Chemistry of 2-monothiacalix[4]arene: shaping of the basic skeleton

Fragment condensation of suitable bisphenols afforded 2-monothiacalix[4]arene on a multigram scale. Using reaction conditions known from classical calixarene chemistry, the macrocycle was alkylated to study the conformational preferences and dynamic behavior of the corresponding tetraalkylated derivatives. While the introduction of propyl residues leads to the immobilization of the respective conformers, shorter substituents (Me and Et) represent dynamic systems depending on temperature. A combination of X-ray crystallography, DFT calculations and NMR techniques demonstrated that the presence of a sulfur atom in the backbone leads to significant changes in the selectivity of the respective conformers. Although six different conformations/atropisomers are theoretically possible, it turns out that only five of them are accessible by direct propylation. Selective preparations of some conformers suggest that this mixed-bridged calixarene could find application in supramolecular chemistry as a potential molecular scaffold.