Cyclodextrin-bonded phases

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. 

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). 

The more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

The Separation of the Isomers of Scopolamine and Phenylephrine on a Cyclodextrin Bonded Phase... 

It is seen that, providing the isomers are eluted at a (kf) value greater than about 2.0, the column will separate those solute pairs having separation ratios as low as about 1.03. This, however, assumes that the column is very well packed and is operated at about the optimum mobile phase velocity. In practice, a more realistic minimum separation ratio would be between 1.035 and 1.04. However, as it will be seen, the cyclodextrin bonded phases can easily provide separation ratios significantly greater than these values. This is achieved by... 

Armstrong, D.W. and DeMond, W., Cyclodextrin bonded phases for the liquid chromatographic separation of optical, geometrical, and structural isomers, J. Chromatogr. Sci., 22, 411, 1984. 

Fujimoto, C., Maekawa, A., Murao, Y, Jinno, K., and Takeichi, T., An attempt directed toward enhanced shape selectivity in reversed-phase liquid chromatography preparation of the dodecylaminated beta-cyclodextrin-bonded phase. Anal. Sci., 18, 65, 2002. 

Table 2 gives chromatographic data for different classes of enantiomeric dmgs resolved by P-CD bonded phases (8). Drugs for which resolution factors (R) greater than 1.0 were obtained include mephenytoin, ketoprofen, chlorpheniramine, and the barbiturates mephobarbital and hexobarbital. Cyclodextrin-bonded phases provide a rapid and specific technique for the pharmacological evaluation of racemic dmgs. 

Chiral stationary phase 138 Covalently attached /3-cyclodextrin bonded phase exploratory separations of PITC-, dansyl-, dabsyl-, and AQC-derivatized amino acids... 

Octadecyl-bonded phases Copper silicate gel Cyclodextrin-bonded phases Hydroxyapatite-covered silica Graphitized carbon... 

Han SM, Armstrong DW, HPLC separation of enantiomers and other isomers with cyclodextrin bonded phases rule for chiral recognition, in Chiral Separations by HPLC (Krstulovic AM, Ed.), Ellis Horwood, Chichester, p. 208 (1989). 

A Guide to Using Cyclodextrin Bonded Phases for Liquid Chromatography, Advanced Separation Technologies, Inc., Whippany, NJ (1996). 

Cyclobond Handbook A Guide to Understanding Cyclodextrin Bonded Phases for Chiral LC Separations, 6th ed., Advanced Separation Technologies, Wippany, NJ, 2002. 

Armstrong, D.W., DeMond, W., and Czech, B.P., Separation of metallocene enantiomers by liquid chromatography chiral recognition via cyclodextrin bonded phases, Anal. Chem., 57, 481, 1985. 

Chang, C.A., Wu, Q., and Tan, L., Normal-phase high-performance liquid chromatographic separations of positional isomers of substituted benzoic acids with amine and P-cyclodextrin bonded-phase columns, J. Chromatogr., 361, 199, 1986. 

T. J. Ward and D. W. Armstrong, Cyclodextrin bonded phases. In Chromatographic Chiral Separations (M. Zief and L. Crane ed.), Marcel Dekker, Inc., New York, 1987. 

TABLE 4 Separation Data for Enantiomeric Compounds Using a 10-cm P-Cyclodextrin Bonded Phase Column 1... 

A special cyclodextrin bonded phase, with an appropriate binder, has been used in TLC plates to separate isomers, including optical isomers.45 Another type of TLC plate made by Macherey-Nagel uses a reverse phase bonded layer with Cu2+ ions and an unspecified chiral reagent. 

Cyclodextrin bonded phases have been demonstrated to be particularly adept in resolving structural isomers (1) such as the ortho, meta and para forms of nitrophenol (Figure 1). These... 

The Naphthylethyl Carbamate Derivative of a Cyclodextrin Bonded Phase Courtesy of ASTEC Inc. 

With chiral stationary phases, chromatographic separation of enantiomeric ferrocene derivatives is possible. An apparatus for the resolution of ferrocenyl alcohols and other compounds on triacetylcellulose has been described [60]. Analytical enantiomer separation of ferrocenyl alcohols, ethers, sulfides, and amines for the determination of enantiomeric excesses is best achieved on cyclodextrin bonded phases [61]. 

Wang, M. Ueda, H. Nagai, T. Simple prediction of stability constants for inclusion complexes of P-cyclodextrin with various drug molecules using P-cyclodextrin bonded phases (Cyclobond 1 Column). Drug Dev. Ind. Pharm. 1990,16 (4), 571-579. 

ICT (Ed.), Cyclobond Handbook, A guide to using cyclodextrin bonded phases, Vol. 4 Frankfurt, ICT Handelsgesellschaft mbH 1988... 

Stalcup, A. M. Comparison ofthe enantioselectivity of phenethyl- and naphthyl-ethyl-carbamate substituted cyclodextrin bonded phases,/. Liq. Chromatogr., 1993,... 

Piperaki, S. Perakis, A. Parissi-Poulou, M. Liquid chromatographic retention behaviour and separation of promethazine and isopromethazine on p-cyclodextrin bonded-phase column. J.ChromatogrA, 1994,... 

Column two 250 mm p-cyclodextrin bonded phase columns in series (Advanced Septu-ation... 

Cyclodextrin bonded phases. Cyclodextrins are cyclic chiral carbohydrates composed of six, seven or eight glucopyranose units the a-, and 7-cyclodextrins, respectively. Only the /3-cyclodextrins have been found to be of use in chiral chromatography. The monomers are configured... 

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