Easy Access to Modified Cyclodextrins by an Intramolecular Radical Approach

A simple method to modify the primary face of cyclodextrins (CDs) is described. The 6(I)-O-yl radical of alpha-, beta-, and gamma-CDs regioselectively abstracts the H5(II), located in the adjacent D-glucose unit, by an intramolecular 1,8-hydrogenatom-transfer reaction through a geometrically restricted nine-membered transition state to give a stable 1,3,5-trioxocane ring. The reaction has been extended to the 1,4-diols of alpha- and beta-CD to give the corresponding bis(trioxocane)s. The C-2-symmetric bis(trioxocane) corresponding to the alpha-CD is a stable crystalline solid whose structure was confirmed by X-ray diffraction analysis. The calculated geometric parameters confirm that the primary face is severely distorted toward a narrower elliptical shape for this rim.