Cyclodextrins meta-para-selectivity

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. 

K. Fujita, A. Shinoda, and T. Imoto (1980), Hydrolysis of phenyl acetates with capped j5-cyclodextrins Reversion from meta to para selectivity. J. Amer. Chem. Soc. 102, 1161-1163. 

However, for the positional isomers of phthalate (56-58), the response selectivity was different for the two types of membranes. Whereas membranes 1 and 2 showed responses in the order of 56 (ortho) > 57 (meta) > 58 (para), membrane 53 interestingly showed a different response order, i.e., 57 (meta) > 58 (para) > 56 (ortho), a selectivity which is quite different from that expected on the basis of simple electrostatic effects. Such a difference in the selectivity is possibly due to host-guest complexation involving not only electrostatic interactions but also inclusion into the P-cyclodextrin cavity, which is capable of recognizing differences in the steric structures of the guests. 

This paper describes the elution behavior of benzene, ortho-, meta-, and para-nitrophenol, naphthalene, and biphenyl in aqueous and methanolic cyclodextrin mobile phases. The inclusion complex formation constants of these selected compounds in e-CD mobile phases containing 0%, 10%, and 20% methanol are reported. The formation constants obtained using a CN column/CD mobile phase were determined and used to predict the elution behavior of the test compounds on C-18 columns. Also, the elution behavior of solutes on C-18, e-CD, and Y-CD stationary phases are compared. 

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