ИСТИНА

Molecular tubes based on calixarenes have attractive structural features that cause unique receptor and conformational properties of these systems: two calix[4]arene macrocycles bound by lower rims through four ethylene linkers form a relatively rigid receptor cavity capable of binding cations by eight ether oxygen atoms. This peculiarity can be used to mimic cellular ion channels behavior [1]. It is known that macrocycles forming a molecular tube undergo slow interconversions between two conformations “flattened cone” characterized by C2v symmetry. The presence of two substituents in distal positions at one of the rims of the calixtube leads to the dominance of one of the conformers [2]. It is noteworthy that in all cases the binding of the cation to the inner cavity of the molecular tube is accompanied by a change in the conformation of calixarene macrocycles with an increase in the symmetry of the molecule from C2v to C4v [1]. In our work, two isomeric calix[4]tubes containing on each rim two tert-butyl groups in distal positions were synthesized for the first time (Figure 1). In one case, the tert-butyl groups are located in perpendicular planes (isomer A), in the other case in the same plane (isomer B). In addition, in calixtube A, the mutual arrangement of the tert-butyl groups is differently directed, which should simplify the transition of the conformation with C2v symmetry to C4v. In turn, in calixtube B, the mutual repulsion of tert-butyl groups is co-directed, which impairs the transition to conformation with C4v symmetry. NMR studies of the kinetics of complex formation of these tubes with KI were carried out. It turned out that the rate of formation of the complex in both cases is about the same.