Enantiomeric high-performance liquid chromatography resolution and absolute configuration of 6β-benzoyloxy-3α-tropanol

The absolute configuration of the naturally occurring isomers of 6β-benzoyloxy-3α-tropanol (1) has been established by the combined use of chiral high-performance liquid chromatography with electronic circular dichroism detection and optical rotation detection. For this purpose (±)-1, prepared in two steps from racemic 6-hydroxytropinone (4), was subjected to chiral high-performance liquid chromatography with electronic circular dichroism and optical rotation detection allowing the online measurement of both chiroptical properties for each enantiomer, which in turn were compared with the corresponding values obtained from density functional theory calculations. In an independent approach, preparative high-performance liquid chromatography separation using an automatic fraction collector, yielded an enantiopure sample of OR(+)-1 whose vibrational circular dichroism spectrum allowed its absolute configuration assignment when the bands in the 1100–950 cm-1 region were compared with those of the enantiomers of esters derived from 3α,6β-tropanediol. In addition, an enantiomerically enriched sample of 4, instead of OR(±)-4, was used for the same transformation sequence, whose high-performance liquid chromatography follow-up allowed their spectroscopic correlation. All evidences lead to the OR(+)-(1S,3R,5S,6R) and OR(−)-(1R,3S,5R,6S) absolute configurations, from where it follows that samples of 1 isolated from Knightia strobilina and Erythroxylum zambesiacum have the OR(+)-(1S,3R,5S,6R) absolute configuration, while the sample obtained from E. rotundifolium has the OR(−)-(1R,3S,5R,6S) absolute configuration.