Syntheses and structures of novel m-xylylene-bridged calix[6]arenes: stabilization of a sulfenic acid in the cavity of calix[6]arene

The syntheses and structures of novel bridged calix[6]arenes 1 with inwardly directed functional groups in the cavity, and application of the bicyclic framework to the stabilization of a sulfenic acid are described. It was found that, unlike their precursors 2, the tetramethoxy compounds 1 have a tendency to adopt the 1,2,3-alternate conformation at least in the crystalline state, except when the central functionality has a large steric demand. Thermolysis of sulfoxide 1g at 80°C in toluene afforded the stable sulfenic acid 1h almost quantitatively; the structure was determined by X-ray analysis. The SOH group was found to be directed into the cavity and surrounded by the calix[6]arene macrocycle with the 1,2,3-alternate conformation, being protected from self-condensation leading to the corresponding thiosulfinate. The high stability of 1h demonstrates that the framework of the bridged calix[6]arene can effectively regulate the reactivity of the functional group fixed in the cavity.