Lanthanide-Crown Ether Mixtures as Chiral NMR Shift Reagents for Amino Acid Esters, Amines and Amino Alcohols

Abstract

Two chiral crown ethers, 2,3:4,5-bis[1,2-(3-phenylnaphtho)-1,6,9,12,15,18-hexaoxacycloeicosa-2,4-diene and 1,2:5,6-di-O-isopropylidene-3,4-[(tert-butylbenzenediyl)bis(oxyethoxy)ethyl-D-mannitol, were evaluated as organic-soluble chiral NMR resolving agents. The crown ethers are useful resolving agents for protonated amino acid esters, amines and amino alcohols. Enantiodiscrimination with the crown ethers is better in methanol than in acetonitrile or chloroform. Organic-soluble lanthanide tetrakis(β-diketonate) anions, which are formed in solution by mixing a lanthanide tris(β-diketonate) and silver β-diketonate, can be added to crown–substrate mixtures to enhance the enantiomeric resolution. The lanthanide-induced enhancement occurs because the enantiomers of the substrate have different association constants with the crown ethers. The enantiomer that associates less favorably with the crown ether is more available to complex with the lanthanide. © 1997 by John Wiley & Sons, Ltd.