The pinched cone conformation of calix[4]arenes: Noncovalent rigidification of the calix[4]arene skeleton

Calix[4]arenes at the upper rim substituted with two (thio)urea moieties, 2a-c and 4, or substituted with two acetamido groups, 5, adopt a pinched cone conformation in CDCl3 solution. The pinched cone conformation is the result of diametrical, intramolecular hydrogen bonds and was studied by H-1 NMR dilution and variable-temperature experiments, NOESY spectroscopy, and FTIR experiments. The pinched cone conformation in these novel calix[4]arenes is stable in C2D2Cl4 up to at least 120 degrees C. The bis(ureido)calix[4]arenes 2b,c give a hydrogen-bonded dimer in the pinched cone conformation.