Chiral nitrogen-containing calix[4]crown-an excellent receptor for chiral recognition of mandelic acid

A chiral nitrogen-containing calix[4]crown 2 bearing optically pure 1,2-diphenyl-1,2-oxyamino residue at lower rim showed excellent chiral recognition between enantiomers of mandelic acid. Using competitive 1H NMR titration the ratio of association constants of (S)- and (R)-mandelic acid with the chiral calix[4]crown was determined to be 102, that is 98% de, which is the best result obtained from artificial receptors for the chiral recognition of mandelic acid up to now.