Studies on molecular recognition in supramolecular systems. Part 31: Circular dichroism spectral studies of molecular and chiral recognition of aliphatic alcohols by 6-modified β-cyclodextrins

The stability constants (KS) for the inclusion complexation of novel mono[6-O-(1- benzotriazole)]-β-cyclodextrin (2) and mono(6-benzylseleno-6-deoxy)-β-cyclodextrin (3) with a series of chiral and achiral (cyclo)alkanols have been determined at 25°C in aqueous phosphate buffer solution at pH 7.20 by circular dichroism spectral titration. It was revealed that 2 and 3 can fairly strictly recognize not only the size/shape but also the chirality of guest molecules. Thus, the log KS value, or the Gibbs free energy change (−ΔG0), increases linearly with increasing number of carbon atoms (NC) in cycloalkanol, affording comparable increments per methylene unit: −dΔG0/dNC=2.0 and 2.2 kJ mol−1 for 2 and 3, respectively. Furthermore, 2 and 3 displayed moderate to excellent isomer selectivities of up to 15.3 for the guest alcohols examined, while moderate enantioselectivities of 1.1–1.4 were obtained with chiral borneol and menthol guests upon complexation with 2 and 3. The present results obtained with 2 and 3 elucidate the effects of substituents on the complexation behavior as well as some of the factors governing size, shape, and chiral selectivities.