Selective Formylation of Azacalixpyridine Macrocycles and Their Transformation to Molecular Semicages

The aromatic electrophilic formylation reaction of azacalix[2]arene[2]pyridine and azacalix[4]pyridine were systematically studied. By simply controlling the ratio of reactants and the reaction temperature, the Vilsmeier-Haack reaction selectively afforded mono-, di-, and tetra-formylated azacalix[2] arene[2]pyridines and azacalix[4]pyridines. The preferential and selective functionalization reactions of macrocycles were discussed in terms of their conformational structure and conjugation effect between aromatic subunits and bridging nitrogen atoms. All resulting functionalized azacalix[2]arene[2]pyridines and azacalix[4]pyridines adopted a 1,3-alternate conformation both in the crystalline state and in solution. Taking advantage of the close proximity of aldehyde groups in 1,3-alternate di- and tetra-formylated azacalixpyridine macrocycles, the McMurry reductive coupling reaction of carbonyls was accomplished to yield unique semicage molecules.