Synthesis and conformational analysis of an α-cyclodextrin [2]-rotaxane

An α-cyclodextrin [2]-rotaxane has been prepared in 10% yield, by threading α-cyclodextrin (α-CD) with (E)-4,4′-diaminostilbene in aqueous solution and capping the included guest through reaction with 2,4,6-trinitrobenzene-1-sulfonate. 1D 1H NMR spectroscopy and DQCOSY and ROESY experiments show that the α-CD rotates freely around the axle of the rotaxane, but is localised over the olefinic moiety of the stilbene. The pKa values of the α-CD [2]-rotaxane were found to be 9.3 and 9.6, which are attributable to deprotonations of the (E)-4,4′-bis(2,4,6-trinitrophenylamino)stilbene moiety. NMR experiments show that these deprotonations do not perturb the conformation of the rotaxane.