Enantioseparation of 3α-acyloxy-6β-acetoxyltropane compounds with Chiralpak AD and Chiralcel OD-H chiral stationary phases

Abstract

Six 3α-acyloxy-6β-acetoxyltropane compounds were enantioseparated by high performance liquid chromatography with amylose-based chiral stationary phase Chiralpak AD and cellulose-based chiral stationary phase Chiralcel OD-H in the normal phase mode, using various mixtures of n-hexane-isopropanol as mobile phases. The enantiomers 6 were completely separated on a Chiralpak AD column. While the enantiomers 1, 4 and 3 got complete, baseline and basic separation respectively on a Chiralcel OD-H column. However, the enantiomers 6 were partially separated on the Chiralcel OD-H column and enantiomers 1 could not be separated on the Chiralpak AD column. This indicated that the cave structure of chiral stationary phase exerted great effect on the resolutions. The enantiomers 5 could not be separated on both of the chiral stationary phases. The main possible mechanism of chiral resolution involves in spatial adaptability and molecular interactions between chiral stationary phases and compounds. The substituents in C-3α position of 3α-acyloxy-6β-acetoxyltropane compounds play an important role in spatial adaptability. And it was suggested that the steric hindrance effect of the substituent in C-3α position was the key factor of determining the selective recognition of chiral stationary phase to the enantiomers of 3α-acyloxy-6β-acetoxyltropane compounds. Besides, the molecular interaction, such as π-π interaction, also exerts great influence to the chiral resolution. This study provides a reference for the enantioseparation of many other tropane derivatives.