No abstract

The simple mono-O-benzyl-p-tert-butylcalix[6]arene (2a) has been reported 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 were found for calix[6]arenes whose O-substituent was larger than butyl, whereas freezing of the conformation was observed for substituents larger than benzyl. Steric effects and preservation of the cyclic array of hydrogen bonds are suggested as the main cause for the high inversion barriers.