Anti to syn isomerization of oxacalix[4]arene bearing two methyl groups at the intra-annular distal positions

The thermal isomerization of anti to syn stereoisomers of oxacalix[4]arene bearing two methyl groups at the intra-annular distal positions was investigated by temperature-dependent NMR spectroscopy. The conversion followed a first-order kinetics, and very slowly proceeded at 473 K in nitrobenzene-d(5) With a half-life of 7.2 h. The free energy of activation (Delta G(-/-) 139 kJ mol(-1)) is much higher than those for the ring inversion of related calix[4]arene derivatives. (C) 2008 Elsevier Ltd. All rights reserved.