Chiralcel enantiomeric separation

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. 

In sub-FC, a detailed study of the influence of mobile phase additives on the chiral resolution of isoxazoline-based Ilb/IIIb receptor antagonists was carried out by Blackwell [145] on Chiralcel OD-H CSPs. The different mobile phase additives used were acetic acid, trifluoroacetic acid, formic acid, water, triethylamine, triethanolamine, n-hexylamine, trimethyl phosphate, and tri-w-butyl phosphate. In general, n-hexylamine and tri-/ -butyl phosphate mobile phase additives resulted in better resolution. The chiral separation of four 1,3-dioxolane derivatives on an amylose-based column has been described [151]. The effects of mobile phase composition, temperature, and pressure have been investigated. The nature of the modifier is the main parameter it has the highest impact on chiral resolution and is more important than the polarity of the mobile phase. Therefore, the organic modifier that gave the best enantiomeric separation was different for each compound. 

Figure 21-7. Preparative enantiomeric separation of 3-benzyloxycarbonyl-2-f-bntyloxazoUdinone on Chiralcel-OD (50 cm x 5 cm) mobile phase hexane/2-propanol = 8/2 (V/V), 50mL/min injection amounts 2g (hatched area) and 3g. (Reprint from reference 50, with permission.)...

Enantiomeric separation of nonpharmaceutical compounds include IV-alkyl-Af-methylaniline W-oxides (ethyl to butyl plus isomers) on a Chiralcel OD column. (A = 210 nm) using 1% to 3% ethanol in hexane [143]. Carrea et al. [144] separated the enantiomers of various substituted chromium and magnesium tricarbonyl metallocenes ( / -benzene and -cyclopentadiene) on a Chiralcel OD column (A = 315 run) with an isocratic mobile phase that varied from 1% to 10% ethanol in hexane depending on the enantiomeric pair involved. Chromium tricarbonyl compounds complexed with a variety of ij -arenes were separated on a Whelk-O column (A = 315 nm) using a 20/80 IPA/hexane as the mobile phase [145]. 

Fig. 11 Comparison of enantiomeric separation of l-(2-naphthalenethioyl)-2-propanol on (a) Chiralcel OD-H and (b) LUX CeUulose-1 in the normal mobile phase (20/80 iso-propanol/heptane). The sample was extracted from a reaction mixture. Please note that a chemical impurity co-eluted with (/ )-enantiomer in (a) was well resolved in (b) (adapted from [148])...

Fig. 14 Representative direct enantiomeric separations of brivanib and its isomers on different CSPs (a) polysaccharide Chiralcel OD-H (coated ceUulose carbamate), (b) Chiralpak 1C (3,5-dichlorophenylcarbamate of cellulose, immobilized on 5-(jtm silica), (c) Chirobiotic V2 (vancomycin), (d) Cyclobond RSP (cyclodextrin), (e) ChiroSil SCA (-) (crown ether), and (f) Sumichiral OA-4800 (Pirkle) column. Peak identification 1, (SR)-, 2, (RS) 3, (RR)-, 4, (S5) 5, positional isomer , chemical impurity (adapted from [150])...

Benzodiazepine enantiomers have also been resolved on the Chiralcel OD CSP. Wang et al. utilized this CSP to determine the enantiomeric composition of camazepam and its metabolites [59]. SFC provided improved resolution of the compounds of interest in a shorter period of time than LC. Phinney et al. demonstrated the separation of a series of achiral and chiral benzodiazepines. An amino column was coupled in series with the Chiralcel OD CSP to achieve the desired separation [41]. 

In another study, the authors reported a comparative study of the enantiomeric resolution of miconazole and the other two chiral drugs by high performance liquid chromatography on various cellulose chiral columns in the normal phase mode [79], The chiral resolution of the three drugs on the columns containing different cellulose derivatives namely Chiralcel OD, OJ, OB, OK, OC, and OE in normal phase mode was described. The mobile phase used was hexane-isopropanol-diethylamine (425 74 1). The flow rates of the mobile phase used were 0.5, 1, and 1.5 mL/min. The values of the separation factor (a) of the resolved enantiomers of econazole, miconazole, and sulconazole on chiral phases were ranged from 1.07 to 2.5 while the values of resolution factors (Rs) varied from 0.17 to 3.9. The chiral recognition mechanisms between the analytes and the chiral selectors are discussed. 

The enantiomeric purity is determined by chiral stationary phase, supercritical fluid chromatographic (CSP-SFC) analysis (Berger Instruments, Daicel Co. CHIRALCEL OD column 4% methanol, 180 psi, 3.0 mUmin flow rate detection at 220 nm). Racemic 1-phenylpropanol exhibited base-line separation of peaks of equal intensity arising from the R-isomer (tp, 2.74 min) and the S-isomer (tp, 3.10 min) whereas the synthetic alcohol showed these peaks in the ratio 97.7 / 2.3. This chromatographic method allowed for identification of the trace contaminants propiophenone (tp, 1.63 min) and benzyl alcohol (tp 3.40 min). 

Huang et al. [96] developed a method for the enantiomeric purity determination of (6 )-ornidazole in raw material and injection solution that was used in an preclinical study. In this publication, a mobile phase of n-hexane, MeOH, and 2-PrOH (95 4 1) was used with a Chiralcel OB-H column. No chiral impurity (/ )-ornidazole was detected above the LOD (0.05%) in either the raw material or the injection solution (see Figure 17.4D and E). The separation of the racemate is presented in Figure 17.4A, and the minor peak in Figure 17.4B corresponds to an enantiomeric impurity of 0.5%. 

FIGURE 17.5 Separation of (5)-timolol and its conceivable chiral and achiral impurities. (A) The mixture solution containing the conceivable chiral and achiral impurities of (5)-timolol, (B) the dissolution solution, and (C) the standard solution at 0.2% enantiomeric impurity. Column Chiralcel OD-H, mobile phase hexane 2-PrOH DEA (965 35 1 v/v/v). Peaks (1) timolol dimer, (2) (i )-timolol, (3) isotimolol, (4) (5)-timolol, (5) dimorpholinothiadiazole, and (6) solvent front. Concentration of analytes 5-10 Xg/mL in (A) and 3 Xg/mL (i )-timolol in (C). (Reprinted from Marini, R.D. et al., Talanta, 68, 1166, 2006. Copyright Elsevier, 2006. With permission.)... 

The crude residue in a 30-mL round-bottomed flask was treated with p-toluenesulfonic acid monohydrate (30 mg) in dry dimethyl formamide/2,2-dimethoxypropane (4.5 mL/4.5 mL) at room temperature for 2 hours (TLC hexane/acetone = 2/1, acetal derived from ketone Rf = 0.82, vy -acetonide Rf = 0.76, awfi-acetonide Rf 0.56). Saturated aqueous sodium hydrogen carbonate (12 mL), water and ether were added to the mixture and the aqueous layer was separated and extracted with diethyl ether (x 2). The combined organic layers were washed with water and with brine (x 2) and dried over sodium sulfate. The solvent was removed under reduced pressure and the resulting residue was analysed to determine the diastereomeric ratio of the aldol products by 1H NMR (in CDCI3, H at C-2, vyn-acetonide 5 4.95, awfi-acetonide 8 5.48). The crude residue was purified by flash silica gel column chromatography (hexane/ether/acetone 30/1/1) to afford the acetonides. The diastereomers were separated by this procedure. The enantiomeric excesses of the acetonides were determined by HPLC after cleavage of the acetonides (DAICEL CHIRALCEL OD, 2-propanol/hexane 20/80, flow l.OmL/min, detection at 254 nm, tR 13.6 min (minor(25,3/ )) and 15.9 min (major(2/ ,3S)). The results obtained from various aldehydes are summarized in Table 11.1. 

Hadley et al. [143] separated six A -alkyl-A -methylaniline iV-oxides (ethyl to butyl and isomers) and their enantiomers on a Chiralcel OD colurrm (2 = 210nm) using 1.5-3.0% ethanol in hexane as the mobile phase. Results are tabulated for each solute and its enantiomer. Nearly complete resolution of all 12 compounds was achieved in one chromatographic run, but only after extremely long chromatographic run times (>240 min). Separation of individual enantiomeric pairs was more effective with most analyses completed in <40 min. 

Enantiomeric forms of carboxy-, acetyl-, hydroxymethyl- and aldehyde-substituted cyclopentadienyl and benzyl complexes of Cr, Mn, and Fe are separated on Chiralcel OC, OD, OT, and OB columns (A —220 tun) using hexane/IPA and hexane/ethanol mobile phases (at the 1-3% level). Better peak resolution was obtained when ethanol was used in the modifier [616]. For both modifiers, resolution increases rapidly as percent organic modifier decreases. Capacity factors of 5-20 were obtained depending upon the sol vent/solute combination chosen. 

Van Overbeke, A. Baeyens, W. Oda, H. Aboul-Enein, H.Y. Direct enantiomeric HPLC separation of several 2-arylpropionic acids, barbituric adds and benzodiazepines on Chiralcel OJ-R chiral stationary... 

Fig. 15 Simultaneous separations of five CBZ-derivatized brivanib alaninate isomers on a 25-cm X 4.6-mm Chiralcel OJ-RH column in reversed phase (a) and an OJ-H column in polar oiganic phase (b). Mobile phase (a) 50 50 20 mM NH40Ac ACN (b) MeOH [166]. The arrows link the enantiomeric pairs (adapted from [166])...

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