Synthesis of new chiral calix[4]arene thiourea derivatives for enantiomeric recognition of carboxylate anions

The study of enantiomeric recognition of amino acid and carboxylic acid compounds is of significance since these compounds are basic building blocks of biological molecules. Enantiomeric recognition and separation of these compounds are among the main topics of supramolecular chemistry since they are basic building blocks of biological molecules and a number of them are known to possess potent biological activities. In this study the synthesis of novel chiral calix[4]arene thiourea derivatives has been reported. The enantioselectivity of chiral receptors was investigated by using UV-Vis spectroscopy. All the chiral calix[4]arene derivatives exhibited certain chiral recognition towards the enantiomers of alpha-hydroxy isovaleric acid (HIVA), mandelic acid (MA), 2-chloromandelic acid (2-ClMA) and N-Boc-alanine (N-BocAl). The receptors with hydrogen bonding sites and aromatic groups showed considerable higher stereoselectivities. As a chiral receptor, calix[4]arene 2-hydroxy-1,2 diphenyl ether thiourea derivative has enantiomeric discriminating ability for 2-chloromandelic acid (up to K-R/K-S = 2.80) at 25 degrees C. The enantiomeric recognition abilities for guests are also discussed from a thermodynamic point of view.