Difunctionalized beta-cyclodextrins: synthesis and X-ray diffraction structure of 6(I),6(II)-dideoxy-6(I),6(II)-bis[2-(2-pyridyl)ethylamino]-beta-cyclomaltoheptaose

The synthesis, solution NMR investigation and solid-state structural charac terization of a new difunctionalized beta -cyclodextrin (beta -CD) are repo rted. 6(I),6(II)-Dideoxy-6(I),6(II)-bis[2-(2-pyridyl)ethylamino]-beta -cycl omaltoheptaose is synthesized for the first time using a regioselective syn thetic procedure. On the basis of an aqueous solution NMR investigation, th e intramolecular interaction of the two pyridine rings with the upper rim o f the cavity is proposed. 6(I),6(II)-Dideoxy-6(I),6(II)-bis[2-(2-pyridyl)et hylamino]-beta -cyclomaltoheptaose, C56H86N4O33, crystallizes in the monocl inic P2(1) space group with cell dimension a=26.303(5), b=15.670(5), c=8.27 6(2) Angstrom and beta =103.60(2)degrees, and 5.5 molecules of water for ea ch independent beta -cyclodextrin molecule. The structure refines to R=0.10 3 for 2270 observed reflections [I greater than or equal to3 sigma (I)] and represents the first example of a complete structural characterization of a branched difunctionalized beta -cyclodextrin. In the solid state, the mac rocycle structure maintains an approximate seven-fold symmetry with only sm all changes occurring in the cyclic carbohydrate conformation where two con secutive primary hydroxy groups are substituted with bulky moieties. The tw o aminoethylpyridine groups linked to contiguous glucosidic units show diff erent behaviour, with one group extending away from the cavity of the beta -CD, the other remaining in the proximity of the 'mouth' of the cavity. How ever, in the crystal the aminoethylpyridine group extending away from the c avity of the beta -CD is deeply inserted into the cavity of the adjacent be ta -CD molecule translated along the c axis, giving rise to long rows of be ta -CD molecules stabilized by these host-guest interactions. The resulting polymeric arrangement has already been observed in crystal structures of o ther monosubstituted beta -CDs.