Synthesis of new diaza-18-crown-6 ethers derived from trans-(R,R)-1,2-diaminocyclohexane and investigation of their enantiomeric discrimination ability with amino acid ester salts

The synthesis of four diaza-18-crown-6 ethers with C2-symmetry derived from trans-(R,R)-1,2-diaminocyclohexane bearing methyl, phenyl and phenoxymethyl moeities attached to a stereogenic centre on the crown ring were achieved. Enantiomeric discrimination of these macrocycles against amino acid methyl ester salts was examined by 1H NMR titration method. They exhibit strong binding ability and some of them show a very high enantioselectivity towards amino acid esters, corresponding to 5.37 kJ/mol of binding energy difference in CDCl3 at 25 °C. Computational modelling showed parallel results with experimental calculations, thus providing a detailed understanding of molecular recognition mode and binding sites between the hosts and the guests.