Enantioselective recognition of a pair of novel receptors for amino acid zwitterions

A pair of artificial enantiotopic receptors 1a, 1b composed of (S,S) or (R,R) chiral bicyclic guanidinium, azacrown ether and (t-butyldiphenylsilyloxy)methyl group for amino acid zwitterions have been synthesized. The liquid–liquid competition extraction experiments and 1H NMR studies indicate that the receptor 1a with (S,S) configuration and 1b with (R,R) configuration selectively recognize l- and d-aromatic amino acids, respectively.