Calixarenes were bridged with a xylenyl unit or capped with a mesitylenyl unit, and their conformational properties were examined by spectroscopic methods. As previously reported by Gutsche et al., the calixarene bridged on 1, 4-phenyl units could enjoy slow ring inversion. The calixarene bridged on 1, 2-phenyl units also showed the ring inversion behavior, whereas in the calixarene bridged on 1, 3-phenyl units, ring inversion was virtually suppressed because of destabilization of the transition state. The capping of 1, 3,5-tri-O-alkylated calixarenes with 1, 3,5-tris(bromomethyl)benzene gave the products in high yields (80–91%). This advantage was accounted for by the C3 symmetrical complementarity of these two reactants. In these capped products, ring inversion was inhibited under the present measurement conditions, and the presence of a unimolecularly closed inner cavity was suggested on the basis of 1H NMR spectroscopy and MM3 calculations.
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