Synthesis of p-tert-Butyl-monoketocalix[4]arenes: Bridge Mono-oxidation with (Diacetoxyiodo)benzene

In order to develop a new bridge oxidation reagent, (diacetoxyiodo)benzene has been used to oxidize a variety of p-tert-butylcalix[4]arene derivatives. The oxidation experiments showed that bridge mono-oxidation with (diacetoxyiodo)benzene can be executed in moderate yields (up to 65%). The optimal solvent, (diacetoxyiodo)benzene amount, temperature, and time for bridge mono-oxidation of 1a are dioxane, 5 equiv, 80 °C, and 12 h, respectively. The substituents with strong electron-withdrawing capacity are detrimental to bridge mono-oxidation. p-tert-Butylcalix[4]arene derivatives in 1,3-alternate and 1,2-alternate conformations can be oxidized into p-tert-butyl-monoketocalix[4]arenes while those in cone and partial cone conformations cannot. A plausible mechanism including two processes was proposed for bridge mono-oxidation. The bridge mono-oxidation results confirm that (diacetoxyiodo)benzene can be used as an alternative and efficient metal-free oxidant for the synthesis of p-tert-butyl-monoketocalix[4]arene in 1,3-alternate and 1,2-alternate conformations. The further derivatizations show that the monobridge carbonyl group can impart a selective bridge-modification to the calix[4]arene skeleton.