Chiral crown ether column

Water and a pepsin enzymatic extract analyzed on chiral crown ether column [Daicel Crownpak CR ( ) with 0.1 mol L HCIO4 as mobile phase on-line ICP-MS detection... 

HPLC, using a Crownpack CR column containing an 18-crown-6-type chiral crown ether, served to separate and resolve the enantiomers of 5,6-dihydroxy-2-aminotetraline (132a) and 6,7-dihydroxy-2-aminotetraline (132b) at pH 2.0 LOQ for enantiomeric impurities was <0.1%308. 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

The simultaneous presence of a chiral selector and a charged non-chiral IPR was studied successfully [129]. The presence of a non-chiral IPR dramatically improved the separation of oppositely charged compounds on a chiral column, probably because the IPR increased retention and hence interactions with the chiral packing, as in the speciation of selenium-containing amino acids, on a crown ether column... 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

Phthalocyanines provided with crown ethers and chiral alkyl side chains (64) have also been reported to self-assemble in nonpolar solvents.77 These molecules form helical columns as could be deduced from the presence of a Cotton effect upon aggregation. To investigate the expression of chirality at the mesoscopic level, sergeants-and-soldiers experiments75 were performed. Chiral phthalocyanine molecules 64 were used as the sergeants and achiral... 

In contrast, CSPs have achieved great repute in the chiral separation of enantiomers by chromatography and, today, are the tools of the choice of almost all analytical, biochemical, pharmaceutical, and pharmacological institutions and industries. The most important and useful CSPs are available in the form of open and tubular columns. However, some chiral capillaries and thin layer plates are also available for use in capillary electrophoresis and thin-layer chromatography. The chiral columns and capillaries are packed with several chiral selectors such as polysaccharides, cyclodextrins, antibiotics, Pirkle type, ligand exchangers, and crown ethers. 

Packed capillary columns with chirally selective stationary phases (e.g., flr acid glycoprotein), as well as wall-immobilized, CD-based stationary phases, have been successfully used in CE chromatographic separations. Also, macro-cylic crown ethers, forming sterically selective complexes with the guest molecule, have been used for the resolution of optically active amines. 

Toda procedure for obtaining enantiomeri-cally pure compounds will find broad application very soon. This development could make preparative HPLC with chiral columns obsolete and be applied to distillable amino acid derivatives as well. After all, analytical resolution of amino acids was quite successful by host/guest complexation chromatography with reversed-phase packings loaded with Cram s chiral 1,1 -binaphthyl crown ethers (similar to 1). [20]... 

As in GC, the uses of LC for the separation of chiral species have significantly increased. Column materials now include chiral phases that may, for example, be based on monolithic silica columns with chemically bonded beta-cyclodextrin, teicoplanin, or cellulose tris(3,5-dimethylphenylcarbamate). Hydro-phobic amino compounds have been separated by LC using a crown ether dynamically coated chiral stationary phase. 

Chiral recognition for 1-phenylethylamine perchlorate(6). Extraction of aqueous 1-phenylethylamine perchlorate(6) with crown ethers(4) and the determination of the amount of extracted amine was carried out by the same procedure as described for a-phenylglycine methyl ester perchlorate. Another 10 ]ll portion of organic phase was derivatized with (-)-(L) N-trifluoroacetylalanine and used for the determination of CRF by GLC(OV-17 coated glass capillary column at 160 °C). 

The chiral recognition properties of crown ethers(4) for racemic Ct-phenylglycine methyl ester perchlorate(5) and 1-phenylethylamine perchlorate(6) were tested by standard extraction experiments.No appreciable amount of crown ethers was detected in aqueous layers. The relative amount of complexed salts to crown ethers in CDCl layers was determined by NMR integrations. When (5) is complexed with (4), the signal of ester methyl protons is moved and separated into two signals for each enantiomei When (6) is complexed with (4), the signal of methyl protons(doublet) is only shifted but not separated. The ratio of enantiomers of complexed salts was determined by HPLC using a chiral column for (5) or by GLC for (6) as diastereomeric isomers of (-)-(L)-N-trifluoroacetylalanine. The results are summarized in Table I and II. 

Before synthetic chiral stationary phases were developed, attempts were made to use naturally occurring chiral materials for the stationary phase. Quartz, wool, lactose and starch were inadequate but triacetylated cellulose has met with some success. The synthetic stationary phases introduced by Pirkle are able to interact with solute enantiomers in three ways, one of which is stereochemically dependent. Typically these interactions are based on hydrogen bonding, charge transfer (rc-donoi -acceptor based) and steric repulsive types. An independent chiral stationary phase therefore consists of chiral molecules each with three sites of interaction bound to a silica (or other) support. Early work in this area demonstrated that 5-arginine bound to Sephadex would resolve 3,4-dihydroxy-phenylalanine, and that direct resolution of chiral helicenes could be accomplished with columns packed with 2-(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)-propionamide or tri-P-naphthol-diphosphate amide. Amino acid esters have also been resolved with a silica bound chiral binaphthyl crown ether, but better separations are achieved with A-acylated amino acid derivatives with amino-acid derived chiral stationary phases. 

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