cyclodextrin column (47 m X 0.23 mm i.d. canier gas H2, 1.70 bar 110 °C isothemial). Reprinted from Journal of High Resolution Chromatography, 13, A. Mosandl et al., Stereoisomeric flavor compounds. XLIV enantioselective analysis of some important flavor molecules , pp. 660-662, 1990, with permission from Wiley-VCH.

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

Gas chromatography (GC) has also been used for preparative purposes, but is restricted to relatively volatile racemates such as anesthetics, pheromones or monoterpenes and, therefore, very few applications are reported. Nevertheless, in the cases to which GC may be applied, it could be considered as an economical alternative to HPLC. Most of the resolutions of enantiomers were performed on cyclodextrin-derived CSPs [109, 144-153], and only on very few occasions were other chiral selectors used [153]. 

Cyclodextrin-based CSPs are among the most popular materials used for the chiral resolution of racemic compounds. These CSPs have a wide range of applications because they can be used successfully in all three mobile phase modes normal, reversed, and polar organic. There are numerous examples of chiral separations on CDs and CSPs based on their derivatives. Some of the important chiral separations are discussed herein. 

The most popular and commonly used chiral stationary phases (CSPs) are polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ether based. The art of the chiral resolution on these CSPs has been discussed in detail in Chapters 2-8, respectively. Apart from these CSPs, the chiral resolutions of some racemic compounds have also been reported on other CSPs containing different chiral molecules and polymers. These other types of CSP are based on the use of chiral molecules such as alkaloids, amides, amines, acids, and synthetic polymers. These CSPs have proved to be very useful for the chiral resolutions due to some specific requirements. Moreover, the chiral resolution can be predicted on the CSPs obtained by the molecular imprinted techniques. The chiral resolution on these miscellaneous CSPs using liquid chromatography is discussed in this chapter. 

Cyclodextrins (CDs) are cyclic and nonreducing oligosaccharides and obtained from starch. The structures and properties of these molecules were discussed in detail in Chapter 3. These molecules are soluble in aqueous mobile phases and, hence, most of the chiral resolution was carried out under the reversed-phase mode. Therefore, cyclodextrins were used frequently as CMPAs for the chiral resolution of a wide variety of racemic compounds. The nontoxicity, nonvolatile, poor UV absorbance, stability over a wide range of pHs, and inexpensive natures of cyclodextrins make them superb CMPAs. 

The approach of CMPAs has also been used in thin-layer chromatography (TLC) for the chiral resolution of a variety of racemic compounds [100-110]. Lepri et al. [104,105] used BSA as a mobile phase additive for the chiral resolution of dansyl amino acids and other drugs by TLC. Armstrong et al. [101,102] used unde-rivatized and hydroxyethyl and hydroxypropyl /I-cyclodextrins for the chiral resolution of dansyl amino acids, alkaloids, and other compounds. Aboul-Enein... 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

Bonato and Paias [136] developed two sensitive and simple assay procedures based on HPLC and capillary electrophoresis for the enantio-selective analysis of omeprazole in pharmaceutical formulations. Racemic omeprazole and (S)-omeprazole were extracted from commercially available tablets using methanol-sodium hydroxide 2.5 mol/1 (90 10). Chiral HPLC separation of omeprazole was obtained on a ChiralPak AD column using hexane-ethanol (40 60) as the mobile phase and detection at 302 nm. The resolution of omeprazole enantiomers by capillary electrophoresis was carried out using 3% sulfated /1-cyclodextrin in 20 mmol/1 phosphate buffer, pH 4 and detection at 202 nm. 

Contrary to conventional HPLC, almost 98% of chiral resolution in CE is carried out using the chiral selector as a mobile phase additive. Again all the common chiral selectors used in NLC can also be used in NCE. But, unfortunately, few chiral molecules have been tested in NCE for enantiomeric resolution of some racemates. To the best of our knowledge only cyclodextrins and protein-based chiral mobile phase additives have been used for this purpose. Manz and coworkers discussed chiral separations by NCE in their reviews in 2004 [21] and 2006 [22], Later on, Pumera [16] reviewed the use of microfluidic devices for enantiomeric resolutions in capillary electrophoresis. Not much work has been carried out on chiral resolution in NCE but the papers that are available are discussed here. 

Gao et al. [27] described enantiomeric resolution of FITC-labeled basic drugs (baclofen, norfenefrine, and tocainide) by NCE using several neutral cyclodextrins as chiral selectors. FITC-baclofen enantiomers were separated completely by y-CD while resolution of FITC-norfenefrine enantiomers was achieved by dimethyl-(3-cyclodextrins. Furthermore, the authors studied the feasibility of using one chiral selector to separate multiple racemic samples on a four-channel chip. Figure 9.8 indicates the role of some chiral selectors for enantiomeric resolution of FITC-baclofen and FITC-norfenefrine race-mates. A perusal of this figure indicates that HP-a-CD and HP-0-CD are... 

Racemic l-dimethylaminopropan-2-ol (18) was acylated with propanoyl chloride, and the reaction product was analyzed by gas chromatography (GC). The resultant (R)- and (5)-esters 19 (R1 = COEt) were resolved by GC using an a-cyclodextrin column. Reaction of the racemic alcohol 18 was then carried out in the presence of Novozyme 435 and vinyl propanoate, and the reaction was followed by GC. After 4 hours the reaction was approximately 2% complete and the ee of the propanoate ester 19 (R1 = COEt) was 95.9%. After 88 hours the conversion had reached 50% and the ee was still 95.6%. The remarkable specificity of Novozyme 435 for the (/O-amino alcohol 1 A )-18 was evident because even after 3.5 days reaction time, only a very small amount of the (5)-ester was detected. The reaction could be scaled up thus, 1 kg of the racemic amino alcohol 18 was treated with vinyl propanoate (0.5 equivalents) and Novozyme 435 (3% by weight). After 3 days, both optically active products were isolated by distillation at reduced pressure. The (5)-amino alcohol (.V)-18 was recovered in 45% yield, which compared favorably with a yield of 32% for resolution on a small scale. The (R)-propanoate 19 (R1 = COEt) distilled as a colorless oil in 36% yield — slightly higher than that obtained from the small-scale resolution. The overall recovery was 81% from the scaled up reaction. 

Additives that specifically interact with an analyte component are also very useful in altering the electrophoretic mobility of that component. For example, the addition of copper(II)-L-histidine (12) or copper(II)-aspartame (54) complexes to the buffer system allows racemic mixtures of derivatized amino acids to resolve into its component enantiomers. Similarly, cyclodextrins have proven to be useful additives for improving selectivity. Cyclodextrins are non-ionic cyclic polysaccharides of glucose with a shape like a hollow truncated torus. The cavity is relatively hydrophobic while the external faces are hydrophilic, with one edge of the torus containing chiral secondary hydroxyl groups (55). These substances form inclusion complexes with guest compounds that fit well into their cavity. The use of cyclodextrins has been successfully applied to the separation of isomeric compounds (56), and to the optical resolution of racemic amino acid derivatives (57). 

The enantiomeric separation of some racemic antihistamines and antimalar-ials, namely (+/-)-pheniramine, (+/-)-bromopheniramine, (+/-)-chlorophen-iramine, (+/-)-doxylamine, and (+/-)-chloroquine, were investigated by capillary zone electrophoresis (CZE). The enantiomeric separation of these five compounds was obtained by addition of 7 mM or 1 % (w/v) of sulfated P-cyclo-dextrin to the buffer as a chiral selector. It was found that the type of substituent and degree of substitution on the rim of the cyclodextrin structure played a very important part in enhancing chiral recognition (174). The use of sulfated P-cyclo-dextrin mixtures as chiral additives was evaluated for the chiral resolution of neutral, cyclic, and bicyclic monoterpenes. While there was no resolution of the monoterpene enantiomers with the sulfated P-cyclodextrin, the addition of a-cyclodextrin resulted in mobility differences for the terpenoid enantiomers. Resolution factors of 4-25 were observed. The role of both a-cyclodextrin and sulfated P-cyclodextrin in these separations was discussed (187). The enantiomeric separation of 56 compounds of pharmaceutical interest, including anesthetics, antiarrhythmics, antidepressants, anticonvulsants, antihistamines, antimalarials, relaxants, and broncodilators, was studied. The separations were obtained at pH 3.8 with the anode at the detector end of the capillary. Most of the 40 successfully resolved enantiomers contained a basic functionality and a stereogenic carbon (173). 

Methods development for chiral analyses has been one of the most challenging separation problems for the analytical chemist in the pharmaceutical industry. Racemic drug substances have a variety of chemical structures and several chiral selectors are available for the analyst to choose in order to obtain the enan-tioselectivity needed for chiral resolution. To alleviate this problem, a fast capillary electrophoresis procedure for the enantiomeric separation of acidic and basic compounds using native and modified cyclodextrins has been described (200). The technique is called cyclodextrin array chiral analysis. A generalized optimi-... 

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