Stationary phases cyclodextrins, preparation

Chiral chromatography methods are considered by many to be superior to conventional methods in that, besides analytical applications, they offer the greatest potential for the preparation of optically pure forms of the isomers [5,27,28]. In these examples the third chiral species is an integral part of the LC (or GC) system and may appear as a plain stationary phase (cyclodextrins),... 

The Preparation of the Pirkle Stationary Phases The Preparation of Cellulose and Amylose Stationary Phases The Preparation of the Macrocyclic Glycopeptides Phases The Preparation of the Cyclodextrin Based Stationary Phases Column Packing Techniques... 

There is a wide variety of commercially available chiral stationary phases and mobile phase additives.32 34 Preparative scale separations have been performed on the gram scale.32 Many stationary phases are based on chiral polymers such as cellulose or methacrylate, proteins such as human serum albumin or acid glycoprotein, Pirkle-type phases (often based on amino acids), or cyclodextrins. A typical application of a Pirkle phase column was the use of a N-(3,5-dinitrobenzyl)-a-amino phosphonate to synthesize several functionalized chiral stationary phases to separate enantiomers of... 

Because plasma and urine are both aqueous matrixes, reverse-phase or polar organic mode enantiomeric separations are usually preferred as these approaches usually requires less elaborate sample preparation. Protein-, cyclodextrin-, and macrocyclic glycopeptide-based chiral stationary phases are the most commonly employed CSPs in the reverse phase mode. Also reverse phase and polar organic mode are more compatible mobile phases for mass spectrometers using electrospray ionization. Normal phase enantiomeric separations require more sample preparation (usually with at least one evaporation-to-dryness step). Therefore, normal phase CSPs are only used when a satisfactory enantiomeric separation cannot be obtained in reverse phase or polar organic mode. 

Elargitai, T., Kaida, Y., and Okamoto, Y., Preparation and chromatographic evaluation of 3,5-dimethylphenyl carbamoylated beta-cyclodextrin stationary phases for normal-phase high-performance liquid-chromatographic separation of enantiomers, J. Chromatogr., 628, 11, 1993. 

Sinner and Buchmeiser prepared a new class of functionalized monolithic stationary phases (norbornene monoliths) by a ROMP process that tolerates a huge variety of functional monomers [69]. They used this approach in order to chemically attach (3-cyclodextrin onto the monomer prior... 

Three approaches can be employed to separate peptide stereoisomers and amino acid enantiomers separations on chiral columns, separations on achiral stationary phases with mobile phases containing chiral selectors, and precolumn derivatization with chiral agents [111]. Cyclodextrins are most often used for the preparation of chiral columns and as chiral selectors in mobile phases. Macrocyclic antibiotics have also been used as chiral selectors [126]. Very recently, Ilsz et al. [127] reviewed HPLC separation of small peptides and amino acids on macrocyclic antibiotic-based chiral stationary phases. 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

The preparative-scale separation of enantiomers on chiral stationary phases (CSPs) by GC cannot match the overwhelming success achieved in the realm of liquid chromatography (LC) (Francotte, 1994, 1996 and 2001). Modern commercial instrumentation for preparative-scale GC is not readily available. In contrast to LC, separation factors a in enantioselective GC are usually small (a = 1.01 - 1.20). This is beneficial for fast analytical separations but detrimental to preparative-scale separations. Only in rare instances are large chiral separation factors (a > 1.5) observed in enantioselective GC. Only in one instance, a separation factor as high as a = 10 was detected in enantioselective GC for a chiral fluorinated diether and a modified 7-cyclodextrin (Schurig and Schmidt, 2003) (vide supra). 

Juza, M, Braun, M., and Schurig, V. (1997) Preparative enantiomer separation of the chiral inhalation anesthetics enfiurane, isoflurane and desflurane by gas chromatography on a derivatized 7-cyclodextrin stationary phase, J. Chromatogr. A 769, 119-127. 

Schiirch, S., Saxer, A., Claude, S., Tabacchi, R., Trusch, B., and Hulliger, J. (2001) Semi-preparative gas chromatographic separation of ah-trans-perhydrotriphenylene enantiomers on a chiral cyclodextrin stationary phase, J. Chromatogr. A 905, 175-182. 

Staerk, D. U., Shitangkoon, A., and Vigh, G. (1995) Preparative gas chromatographic separation of the enantiomers of methyl-chloropropionate using a cyclodextrin-based stationary phase. J. Chromatogr. A 702, 251-257. 

S. N. El-Gizawy, A. N. Ahmed, and N. E. El-Rabbat, High performance liquid chromatographic determination of multivitamin preparations using a chemically bonded cyclodextrin stationary phase, Anal. Lett., 24 1173 (1991). 

Cvclodextrin-Silica stationary Tfriaaag. since excellent reviews deal with the preparation, properties, and analytical applications of cyclodextrin-silica stationary phases (11,12.14-16), the following paragraphs will discuss these topics very briefly, only to the extent that the information will be used in the rest of this chapter. 

Although they are extremely useful analytically, the protein based stationary phases 3-6,371 have found little application in preparative HPLC because they suffer from low loading capacity, due primarily to the low number of active sites. The natural macrocylic molecules Cyclodextrin 3 8,3 91 and antibiotics such as Vancomycin 3 10] have shown some promise. Synthetic chiral crown ethers 311 are particularly useful for the separation of chiral primary amines. 

An advantage of cyclodextrins over the common stationary phases is the high selectivity toward the isomeric substances. It has been demonstrated that many positional and geometric isomers can be separated by packed-column GSC in a very short time (i.e. analysis time does not exceed 2 minute) in separation of a mixture of o-, m- and p-isomers. From the analytical viewpoint, the low efficiency of the columns used is a disadvantage. Also, there ar other drawbacks of the gas-solid chromatography using CO s nonlinearity of the separation isotherm over a wider concentration range and poor reproducibility in the preparation of the CD columns utilized. 

Enantiomeric Stationary Phases. Chiral nonracemic chromatographic stationary phases prepared from p-cyclodextrin, derivatized with (R)- and (S)-NEI, and covalently bonded to a silica support are useful for the direct separation of enantiomers of a wide variety of compounds in both normal-phase and reversed-phase HPLC. ... 

The developments of the Lewis base-modified zirconia and mixed-oxide containing zirconia as stationary phases for high-performance liquid chromatography (HPLC) are reviewed. In this context, the preparation methods of porous spherical zirconia, and zirconia supports for HPLC based on modification with fluoride, phosphate, phos-phonate, carboxylic acid, phenols, and protein, as well as cyclodextrin derivative, are covered. The application of modified-zirconia in capillary electrochromatography (CEC) is also discussed. 

For the separation of chiral molecules into their respective enantiomers, several approaches are possible by HPLC. These include precolumn derivatization to form diastereomers, followed by the use of normal-phase or reversed-phase HPLC, or addition of the derivatization reagent to the chromatographic mobile phase to form dynamic diastereomers during the separation process. Alternatively, specialty columns prepared with cyclodextrins or specific chiral moieties as stationary phases may be used. 

Araki, T, Y Kashiwamoto, S Tsunoi, M Tanaka (1999). Preparation and enantiomer separation behavior of selectively methylated y-cyclodextrin-bonded stationary phases for high-performance liquid chromatography. Journal of Chromatography A, 845( 1-2), 455 62. 

The cyclodextrin based chiral stationary phases are some of the more popular materials used for contemporary chiral separations. One of their distinct advantages lies in their unrestricted and successful use with all types of solvent. In particular, they can be used very effectively in the reversed phase mode (a method of development that is not possible with some other chiral stationary phases) and, as well as being usable in a normal phase. They can also be used in the so-called polar organic mode, where the polar constituents of the mobile phase can be anhydrous diethylamine or glacial acetic acid, but in the complete absence of water. The cyclodextrins and their derivatives are widely used for all types of chiral separations, they have a good sample capacity, and can often be used for preparative separations. Cyclodextrin-based phases are readily available, usually covalently bonded to spherical silica gel particles 5 pm in diameter. There are numerous examples of the use of cyclodextrins in chiral separations and the following are some applications that illustrate their general use. 

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