Structural isomers cyclodextrin

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. 

Armstrong, D.W., DeMond, W., Alak, A., Hinze, W.L., Riehl, T.E., and Bui, K.H., Liquid chromatographic separation of diastereomers and structural isomers on cyclodextrin-bonded phases, Anal. Chem., 57, 234, 1985. 

Armstrong et al. developed a chromatographic technique which could be used to evaluate the stoichiometry and all relevant binding constants for most substrate-CD systems (8). This method was not dependent on a solute s spectroscopic properties, conductivity, electrochemical behavior, or solubility. This work presented theory and chromatographic evidence for multiple cyclodextrin complex formation. Previous theoretical work considered only 1 1 complex formation (9-12). A two to one complexation equation was derived by expanding on the equation first used in 1981 to describe the 1 1 complexation behavior of a solute in a pseudophase system (13.14). Using this method, it was demonstrated that closely related compounds such as structural isomers of nitroaniline could exhibit different binding behaviors (8). 

Gas Chromatographic Separation of Structural Isomers on Cyclodextrin and Liquid Crystal Stationary Phases... 

X-ray crystal structures were used for the production of computer projected images of inclusion complexes of structural isomers, enantiomers and dlastereomers with a- or B-cyclodextrin. These projections allow for a visual evaluation of the interaction that occurs between various molecules and cyclodextrin, and an understanding of the mechanism for chromatographic resolution of these agents with bonded phase chromatography. 

The wide interest in the use of cyclodextrins as a separation medium has led to a number of useful applications. The ability of these molecules to bind other molecules to form an inclusion complex, has provided for their use in typically difficult separations of enantiomers, diasastereomers, and structural isomers. Through the coupling of cyclodextrin to a solid support, such as silica gel, a chromatographic resin can be made, and has been developed as a useful chromatographic procedure. 

This review will illustrate examples of computer projected models of inclusion complexes of structural isomers (ortho, meta, para nitrophenol), enantiomers (d- and 1- propranolol) and diastereomers [cis and trans. l(p-B-dimethylaminoethoxy-phenyl-butene), tamoxifen] in either a- or B-cyclodextrin. The use of these computer projections of the crystal structures of these complexes allows for the demonstration and prediction of the chromatographic behavior of these agents on immobilized cyclodextrin. 

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... 

There have been several reports which have demonstrated the chromatographic separation of ortho, meta and para structural isomers on 6-cyclodextrin matrices (jj). One common feature of these separations is that the order of retention (greatest to lowest) falls in the order of para > ortho > meta, although the ortho and meta isomers elute very close to one another. This is in contrast to what is obtained, as illustrated above for nitrophenol, with o-cyclodextrin, where the binding affinity for the meta isomer is greater than the ortho. In an attempt to understand this... 

Aqueous and methanolic solutions of cyclodextrins have been employed as mobile phases in high performance liquid chromatography (HPLC), and as stationary phases by bonding the cyclodextrin to silica packing by several workers (1-8). They have been shown to be especially suitable for separation of structural isomers, cis-trans geometric isomers, and enantiomers. Cyclodextrins (CD) are toroidal-shaped, cyclic, oligosaccharides made up of o-l,4 linked,... 

Chen,CY,CH Lin and JHYang(2005).Use of chemically bondedj8-cyclodextrin silica stationary phase for liquid chromato graphic separation of structural isomers. Journal of the Chinese Chemical Society, 52(4), 753-758. 

D.W. Armstrong and W. DeMond, Cyclodextrin Bonded Phases for the Liquid Chromatographic Separation of Optical, Geometrical, and Structural Isomers, J. Chromatog. Sci., 22(1984)411. 

D.W. Armstrong, W. DeMond, A. Alak, W.L. Hinze, T.E. Riehl and K.H. Bui, Liquid Chromatographic Separation of Diastereomers and Structural Isomers on Cyclodextrin- Bonded Phases,... 

Cyclodextrins have previously been successfully employed in separation science. For instance, the partial separation and enrichment of optical and structural isomers as well as routine compounds based on selective precipitation with CDs have been reported [5,7-8]. Additionally, solutions of CDs have served as the mobile phase in a few thin-layer and high performance liquid chromatographic separations [5,9,10]. However, their most widespread application in chromatography has been as part of the stationary phase [5,6]. Various polymeric CD materials, CD gels or resins, as well as CD coated columns have been utilized as the stationary phases in the separation of many important classes of compounds [5,6,11-13]. Unfortunately, the use of these CD phases has been largely restricted to column or gas chromatography due to their low efficiency and/or poor mechanical strength [14-16]. 

Armstrong et al. [110) studied 19 sets, with two to five compounds in each set, of structural isomers (e.g., cresols and xylenes to methylindoles and prostaglandins) on a ) -cyclodextrin colunm (X = 254 nm or 280 nm). Good peak shapes and resolution were obtained for all sets using mobile phases ranging from 30/70 methanol/water (for cresols) to 90/10 methanol/water (for vitamin D2). It was noted that retention for disubstituted benzenes (e.g., the hydroxy/methyl, dimethyl, nitto/hydroxy, nitro/amine and amine/carboxy substituent pairs) usually decreased in the order para > ortho > meta on the ) -cyclodextrin column. 

The presence of double bonds and the possibility of an oxygen atom at different positions in the molecule account for the occurrence of isomers. There are structural isomers and frequently stereoisomers. Structural isomers as well as cis-trans isomers are usually separated in gas chromatography with standard columns, the method of choice for the analysis of essential oils. However, the separation of enantiomers needs specific chiral columns based on cyclodextrins. Therefore, in most publications where these columns were not used, there is no differentiation between the enantiomers. 

Lepri, L, Coas, V, and Desideri, P. (1994). Planar chromatography of optical and structural isomers with eluents containing modified beta-cyclodextrins. J. Planar Chromatogr.—Mod. TLC 7 322-326. 

Alak and Armstrong (107,108,112,113) investigated the influence of different silicas and binders on the separation behavior of P-cyclodextrin TLC plates. Besides nine racemates, three diastereomeric compounds and six structural isomers were separated. Wilson (109) impregnated silica plates with a 1% solution of P-CD in ethanol-dimethylsulfoxide (80 20 by volume) racemic mandelic acid was barely separated, and the antipode separation of P-blockers was not possible. Armstrong et al. (110) were the first to describe application of P-cyclodextrin as a chiral eluent additive for separations on reversed-phase TLC plates. The success of separation was strongly dependent on type and quantity of modifier applied, but above all on the concentration of P-CD. The low solubility of 3-CD in water (0.017 M, 2S C) can be improved by addition of urea sodium chloride stabilizes the binder of the RP plates. Compared to 3-CD bonded phases, a reversed retention behavior was noticed, the D-enantiomer eluting above the L-isomer. The separation of steroid epimers and other diastereomeric classes of compounds is also possible with this technique. Hydroxypropyl and hydroxyethyl P-... 

Mandal et al. (in preparation) incorporated p-cyclodextrin in an aqueous solution (0.7 M P-cyclodextrin, 7.5% NaOH, 37.5% urea, pH = 12) and employed it as a CLM in a permeator containing 300 symmetric hydrophobic feed fibers and 300 similar strip fibers. The feed was an equimolar (0.005 M) mixture of structural isomers o-nitroaniline and /7-nitroaniline in 80% octanol-20% heptane with the same solvent mixture as the strip liquid (Figure 3a). They obtained a selectivity of almost 5 for /7-nitroaniline over the ortho isomer. The operation was stable. The longest run continued for a period of almost two days with the feed solution and the strip solvent in coxmtercurrent flow. This successful separation indicated that employing a reflux arrangement at the end of the extraction cascade, it would be possible to continuously separate structural isomers into high purity (99%). ... 

Lepri, L. et ah. Separation of optical and structural isomers by planar chromatography with development by /3-cyclodextrin solutions, J. Planar Chromatogr., 3 311, 1990. 

---