A-cydodextrin

D. H. Macartney, The Self-Assembly of a [2]Pseudoro-taxane of a-Cydodextrin by the Slippage Mechanism , J. Chem. Soc Perkin Trans. 21996, 2775-2778. 

Figure 13. a-Cyclodextrin-based bimotecular ion channels, (a) Hexatcar-boxymethyh-substituted a-cydodextrin combined with dioctadecyldimethylam-monium cation to afford hexa ion pairs 9, hexa(stearoyl)substituted a-cyclodextrin tO, and hexa trimethylcholyl)-substituted a-cydodextrin 11. (b) A hypothetical side view for the passage of metal ion through a hydrophilic channel wall comprised of ether network convergently extended from steroidal backbone. 

Several procedures are used to control the ratios of cyclodextrins produced. One is addition of a substance to the reaction mixture that can gready affect the formation of one specific cyclodextrin over another. For example, in the presence of 1-decanol and 1-nonanol, a-cydodextrin is produced almost exclusively whereas hexane or toluene promote the production of p-cyclodextrin. Conversely both cyclodextrins are produced simultaneously in the... 

Fig. 15. Prototype examples of (a) cydodextrins and (b) calixarenes, showing conformational structures and dimensions.

In one of our earliest approaches to such biomimetic selectivity, directed by the geometry of the catalyst-substrate complex, we examined the directed substitution of an aromatic ring bound into the cydodextrin cavity [142,143]. Anisole was chlorinated by HOC1 entirely in the para position when it was bound into a-cydodextrin, while in... 

Complex formation with substrate (S) can proceed directly, by route A, to yield a relaxed a-cyclodextrin with all six 0(2) -0(3 ) hydrogen bonds engaged (as in the a-cyclodextrin methanol complex, Fig. 18.8), or the macrocycle can first open up to a relaxed form, route B, with the enclosed water molecules disordered over several sites so as to fill, statistically, the 5 A diameter a-cydodextrin cavity (as observed in the a-cyclodextrin 7.57H20 crystal struc- ture, Fig. 18.6 b). The water is now in an activated form and can be replaced directly by the j substrate. In a third possible mechanism, route C, the substrate aggregates first at the periphery of tense a-cyclodextrin, and in a second step replaces the two enclosed water molecules. 

Lil,i I2 8H2O (a-cydodextrin)2 (tetragonal pyramidal geometry of oxygen atoms) Li—0 NR 130... 

Formation of Host-Cuest Complexes of a-Cydodextrins with Azo Dyes ... 

Figure 4 Schematic representation of two important classes of macrocyclic host molecules (a) cydodextrins 4 and (b) CB[n] 5. Important guests and associated K, ranges are included for comparison.

Two [2]rotaxanes (Figure 1.22), each comprising a-cydodextrin as the rotor, stilbene as the axle, and 2,4,6-trinitrophenyl substituents as the capping groups, were prepared and their conformations were examined in solution and in solid state using NMR spectroscopy and X-ray crystallography, respectively [132]. In the solid state, the axles of rotaxanes form extended molecular fibers that are separated... 

Phosphate Ester Hydrolysis Catalyzed by a Cydodextrin-based Enzyme Mimic... 

Enantioselectivity The ability of a sorbent (typically a cydodextrin) to separate chiral compounds. 

Figure 1.14 Two isomers in which pyridoxamine is doubiy bound to a cydodextrin. They show very large rate accelerations of transaminations in water and striking differences in substrate selectivity.

Breslow, R. and Zhang, B., %ry fast ester hydrolysis by a cydodextrin dimer with a catalytic linking group, /. Am. Chem. Soc., 1992,114, 5882 5883. 

Figure 7.8. Ligands useful for ISEs with liquid membranes. a dycyclohexyl-18-crown-6. b Cryptand-222. c Monensin. d ETH 295. e a-cydodextrin. f calix[4]arene...

Armstrong DW, Yang X, Han SM, Menges RA. Dired liquid diromatogmphic separation of racemates with an a-cydodextrin bonded phase. Anal Chem 1987 59 2594—6. 

Direct protection of some hydroxyls on the primary rim of native a-cyclodextrin 1 using trityl chloride is an interesting method to access multifunctionalized derivates. For example, a-cydodextrin 50 selectively protected on the positions 6A,6C,6E can be obtained in 23% yield [55], using trityl chloride in pyridine at 55 °C (Scheme 9.21). All reaction conditions have to be precisely controlled, as for example, an increase in the temperature leads to the additional formation of regioisomers 6A,6B,6C, 6A,6B,6D, and 6A,6B,6E [56]. The free hydroxyls can then be methylated, and after deprotection of the trityl groups, 6A,6C,6E-trihydroxy a-cyclodextrin is obtained, these hydroxyls being available for further transformation. 

The same strategy can be applied to the bridged cyclodextrin 82, giving the 6A,6D-bridge,6C-hydroxyl a-cydodextrin 94 in 52% yield (Scheme 9.43) [78]. The bridge can be removed using a palladium-catalyzed double de-O-aUylation to give the 6A,6C,6D-trihydroxy a-cydodextrin 92, or cyclodextrin 93 if the hydroxyl has been protected with a silyl ether before. 

The effect of methylated cydodextrins on the RhH(CO)(TPPTS)3 complex in hydroformylation conditions (50 bar, CO H2 = 1 1, and80°C) were investigated by high-pressure P NMR spectroscopy. It was found that the formation of the stable inclusion complex between methylated P-cyclodextrin and TPPTS influences the TPPTS dissociation equilibrium. The methylated a-cydodextrin does not interact with the TP PTS and the methylated y-cydodextrin can only weakly bind to the TP PTS. These results explain why a decrease in the normal-to-branched aldehydes ratio is always observed when cydodextrins are used as mass-transfer agents in aqueous biphasic hydroformylation processes [104]. 

FIGURE S9.S Monomer(E)- l/dimer(3) equilibrium of a permethylated a-cydodextrin azobenzene dyad. The equilibrium is disturbed by ( )-l- (Z)-l photoisomerization and may be reformed after thermal (2)-l—>( )- isomerization. (From Fujimoto, T Nakamura, A. Inoue.Y., Sakata, Y, and Kaneda, T Tetrahedron Lett., 42(45), 7987,2001. Reprinted with permission from Elsevier Science. Copyright 2001.)... 

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