Synthetic receptors

Abstract

Novel, rigid cyclophanes have been designed and synthesised as receptors for the biologically important neurotransmitter, acetyl choline. The receptor cavities were less symmetrical than those of similar cyclophanes which had been previously prepared, as they incorporated two quite different functional groups on opposite sides of the cavity. NMR experiments indicated that a cyclophane which incorporated a benzoate residue bound acetyl choline, with an exchange between the free and bound species which was slow on the NMR time scale. A second cyclophane, which incorporated both a benzoate and a pyridinium residue, also bound acetyl choline, but the exchange between the free and bound species was fast on the NMR time scale. NMR experiments also indicated that a third cyclophane, which incorporated only uncharged pyridyl and benzyl residues, did not bind acetyl choline. However, acetyl choline was efficiently transported across a PVC membrane by this neutral cyclophane with very little interference from ammonium and group I and group II metal cations. Urea and thiourea residues were incorporated into crown-type macrocyclic frameworks. The crystal structures of two macrocycles incorporating thiourea residues (18N(_4)O(_2).2CS and 24N(_4)O(_4).2CS) were determined, as was the crystal structure of the 18N(_4)O(_2).2CS/silver(I) complex. Preliminary experiments indicate that the bisthioureas 18N(_4)O(_2).2CS and 24N(_4)O(_4).2CS form quite stable complexes with the 'soft' metal cations silver (I), zinc (II), cadmium (II) and mercury (H), and that the analogous bisureas 18N(_4)O(_2).2CO and 24N(_4)O(_4).2CO form stable complexes with sodium and potassium, for which they are selective.