Calix[4]arenes with siloxanes bridging opposite rings.

Several C-silylated (allyldimethylsilyl) calixarenes (1, 14, and 18) were treated with commercial TBAF in THF, giving the novel calixarenes 2, 15, and 19, in which the opposite rings were bridged by a Si-O-Si siloxane group. Compound 19 and the derived phenol 20 (as well as the dibromocalixarene precursor 17) were mixtures of cone and partial cone conformations according to NMR, while 2 and 15 (as well as 14 and the cone components of 17, 19, and 20) were in flattened cone conformations.