Kinetic resolutions concentrate the minor enantiomer and aid measurement of high enantiomeric purity.

Abstract

Although many methods can measure enantiomeric purity, only a few can measure high enantiomeric purity, >98% ee, because in most methods the signal for the major enantiomer overwhelms the signal for the minor enantiomer. We use a kinetic resolution to concentrate the minor enantiomer into the product and thereby extend the ability of all existing techniques to measure high enantiomeric purity. The original enantiomeric purity is calculated using the enantioselectivity of the kinetic resolution and the extent of conversion. We verified this method with samples of (1S)-menthol using an acetylation with vinyl acetate catalyzed by lipase from Candida rugosa. The enantiomeric purities determined by capillary gas chromatography directly and after kinetic resolution agreed for samples with 90–99.9% ee. Error analysis suggests that the usual accuracies for conversion and enantiomeric ratio are sufficient for accurate determination of enantiomeric purity with this method. In another example, we used a kinetic resolution followed by a simple optical rotation measurement to accurately quantify 98.5% ee for a commercial sample of (S)-(+)-6-methoxy-α-methyl-2-naphthaleneacetic acid (naproxen). Thus, this kinetic resolution method allows simple techniques such as optical rotation to measure high enantiomeric purity.