Tetazac: A Novel Artificial Receptor for Binding co-Amino Carboxylates

Our concept to produce linearly connected polytopic artificial receptors was developed in order to enhance binding selectivity while conserving a reasonable synthesizability of the host compound. A first step in this direction involves the synthesis of Tetazac (6) consisting of two subunits connected by one p-xylene bridge. The attachment of a primary ammonium group binding aza crown moiety 3a to a tetrahedral anion host 1 should give a receptor configuration suitable for complexing w-amino carboxylates like GABA (4) in protic solutions. The synthesis of 6 followed a convergent strategy and produced the target receptor in 28 steps in 2% overall yield. Complexation studies by NMR and ion-selective potentiometry indicate that either subunit in the ditopic host 6 is functional in binding their respective monotopic guests. Collaborative action of the substructures in the binding of zwitterionic species was deduced from comparing the association constants in 90% aqueous methanol determined by competition with K⁺ ion to those obtained with the structurally closely related but monotopic host Amazac (7), Though absolute K. values are higher with 7, the ditopic host 6 shows greater selectivity for hydrophobic or zwitterionic ammonium salts by a factor of 3 or 2.5, respectively. No difference in binding 7-aminobutyric acid (GABA, 4) or 6-aminohexanoic acid (24) to 6 was found (log 7C_as = 2.4), indicating the flexibility of this artificial ditopic receptor.