Abstract
preparative gas chromatography even on optically impure chiral stationary phases, the enantiomer eluting as the second fraction may be contaminated with the preceding eluent still present in the post-column flow system. The second eluting enantiomer may be reverted to the first fraction by using the stationary phase of opposite chirality. It is beneficial in this respect that carvone as the chiral recognition principle in 2 is commercially available in both antipodal forms with high enantiomeric purity from natural sources. According to Figs. 2 and 3, the parent compound 1 a as well as the three enantiomeric pairs of the 2.7-dimethyl-l.6dioxaspiro[4.4]nonanes l d, l e and i f are also quantitatively separated employing the same column. The small "p la teau" which appears between the enantiomeric fractions o f l a is not due to incomplete enantiomer separation but is caused by the inherent configurational lability (leading to enantiomerization) of spiroketals [7]. Fortunately, the semi-preparative enantiomer separation of 1 can be performed at low temperature (45 ~ where enantiomerization is minimized.
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