Divergent RRM Using a Single Chiral Reagent Ketone Reduction

Divergent RRM Using a Single Chiral Reagent Ketone Reduction 

Guette and Horeau were able to derive a mathematical expression that relates the molar fraction of each product to the observed enantiomeric purities. The expression shown in Equation 6.1 is based on a more recent version developed by Kagan [7, 9] that apphes to the case where a racemic substance is partially converted into two distinct products (corresponding to diastereomers 12 and 13 in the camphor reduction). The molar fractions Xj (unreacted starting material), 

For incomplete reactions, the enantiomeric purity of the exo- and endo-alcohols 12 and 13 can vary with percentage (%) conversion because one enantiomer of racemic camphor (+/-)-ll may react faster (simple KR) this situation is described in Equation 6.1. However, the ratio of 12 13 at 100% conversion of racemic 11 is independent of KR. This ratio is determined by the inherent diastereoselectivity of the chiral reagent towards each enantiomer and does not rely on rate differences between the enantiomers of 11. The single-reagent divergent RRM experiment thus differs from the two-reagent experiment (PKR) because the enantioselectivity in PKR is determined by the rate difference between enantiomers in two distinct reactions. This issue will be revisited later as part of the broader discussion of PKR, 

The reported 100% optical purity for both of the alcohols 24 and 25 was based on NMR assay methods, so it may be prudent to assume a lower limit in the range of 95% ee. Nevertheless, it is clear that highly enriched products were obtained and separated. Furthermore, Davies and Jones were able to demonstrate an application to the synthesis of chiral secondary alcohols. Thus, desulfurization of 24 and 25 afforded enantiomerically pure 3-hexanol in good yield. 

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Divergent RRM Using a Single Chiral Reagent Ketone Reduction 227 OH OH... 

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