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

Partial oxidation of the 1,3-alternate atropisomer
of p-tert-butylcalix­[4]­arene tetraacetate with CrO3 afforded mainly a mixture of trioxo- and tetraoxo-calix[4]­arene
tetraacetate derivatives. The trioxotetrahydroxy derivative 6 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-alternate conformation.
Acetylation or alkylation of the tetrahydroxytrioxocalix[4]­arene 6 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-alternate 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 9b with MeLi followed
by 2-fold elimination of water afforded calixarene 11 with a pair of distal exocyclic double bonds at the bridges. Both
the tetramethyl ether derivatives 9b and 11 exist in solution as a mixture of the 1,2-alternate and 1,3-alternate conformers, but in the crystal
both adopt a 1,2-alternate conformation.