Base-catalyzed oxygenation of tert-butylated phenols. 2. Formation of 3-aryl-2,5-di-tert-butyl-2,4-cyclopentadienones by base-catalyzed oxygenation of 4-aryl-2,6-di-tert-butylphenols: x-ray structure determination of 2,5-di-tert-butyl-3-(4-chlorophenyl)cyclopentadienone