engleski

DETERMINATION OF QUINUCLIDINE ESTER ENANTIOMERIC PURITY BY 1H NMR SPECTROSCOPY

It is known that chiral 1, 1`-bis-2-naphthols are useful agents for the NMR determination of enantiomeric purity of some amino alcohols and other compounds [1]. Therefore, we investigated the potential of (R)-1, 1-bis-2-naphthol (RBN) as a chiral solvating agent in the case of prepared tertiary and quaternary quinuclidine esters. We have prepared racemates and enantiomerically pure esters of acetic, pivalic, butyric and benzoic acid as well as their quaternary N-methyl and N-benzyl derivatives. Optical rotation measurements and 1H NMR using RBN as a chiral solvating agent were used to check optical purities of the prepared compounds. Spectra were recorded with a Gemini 300 MHz NMR spectrometer, using CDCl3 as a solvent and adding 5 molar equivalents of RBN. A significant shift of all quinuclidinium protons toward the higher field was observed for all compounds, indicating that these protons are under the influence of the ring current effect of the binaphthyl system. The splitting of the quinuclidinium proton resonances into two signals was more significant in the case of the racemic quaternary derivatives and the magnitude of the non-equivalence was greatest for the splitting of the quinuclidinium 4-H resonance of the quaternary benzoates [2]. Among the other tested esters the splitting of the 4-H was only observed in the N-benzyl derivatives but magnitude of the non-equivalence between enantiomers was much lower indicating the importance of the p-p interactions among the ester and RBN. Proton resonances of 4-H were assigned by the use of 2D 1H COSY spectra. From the NMR spectra of each enantiomer it was determined that the 4-H signal at a higher field belongs to the (S)-enantiomer. [1] F. Toda et al., Bull. Chem. Soc. Jpn. 61 (1988) 4167. [2] I. Primožić et al., Eur. J. Org. Chem. (2003) 295.

quinuclidine esters; 1; 1`-bis-2-naphthols; 1H NMR; enantiomeric purity

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