Cyclodextrin capped isomers

Generally, separation of regioisomers of capped cyclodextrins is difficult. Only in limited ideal examples is each isomer easily separated through the column and purified by recrystallization or reprecipitation without any difficulty. The phenanthrenedisulfonate cap is a typical example of this kind (72). But usually column separation (even a very efficient HPLC column) is not very helpful. Regioselective capping is very important in these sense and from a mixture of regioisomers, the most abundant isomer is usually purified by careful and repeated reprecipitation. 

Either the C -capped or the O -capped material can be heated with an excess of imidazole to produce /3-cyclodextrin bisimidazole (VII). On the basis of the above discussion, we believe that the material produced from the C-capped compound is a 6A,6C and 6A,6D isomeric mixture, while that produced from the O-capped compound is largely the 6A,6D isomer. As it turns out, we have not detected yet significant differences between these two materials as catalysts in our kinetic and product studies. 

In the studies of the hydrolyses of substituted phenyl acetates by a- or jS-cyclodextrins, it was observed that meta-substituted phenyl esters were more rapidly hydrolyzed than the corresponding para-isomers, a phenomenon termed m ta-selectivity. This observation indicates that the binding mode is probably asymmetric. This effect is apparently dependent on the depth of the cavity, and Fujita et al. (177) showed that appropriate simple modifications of j8-cyclodextrin such as capping the host, for instance, can alter this selectivity and leads to conversion of the well-established metaselectivity to para-selectivity. 

In another attempt, azobenzene-appended y-cyclodextrin 11 was prepared. Upon the irradiation with 320- to 390-nm light, 70% of the E-isomer in water isomerized to the Z-isomer, which exhibited an enhanced binding ability for (-)-borneol and (-l-)-fenchone. Benzo-18-crown-6 12 linked to a phenol was synthesized. Upon irradiation by UV light, the azo linkage isomerized to the Z-isomer. It slowly reverted to the E-form in the dark and rapidly upon irradiation with visible light. An enhanced extraction of Ca + by the Z-isomer has been observed. The Z structure constitutes a photoresponsive anion cap. ... 

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