MOLECULAR ENGINEERING .2. INFLUENCE OF SIDE-CHAIN SUBSTITUENTS IN LARIAT-TYPE UPPER-RIM CALIX[4]CROWNS ON THEIR BINDING-PROPERTIES AND THE REVERSAL OF THESE

Cone-structured upper-rim 1,3-calix[4]crowns have been synthesized fro m the distal dibromo diol 6 which is itself regioselectively synthesiz ed from p-bromocalix[4]arene hexyl ether by transmetallation, quenchin g with B(OMe)(3) and oxidation. The dibromo substituents in these cali x[4]crowns have been transformed into hydrophobic or hydrophilic group s to give lariat-type hosts. Picrate extraction experiments show that alkylammonium ions bind more strongly than alkali-metal cations to hos ts having hydrophobic side-arms such as bromo or p-methoxyphenyl, whic h means that the hydrophobic binding of the calix[4]arene unit and the hydrophilic binding of the crown unit synergistically increase the af finity of the calix[4]crowns for alkylammonium ions. H-1 NMR spectra a lso support the conformational change of the calix[4]crowns from pinch ed-cone to cone upon complexation. With amide or ester side-arms the a ffinity tendency is reversed. It is presumed that the hydrophilic side -arms have a high affinity for the charged part of the guest, which le ads to a collapse of the hydrophobic cavity.