Calix[4]pyrroles having extended aromatic cavities have been functionalized with 4 ureas in the para position of their meso phenyl substituents. This elaboration of the upper rim was completed in 2 synthetic steps starting from the α,α,α,α-tetranitro isomer of the calix[4]pyrrole obtained in the acid catalyzed condensation of p-nitrophenyl methyl ketone and pyrrole. In dichloromethane solution and in the presence of 4,4′-bipyridine N-N′-dioxide the tetraurea calix[4]pyrrole dimerizes reversibly forming a cyclic array of 16 hydrogen bonds and encapsulating 1 molecule of bis-N-oxide. The encapsulated guest is bound in the cavity by hydrogen bonding to the 2 endohedral calix[4]pyrrole centers. Further evidence for dimerization of the tetraurea calix[4]pyrroles is provided by ¹H-NMR experiments and by the formation of mixed capsules.

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