Chiral anion recognition using calix[4]arene-based ureido receptors in a 1,3-alternate conformation

The introduction of chiral alkyl substituents into the lower rim of calix[4]arene immobilised in the 1,3-alternate conformation led to a system possessing a preorganised ureido cavity hemmed with chiral alkyl units in the near proximity. As shown by the ¹H NMR titration experiments, these compounds can be used as receptors for chiral anions in DMSO-d ₆. The chiral recognition ability can be further strengthened by the introduction of another chiral moiety directly onto the urea N atoms. The systems with double chiral units being located around the binding ureido cavity showed better stereodiscrimination, with the highest selectivity factor being 3.33 (K _L/K _D) achieved for N-acetyl-ʟ-phenylalaninate. The structures of some receptors were confirmed by single crystal X-ray analysis.