Synthesis of new chiral calix[4]azacrowns for enantiomeric recognition of carboxylic acids

Two novel chiral calix[4]azacrown ethers 4 and 5 bearing a furfuryl group on the nitrogen atom were developed by the reaction of dibromo- or ditosyl derivatives of p-tert-butylcalix[4]arenes 2 and 3 with a chiral diol, 1. The enantioselective recognition of these receptors towards the enantiomers of racemic carboxylic acids has been studied by 1H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds 4 and 5 with each of the enantiomers of guest molecules were determined by using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate that both of the hosts form 1:1 instantaneous complexes with (R)- or (S)-mandelic acid and (l)- or (d)-dibenzoyltartaric acid. The receptors exhibited different chiral recognition abilities towards the enantiomers of racemic guests.