INTERPLAY OF STERIC HINDRANCE AND HYDROGEN-BONDING TO RESTRICT MONO-O-SUBSTITUTED P-TERT-BUTYLCALIX[6] ARENES IN CONE CONFORMATION

The simple mono-O-benzyl-p-tert-butylcalix[6]arene (2a) has been repor ted to display a cone conformation with a slow inversion rate at room temperature. Here, a series of mono-O-substituted p-tert-butylcalix[6] arenes (2a-g, 3-9) has been studied by 2D and variable-temperature NMR spectroscopies. Cone conformations with slow macrocyclic inversion we re found for calix[6]arenes whose O-substituent was larger than butyl, whereas freezing of the conformation was observed for substituents la rger than benzyl. Steric effects and preservation of the cyclic array of hydrogen bonds are suggested as the main cause for the high inversi on barriers.