Neutral cyclodextrins

The condensation reactions described above are unique in yet another sense. The conversion of an amine, a basic residue, to a neutral imide occurs with the simultaneous creation of a carboxylic acid nearby. In one synthetic event, an amine acts as the template and is converted into a structure that is the complement of an amine in size, shape and functionality. In this manner the triacid 15 shows high selectivity toward the parent triamine in binding experiments. Complementarity in binding is self-evident. Cyclodextrins for example, provide a hydrophobic inner surface complementary to structures such as benzenes, adamantanes and ferrocenes having appropriate shapes and sizes 12) (cf. 1). Complementary functionality has been harder to arrange in macrocycles the lone pairs of the oxygens of crown ethers and the 7t-surfaces of the cyclo-phanes are relatively inert13). Catalytically useful functionality such as carboxylic acids and their derivatives are available for the first time within these new molecular clefts. 

Matthijs, N., Van Hemehyck, S., Maftouh, M., Massart, D.L., Vander Heyden, Y. Electrophoretic separation strategy for chiral pharmaceuticals using highly-snlfated and neutral cyclodextrins based dual selector systems. An. Chim. Acta 2004, 525, 247-263. 

Li-dodecyl sulfate, borate, beta-cyclodextrin MEKC/Chiral system with direct UV detection Neutral and charged solutes 52... 

In 1990, the applicability of cyclodextrin-modified MEKC (CD-MEKC) was successfully explored for the first time by adding a neutral CD to the micellar solution for the separation of highly hydrophobic and closely related compounds such as chlorinated benzene congeners, polychlorinated biphenyl (PCB) congeners, and tetrachlorodibenzo-p-dioxin (TCDD)... 

Of course, encapsulation can also be governed by other factors besides the charge control. Thus, the anion-radical of fullerene Cgg exhibits better inclusion ability with cyclodextrins than neutral Cgg (Liu et al. 2007). Polarizability of the anion-radical is much larger than that of neutral fullerene. This factor enhances dispersion forces and assists encapsulation of the charged particle by cyclodextrin. 

From Eq. (13.3), it is clear that neutral molecules will have a net velocity. In normal electrophoresis, cations will migrate faster than neutrals, and neutrals will migrate faster than anions. Anions are electrophoretically migrating in a direction opposite to EOF. Separations of neutral molecules, such as organic explosives, can only be achieved by using buffer additives, such as micelles, ionic cyclodextrins, and bile salts. The interaction of neutral analytes with these ionic buffer additives results in a modified mobility that enables separation. 

Krishnaveni NS, Surendra K, Reddy MA, Nageswar YVD, Rao KR (2003) Highly efficient deprotection of aromatic acetals under neutral conditions using beta-cyclodextrin in water. J Org Chem 68 2018-2019... 

S Izumoto, H Nishi. Enatiomer separation of drugs by capillary electrophoresis using mixtures of /3-cyclodextrin sulfate and neutral cyclodextrins. Electrophoresis 20 189-197, 1999. 

Y Tanaka, M Yanagawa, S Terabe. Separation of neutral and basic enantiomers by cyclodextrin electrokinetic chromatography using anionic cyclodextrin derivatives as chiral pseudo-stationary phases. J High Res Chromatogr 19 421-433, 1996. 

TH Seals, C Sheng, JM Davis. Influence of neutral cyclodextrin concentration on plate numbers in capillary electrophoresis. Electrophoresis 22 1957-1973, 2001. 

PK Owens, AF Fell, MW Coleman, M Kinns, JC Berridge. Use of H -NMR spectroscopy to determine the enantioselective mechanism of neutral and anionic cyclodextrins in capillary electrophoresis. J Pharm Biomed Anal 15 1603-1619, 1997. 

Cyclodextrins are neutral compounds which migrate at the same rate as the EOF. They have large hydrophobic cavities in their structures into which molecules can fit. The ease with which a molecule fits into the cavity of the cyclodextrin is dependent on its stereoehemistry. Cyelodextrins have been used as additives both in chiral, where opposite enantiomers form transient diastereomeric complexes with the optically active cyclodextrins, and non-chiral separations where the cyclodextrins affect diastereoisomers to a different extent. 

There are several types of natural and synthetic molecular hosts, such as cyclodextrin and cyclophane, that are shaped to accommodate neutral and charged organic molecules in the three-dimensional cavity. The inclusion complexation by molecular hosts is driven by various weak forces like van der Waals, hydrophobic, hydrogen bonding, ion-dipole, and dipole-dipole interactions, and therefore the molecular recognition process seems much more complicated. In expanding the scope of the present theory, it is intriguing and inevitable to perform the extrather-... 

Cyclodextrins for which the complexation of neutral molecule was proved in innumerable... 

The extracts contained almost pure pheromone, with only minor amounts of other components. By comparison with synthetic samples, derivatization, and gas chromatography on a chiral cyclodextrin phase, the pheromone was identified as a 95 5 mixture of (5)- and (/ )-enantiomers of dimethyl citrate (cupilure (3), Fig. 4.2). Only the (5)-enantiomer was active (Tichy et al., 2001). Cupilure is probably present in ionized form on the silk (pK 3.5), because solvent extracts were neutral. The ionized carboxyl group may be conjugated to basic amino acids of the silk proteins. This would also explain why the pheromone is not volatile, despite its relatively low molecular weight, and is easily washed from the silk by water, including rain. In the tropical habitat of C. salei this would ensure the presence of cupilure only on freshly laid silk. 

This technique is a variant of CZE. A cationic or anionic surfactant compound, such as sodium dodecylsulphate, is added to the mobile phase to form charged micelles. These small spherical species, whose core is essentially immiscible with the solution, trap neutral compounds efficiently by hydrophylic/hydrophobic affinity interactions (Fig. 8.7). Using this type of electrophoresis, optical purity analysis can be conducted by adding cyclodextrins instead of micelles to the electrolyte. This is useful for separating molecules that are not otherwise separable. Under such conditions, the enantiomers form inclusion complexes of different stability with cyclodextrin (cf. 3.6). 

If pH does not provide adequate separation or if analytes are neutral, use surfactants for micellar electrokinetic capillary chromatography. For chiral solutes, try adding cyclodextrins. 

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