The first successful crystallographic characterization of a cyclodextrin dimer: efficient synthesis and molecular geometry of a doubly sulfur-bridged beta-cyclodextrin.

Beta-cyclodextrin is transannularly disulfonylated at the 6(A)- and 6(B)-positions, and then converted to the corresponding 6(A),6(B)-diiodide and 6(A),6(B)-dithiol. Cross-coupling of the latter two species yields a single head-to-head-coupled beta-cyclodextrin dimer 5 with two sulfur linkers at adjacent 6-methylene carbons. NMR and X-ray analysis revealed the trans-type ("aversive") linkage of both beta-cyclodextrin units. In the solid-state structure of 5.5 MeOH.23 H(2)O, the undistorted cyclodextrin macrocycles feature almost parallel ring planes pointing away from each other, leaving 5 with a "handcuff-like" appearance of approximate C(2) symmetry. This work represents the first successful crystallographic study on a cyclodextrin dimer.