Efficient Fragment Coupling Approaches toward Large Oxacalix[<i>n</i>]arenes (<i>n</i> = 6, 8)

The first rational, stepwise synthesis of enlarged oxacalix[n]arenes (n > 4) is described. Variously substituted oxacalix[3]arene[3]pyrimidines were prepared rather selectively by a straightforward [3 + 3] fragment coupling approach after a thorough search for the optimum nucleophilic aromatic substitution conditions. Similar procedures also allowed facile synthesis of unsymmetrical oxacalix[4]- and oxacalix[8]arenes.