Cyclodextrins chiral HPLC

The type of CSPs used have to fulfil the same requirements (resistance, loadabil-ity) as do classical chiral HPLC separations at preparative level [99], although different particle size silica supports are sometimes needed [10]. Again, to date the polysaccharide-derived CSPs have been the most studied in SMB systems, and a large number of racemic compounds have been successfully resolved in this way [95-98, 100-108]. Nevertheless, some applications can also be found with CSPs derived from polyacrylamides [11], Pirkle-type chiral selectors [10] and cyclodextrin derivatives [109]. A system to evaporate the collected fractions and to recover and recycle solvent is sometimes coupled to the SMB. In this context the application of the technique to gas can be advantageous in some cases because this part of the process can be omitted [109]. 

A chiral GC column is able to separate enantiomers of epoxy pheromones in the Type II class, but the applications are very limited as follows a custom-made column packed with a p-cyclodextrin derivative as a liquid phase for the stereochemical identification of natural 3,4- and 6,7-epoxydienes [73, 74] and a commercialized column of an a-cyclodextrin type (Chiraldex A-PH) for the 3,4-epoxydiene [71] (See Table 3). The resolution abilities of chiral HPLC columns have been examined in detail, as shown in Table 7 and Fig. 14 [75,76, 179]. The Chiralpak AD column operated under a normal-phase condition separates well two enantiomers of 9,10-epoxydienes, 6,7-epoxymonoenes and 9,10-epoxymonoenes. Another normal-phase column, the Chiralpak AS column, is suitable for the resolution of the 3,4-epoxydienes. The Chiralcel OJ-R column operated under a reversed-phase condition sufficiently accomplishes enantiomeric separation of the 6,7-epoxydienes and 6,7-epoxymonoenes. 

All aldehydes used in the experiment were freshly distilled or washed with aqueous NaHC03 solution to minimize the amount of free acid. Chiral HPLC was performed using a chiral OJ-H column (0.46 cm x 25 cm, Daicel industries) with a water 717 auto sampler and a UV-vis detector (254 nm). The eluting solvent used was different ratios of hexane and 2-propanol. Chiral gas chromatography analysis was performed in a Shimadzu auto sampler with cyclodextrins columns as chiral stationary phase (fused-silica capillary column, 30 m X 0.25 mm x 0.25 gm thickness, /3-Dex-120 and /3-Dex-325 from Supelco, USA) using He as a carrier gas (detector temperature 230 °C and injection temperature 220 °C). 

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. 

Determination of Optical Yields. Optical yields of the siloxycyclopentenones derived from CPDK were determined by chiral HPLC (Chiracel OC column (J. T. Baker)) with the exception of the triphenylsilane derivative which was determined by optical rotation. 2-Butanol was derivatized to the corresponding diastereomeric urethanes with /Mnethylbenzylisocyanate according to literature procedures (32) the optical yield was then determined by G.C analysis using a Chirasil-L-Val column (Chrompack). The optical purity of the remaining alcohols (with the exception of a-tetralol optical rotation) was determined by chiral G.C. analysis of the underivatized alcohol using a CP-Cyclodextrin-B-2,3,6-M-19 column (Chrompack). Baseline resolution of the enantiomeric alcohols was achieved in all cases and it was observed that the / -isomer was eluted first without exception (confirmed by both optical rotation and G.C. analysis of independently prepared optically pure samples). 

Interest in phosphorus-containing calixarenes continues. Structures reported include hexa(diethoxyphosphoryloxy)calix[6]arene (8), inherently chiral 1,2-bridged calix[4]arene diphosphates, and a calixarene like C3 symmetric receptor with a phosphate function at the cavity bottom. " The purification of phosphate substituted calixarenes has been studied by chiral HPLC and by normal reverse phase HPLC. Mono(6-0-diphenoxyphosphoryl)-P-cyclodextrin (9) and mono(6-0-ethoxyhydroxyphosphoryl)-p-cyclodextrin (10) have been synthesised and show enantioselective inclusion of D and L amino acids e.g. 3.6 for D/L serine in the case of 9). ... 

H.Y. Aboul-Enein, MR. Islam and S.A. Bakr, Direct HPLC Resolution of Racemic Nomifensine Hydrogen Maleate Using a Chiral Beta-Cyclodextrin-Bonded Stationary Phase, J. Liq. Chromatogr., 11 (7)(1988)1485. 

The separation selectivity tuning in dual column chromatography may be illustrated by HPLC separation of the enantiomers of 2/-3,5-dinitrobenzoyl derivatives of some amino acids in two chiral P-cyclodextrin columns of opposite separation selectivity coupled in series. Fig. 3 shows... 

Cyclodextrins (and their derivatives) have been used widely in analytical chemistry, in particular, in separation technology. They have been incorporated into chromatographic applications, such as thin layer chromatography, gas chromatography, capillary electrophoresis and high performance liquid chromatography (HPLC), for the separation of similar chemical substances and even enantiomers (cyclodextrins are chiral). Conventional chiral HPLC columns are costly. A normal column containing an immobilised chiral cyclodextrin, such that one enantiomer forms a more stable complex over the other, can drastically reduce the cost. 

Chiral-HPLC started in the 1980s, and the main applications have been the separation of sugars, amino acids (small peptides), and their derivatives. Different types of natural and modified cyclodextrins (CD) have been immobilized as enantioselective ligands onto conventional silica particles. Cyclodextrins are rings of glucose units, with a toroidal three-dimensional (3D) structure. Their hydrophilic (abundant hydroxyl groups) surface makes... 

Allethrin D-allethrin has eight isomers (2 = 8), four cis (C, D, E, and F) and four trans (A, B, G, H) isomers. Mancini et al. (2004) separated cisitrans isomers from each other on an achiral silica HPLC column using n-hexane tert-butyl methyl ether (96 4) (v/v) as the mobile phase (Table Cl, Appendix C). The trans isomers were separated (G, H, A, B, respectively) from each other on a CHIRAL-CEL OJ using n-hexane-tert-butyl methyl ether (90 10) (v/v). This same colunm was used to separate the cis isomers (F, D, C, and E, respectively) using n-hexane isopropanol (99.3 0.7) (v/v). Kutter and Class (1992) were able to separate the trans allethrin isomers on a chiral p-cyclodextrin RP-HPLC column, but were unable to separate the cis isomers. 

Fig. 3. The chiral separation obtained for oxa2epam on a sulfated cyclodextrin hplc column (4.6 mm ID x 25 cm) using a 10% acetonitrile/buffer (25 mM...

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in Hquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

Cyclodextrin stationary phases utilize cyclodextrins bound to a soHd support in such a way that the cyclodextrin is free to interact with solutes in solution. These bonded phases consist of cyclodextrin molecules linked to siUca gel by specific nonhydrolytic silane linkages (5,6). This stable cyclodextrin bonded phase is sold commercially under the trade name Cyclobond (Advanced Separation Technologies, Whippany, New Jersey). The vast majority of all reported hplc separations on CD-bonded phases utilize this media which was also the first chiral stationary phase (csp) developed for use in the reversed-phase mode. 

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]. 

The PO mode is a specific elution condition in HPLC enantiomer separation, which has received remarkable popularity especially for macrocyclic antibiotics CSPs and cyclodextrin-based CSPs. It is also applicable and often preferred over RP and NP modes for the separation of chiral acids on the cinchonan carbamate-type CSPs. The beneficial characteristics of the PO mode may arise from (i) the offset of nonspecific hydrophobic interactions, (ii) the faster elution speed, (iii) sometimes enhanced enan-tioselectivities, (iv) favorable peak shapes due to improved diffusive mass transfer in the intraparticulate pores, and last but not least, (v) less stress to the column, which may extend the column lifetime. Hence, it is rational to start separation attempts with such elution conditions. Typical eluents are composed of methanol, acetonitrile (ACN), or methanol-acetonitrile mixtures and to account for the ion-exchange retention mechanism the addition of a competitor acid that acts also as counterion (e.g., 0.5-2% glacial acetic acid or 0.1% formic acid) is required. A good choice for initial tests turned out to be a mobile phase being composed of methanol-glacial acetic acid-ammonium acetate (98 2 0.5 v/v/w). 

With capillary electrophoresis (CE), another modern primarily analytically oriented separation methodology has recently found its way into routine and research laboratories of the pharmaceutical industries. As the most beneficial characteristics over HPLC separations the extremely high efficiency leading to enhanced peak capacities and often better detectability of minor impurities, complementary selectivity profiles to HPLC due to a different separation mechanism as well as the capability to perform separations faster than by HPLC are frequently encountered as the most prominent advantages. On the negative side, there have to be mentioned detection sensitivity limitations due to the short path length of on-capillary UV detection, less robust methods, and occasionally problems with run-to-run repeatability. Nevertheless, CE assays have now been adopted by industrial labs as well and this holds in particular for enantiomer separations of chiral pharmaceuticals. While native cyclodextrins and their derivatives, respectively, are commonly employed as chiral additives to the BGEs to create mobility differences for the distinct enantiomers in the electric field, it could be demonstrated that cinchona alkaloids [128-130] and in particular their derivatives are applicable selectors for CE enantiomer separation of chiral acids [19,66,119,131-136]. 

Wamke, M.M. et al.. Use of native and derivatized cyclodextrin based and macrocychc glycopeptide based chiral stationary phases for the enantioseparation of pterocarpans by HPLC, J. Liq. Chrom. Rel. TechnoL, 28, 823, 2005. 

After elution from the SPE cartridges, the eluents will be evaporated slowly under nitrogen gas. The concentrated samples containing metolachlor will be analyzed by GC/MS while the fractions containing the polar metabolites will be analyzed by HPLC. Both the GC and HPLC will be equipped with chiral columns. For GC/MS, a fused silica column coated with tert-butyldimethylsilyl-P-cyclodextrin will be used. This column has been shown to partially separate metolachlor isomers (13). 

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