(+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid and its ytterbium(III) complex as chiral NMR discriminating agents

The compound (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (I) and its yt terbium(III) complex are evaluated as chiral NMR discriminating agents. The crown ether is a useful chiral discriminating agent for protonated amino a cid esters, amines, and amino alcohols. The crown can also be used with neu tral primary amines since amines are protonated through a neutralization re action with a carboxylic acid moiety of the crown. Enantiodiscrimination wi th the crown is observed in methanol and acetonitrile. Addition of ytterbiu m(III) nitrate to crown-substrate mixtures causes upfield shifts in the NMR spectrum of the substrate and often enhances the enantiomeric discriminati on. Evidence indicates that the ytterbium(III) bonds to the carboxylic acid moieties of the crown, but enhancements in enantiomeric discrimination res ult from either the different association constants of the enantiomers with the crown or diastereomeric nature of the resulting crown-substrate comple xes. The ytterbium complex with the crown is suitable for use in methanol b ut precipitates in acetonitrile.