Calix[4]quinone. Part 2: Intramolecular Michael-addition of calix[4]diquinone

The oxidation of calix[4]arene dibenzoate 1 with chlorine dioxide yielded the corresponding calix[4]diquinone 2 and an intramolecular Michael-addition product 3. Reaction of diquinone 2 with ethylene glycol under acidic conditions produced the half-protected ketal derivative 4. The removal of benzoate moieties from compound 4 in basic conditions produced a phenoxide anion, which underwent intramolecular Michael-addition and yielded product 5. In acidic ketal cleavage conditions, the ketal moieties of product 5 were removed, but the intramolecular Michael-addition structure was maintained in the product 6.