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

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