UPPER-RIM UREA-DERIVATIZED CALIX[4]RENES AS NEUTRAL RECEPTORS FOR MONOCARBOXYLATE ANIONS

Several new anion receptors have been synthesized by introducing one o r two urea or thiourea units at the upper rim of calix[4]arenes, block ed in the cone conformation. The difunctionalized receptor 13 shows se lectivity in the recognition of acetate anion, which interacts with th e two urea units through four hydrogen bonds. The monofunctionalized r eceptors 21 and 22 complex more strongly aromatic carboxylates or buty rate anion since the primary hydrogen bonding contacts are strengthene d by weak pi/pi or CH3/pi host-guest interactions.