Cyclodextrin rings

Anderson and coworkers [183-185] reported water-soluble polyrotaxanes 149 and 150, containing sulfonated PPV chains surrounded by mechanically bound a- and (3-cyclodextrin macrocycles (Chart 2.32). The cyclodextrin rings play the role of a wire insulator, preventing aggregation and interchain quenching. The effect was demonstrated by atomic force... 

The inclusion modes of flurbiprofen with beta cyclodextrin and with heptakis(2,3,6-tri-0-methyl)-beta cyclodextrin have been studied by Imai and coworkers. They showed that, although the Cotton effects in the circular dichroic spectra induced by beta cyclodextrin in / (-) and 5(+) flurbiprofen are identical, those induced by heptakis(2,3,6-tri-0-methyl)-beta cyclodextrin differ from each other and from those induced by beta cyclodextrin. C.p.-m.a.s. C-n.m.r. experiments showed that the cyclodextrin ring is probably more distorted in the flurbiprofen inclusion complex with methylated beta cyclodextrin than in that with beta cyclodextrin. 

Cyclodextrins are formed in the degradation of starch and dextran by the action of cyclodextrin/glucanosyltran-sferases [EC 2.4.1.19]. These enzymes also catalyze so-called acceptor reactions in which the cyclodextrin ring is opened and an acceptor molecule (e.g., glucose) is added to the reducing end of the maltodextrin chain. 

Among the most recent and sophisticated flavor encapsulation processes,cyclodextrin complexation represents a special way of encapsulation entrapment of flavors on the lowest possible scale,which is called molecular encapsulation.In this process,every flavor constituent is surrounded by a cyclodextrin ring which offers an almost perfect protection against damaging effects of the environment.The most significant advantages of the application of molecular encapsulation in the food industry and cosmetics are as follows/1/ ... 

Two remarkable and unique properties of molecular encapsulation are protection against atmospheric oxidation and disproportionations between components of flavors. In addition,intermolecular transformations are almost entirely eliminated due to the surrounding cyclodextrin ring. 

Cyclodextrin rings can be threaded by a long chain, and rotaxanes and polyrotax-anes are formed when stopper moieties are connected to the ends of the threading chain [93], Tamura and Ueno prepared a rotaxane, in which naphthalene-ap-... 

The situation is complex. In another study we examined the cyclization of compound 54 catalyzed by cyclodextrin bis-imidazoles [140]. This dialdehyde can perform the intramolecular aldol reaction using the enol of either aldehyde to add to the other aldehyde, forming either 55 or 56. In solution with simple buffer catalysis both compounds are formed almost randomly, but with the A,B isomer 46 of the bis-imidazole cyclodextrin there was a 97 % preference for product 56. This is consistent with the previous findings that the catalyst promotes enolization near the bound phenyl ring, but in this case the cyclization is most selective with the A,B isomer 46, not the A,D that we saw previously. Again the enolization is reversible, and the selectivity reflects the addition of an enol to an aldehyde group. The predominant product is a mixture of two stereoisomers, 56A and 56B. Both were formed, and were racemic despite the chirality of the cyclodextrin ring. 

Metalloporphyrins can catalyze the hydroxylations of solvent species such as cyclohexane. From our studies with cyclodextrin dimers, we concluded that by attaching cyclodextrin rings to metalloporphyrins we should be able to bind substrates in water and achieve selective hydroxylations directed by the geometries of the complexes. This was successful. 

We have also synthesized a catalyst related to 131 in which the cyclodextrin rings were replaced with synthetic macrocyclic binding groups [196], Also, we have examined catalysts related to 131 in which substrate binding involved metal ion coordination, not hydrophobic binding into cyclodextrins or macrocycles [198]. 

The ability of some components of nucleic acids, especially those with an adenine base, to form complex with 8-cyclodextrin, can also be readily used for chromatographic separations of various nucleotides and nucleosides (59). A substantial problem associated with application of cyclodextrin polymer gels, is that the accessibility of the cyclodextrin cavities on the surface and within the interior of the polymer particle is rather different. The rate of entrapment and release of solutes from the streaming liquid is obviously a diffusion controlled process. Consequently, a longer time is needed to reach an equilibrium within the particle than on its surface. The accessibility of the cyclodextrin rings will be more uniform, if the cyclodextrin is immobilized on the surface of non-complexing polymer particles (polyacrylamide, agarose (60,61) cellulose (62), and silica (63)). Therefore, a better separation (however lower capacity) is expected. 

Seo et have shown the dependence of stability constant and overall solubility upon cyclodextrin ring size for spironolactone. From the data given in Fig. 19, it can be seen that for the relatively large spironolactone molecule (Fig. 9) the stability constant increases with size of the host cavity that is reflected through an increase in the linear slopes. The lowest maximum solubility was obtained with the p-cyclodex-trin, which has the lowest solubility of the three cyclo-dextrins. ... 

In channel structure ICs, the cyclodextrin rings are stacked on top of each other to produce cylindrical central cavities. In cage structures, the cavity of... 

Other studies have included the formation of complexes using spray-drying techniques, including the preparation of cyclodextrin complexes. DSC may be used to determine the extent of drug inclusion within the cyclodextrin ring system, again via monitoring of the decrease in the heat of fusion (97). 

Fig. 16A-D. Mechanical switching in rotaxanes. A Rotaxanes may exist in isomeric states by the movement of the ring component between dissymmetric sites on the string component. B A redox- or pH-switchable [2]rotaxane. While the cyclophane complexes the native benzidine site (spectrum, curve a), the reduced or protonated benzidine repels the cyclophane, causing it to move to the dioxybiphenylene site (spectrum, curve b). C An azobenzene-based switchable [2]rotaxane. The cyclodextrin ring complexes the azobenzene site in the trans-state, but it is repelled from the ds-azobenzene. The state of the system is measurable by circular dichroism (plot). D A pH-switchable rotaxane. When the amine on the string component is protonated, it complexes the crown ether ring by hydrogen-bonding interactions (40a). When the amine is deprotonated, however, the ring component moves to the bipyridinium unit, where it is complexed by n donor-acceptor interactions (40b). The plots in B and C are adapted from [67] and [69], respectively, with permission...

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