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 (1)H 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.