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 CDCl 3 solution. The pinched cone conformation is the result of diametrical, intra molecular hydrogen bonds and was studied by 1 H NMR dilution and variable-temperature experiments, NOESY spectroscopy, and FTIR experiments. The pinched cone conformation in these novel calix[4]arenes is stable in C 2 D 2 Cl 4 up to at least 120 °C. The bis(ureido)calix[4]arenes 2b, c give a hydrogen-bonded dimer in the pinched cone conformation.