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 H-1 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)- OF (S)-mandelic acid and (L)- or (D)-dibenzoyl-tartaric acid. The receptors exhibited different chiral recognition abilities towards the enantiomers of racemic guests. (C) 2008 Elsevier Ltd. All rights reserved.