Syntheses and NMR Spectroscopic Studies of Bridged and Capped Calix[6]arenes: High-Yield Syntheses of Unimolecular Caged Compounds from Calix[6]arene

Calix[6]arenes 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 calix[6]arene bridged on 1, 4-phenyl units could enjoy slow ring inversion. The calix[6]arene bridged on 1, 2-phenyl units also showed the ring inversion behavior, whereas in the calix[6]arene 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 calix[6]arenes 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.