Tuning conformations of calix[4]tubes by weak intramolecular interactions

The conformational distribution in distally functionalized adamantylated calix[4]tubes was analyzed with NMR and quantum-chemical calculations. Without any intramolecular interactions, ester-, acid-, amino-, alcohol-, isocyanate-, phthalimide- and urea-substituted calixtubes preferred less sterically hindered flattened cone conformers in solution. In contrast, in a non-polar medium (CDCl3) for calixtubes bearing two 3-carboxymethyl-1-adamantyl or 3-(2-ureidoethyl)-1-adamantyl units, the less sterically preferable flattened cone conformation with bulky substituents connected by intramolecular hydrogen bonds was found to be more favorable. On the other hand, for a calixtube with two positively charged units, only the conformer with distanced substituents was exclusively detected. Thus, the conformational equilibrium in functionalized calix[4]tubes can be controlled by intramolecular hydrogen bonding between appropriately arranged carboxylic groups or urea moieties, or by electrostatic repulsion of positively charged substituents.