Cinchona alkaloids as privileged chiral solvating agents for the enantiodiscrimination of N-protected aminoalkanephosphonates—a comparative NMR study

Abstract

A series of chiral amines of increasing structural complexity, such as 1-(1-naphthyl)ethylamine, ephedrine, quinine, quinidine, 9- O- tert-butylcarbamoylquinine, and 9- O- tert-butylcarbamoylquinidine were studied as chiral solvating agents for the enantiodifferentiation of N-benzyloxycarbonyl derivatives of 1-aminoalkanephosphonic and phosphinic acids by means of 31P NMR spectroscopy. Among the cinchona alkaloids, that induced the most significant chemical shift non-equivalences, non-substituted quinidine exhibited the best effectiveness, excellent for analytical purposes (for example Δ δ = 0.348 up to 1.008 ppm for phosphinic acid analogs). The signal separation of the diionic phosphonic acid enantiomers could be further optimized by the addition of an excess of the solvating agents. The newly obtained results appeared better when compared to the data set previously reported for the same analytes with the application of cyclodextrins.