Tuning and enhancing enantioselective quenching of calixarene hosts by chiral guest Amines

The synthesis of a propranolol amide derivative of p-allylcalix[4]arene is described, which has been designed to behave as a molecular sensor capable of distinguishing chiral amines on the basis of their shape and chirality. This molecule can discriminate between the enantiomers of phenylalaninol through the quenching of the fluorescence emission in methanol in contrast to an (S)-dinaphthylprolinol calix[4]arene derivative, which can discriminate between the enantiomers of phenylglycinol, but not phenylalaninol. The separation between the naphthyl fluorophores and the hydrogen-bonding sites within the chiral cavity can be tuned to recognize guest amines with similar separation between aryl groups and hydrogen-bonding sites. The formation of metal ion complexes of the p-allylcalix[4]arene propranolol amide derivative is shown to induce a more regular and rigid cone conformation in the calix[4]arene macrocycle, which generates a significant enhancement in the observed enantiomeric discrimination.