Pyrrolamidocalix[4]arenes: new receptors for anion recognition

Pyrrolamidocalix[4]arenes 1–4, members of a new class of anion receptors bearing pyrrolic units at the upper rim of calix[4]arene macrocycle, have been readily synthesized in good yields. Derivatives 1 and 3, with unsubstituted pyrrole units, show a good selectivity for H 2 PO 4 - over F− and AcO−, while the presence of electron-withdrawing NO2 substituents in 2 and 4 inverts the selectivity favoring more basic AcO− and F−. In addition, it is demonstrated that the flexibility of calix[4]arene skeleton, present in 1 but absent in 3, is very important in the fitting process that leads the amidopyrrole moieties to wrap the tetrahedral H 2 PO 4 - guest.