Incorporation of Three or Two Distal Double Bonds at the Methylene Bridges of the Calix[4]arene Scaffold

Partial oxidation of the 1,3-<i>alternate</i> atropisomer of <i>p-tert-</i>butylcalix­[4]­arene tetraacetate with CrO₃ afforded mainly a mixture of trioxo- and tetraoxo-calix[4]­arene tetraacetate derivatives. The trioxotetrahydroxy derivative <b>6</b> was isolated from the mixture after hydrolysis of the crude product, followed by trituration with ethanol. Trioxocalix[4]­arene adopts in the crystal a 1,2-<i>alternate</i> conformation. Acetylation or alkylation of the tetrahydroxytrioxocalix[4]­arene <b>6</b> with acetic anhydride and 1-bromobutane, respectively, afforded exclusively a single atropisomer of the product, which in both cases were characterized as the 1,3-<i>alternate</i> form. Addition of MeLi to the tetramethyl and tetrabutyl ether of the trioxocalix[4]­arenes followed by 3-fold elimination of water yielded calixarene derivatives possessing three exocyclic double bonds at the bridges. Reaction of the dioxotetramethoxy calix[4]­arene <b>9b</b> with MeLi followed by 2-fold elimination of water afforded calixarene <b>11</b> with a pair of distal exocyclic double bonds at the bridges. Both the tetramethyl ether derivatives <b>9b</b> and <b>11</b> exist in solution as a mixture of the 1,2-<i>alternate</i> and 1,3-<i>alternate</i> conformers, but in the crystal both adopt a 1,2-<i>alternate</i> conformation