ИСТИНА

Control on macrocycles’ conformations is one of the most important parts of calixarene-based receptors’ design. In calix[4]tubes the rigidified calix[4]arene counterparts possess C2v symmetry not only in the solid state (that is common for most of cone calix[4]arene tetraethers) but also in solutions as follows from characteristic signal doubling in NMR spectra. Still the conformational interconversion is not blocked but significantly decelerated, and exchange rates can be easily derived from EXSY. Due to, at least, different sterical crowding of calixarene moieties, di-substituted calixtubes have non-equal distribution of C2v-symmetrucal flattened cone conformers. We prepared a series of distally di-substituted calixtubes bearing two ester, acid, amide, alcohol, amine or urea functionalities linked through adamantane units,1 and analyzed their conformer A/conformer B ratios by 1D and 2D NMR experiments. Less sterically hindered conformers B were found dominant in most cases, but not for calixtubes with 3-carboxymethyl-1-adamantyl and 3-ureidoethyl-1-adamantyl units which can stabilize conformer A in non-polar medium due to intramolecular hydrogen bonding (see Figure).