The Preparation and Crystal Structures of Octaoxoketocalix[8]arene Derivatives: The Ketocalixarene Counterparts of the Largest “Major” Calixarene

The purpose of this study was to synthesize and structurally characterize ketocalixarenes (i.e., calixarenes where the bridging methylene bridges are replaced by carbonyl groups) derived from the largest "major" calixarene, namely <i>p</i>-<i>tert</i>-butylcalix[8]arene <b>3a</b>. Ketocalix[8]arenes were synthesized by the oxidation of protected <i>p</i>-<i>tert</i>-butylcalix[8]arene derivatives. Octamethoxy-p-tert-butylketocalix[8]arene <b>6b</b> was prepared by the photochemical reaction of the calixarene <b>3b</b> with NBS in a CHCl₃/H₂O mixture. The oxidation of the methylene groups of octaacetoxy-<i>p</i>-<i>tert</i>-butylcalix[8]arene <b>3c</b> was conducted by a reaction with CrO₃ in Ac₂O/AcOH. The basic hydrolysis of the acetate groups of the oxidation product yielded octahydroxy-<i>p</i>-<i>tert</i>-butylketocalix[8]arene <b>6a</b>. In the crystal, the molecule adopts a saddle-like conformation of crystallographic <i>C</i>₂ and idealized <i>S</i>₄ symmetry. Strikingly, the array of OH/OH intramolecular hydrogen bonds present in the parent <b>3a</b> is completely disrupted in <b>6a</b>.