Positive and negative allosteric effects of thiacalix[4]arene-based receptors having urea and crown-ether moieties

Heteroditopic receptors (4(a-e)) based on a thiacalix[4] arene in the 1,3-alternate conformation, which have two urea moieties linking various phenyl groups substituted with either electron-donating or -withdrawing groups at their m-, or p-positions with a crown-ether moiety at the opposite side of the thiacalix[4] arene cavity, have been synthesized. The two examples with p-CH3- (4(b)) and p-NO2-substituted (4(e)) phenyl groups have been characterized by X-ray crystallography. The binding properties of receptor 4(e) were investigated by means of H-1 NMR spectroscopic and absorption titration experiments in CHCl3-DMSO (10 : 1, v/v) solution in the presence of K+ ions and various anions. Interestingly, it was found that receptor 4(e), which possesses two p-nitrophenyl ureido moieties, can complex most efficiently in the urea cavity or the crown-ether moiety; and the plausible allosteric effect of receptor 4(e) was also studied.