Restricted Rotation of tert-Butyl Groups in Sterically Crowded Methylene-Functionalized Calix[4]arenes

The crowded methylene-functionalized calix[4]arenes 4 and 5 display restricted rotation of the tert-butyl substituents at the bridges. At low temperatures, separate signals were observed for the methyls of these groups in the ¹H NMR spectra.