Asymmetric parallel kinetic resolutions

Parallel kinetic resolution is a developing area of asymmetric synthesis which facilitates the simultaneous conversion of both enantiomers of a racemic mixture into useful products. Pedersen and co-workers have recently reported the first Horner-Wadsworth-Emmons procedure that allows the parallel kinetic resolution of racemic aldehydes. 

Asymmetric carbonyl olefinations through parallel kinetic resolution. 

For example Vedejs, E., Daugulis, O., and Diver, S. T., 1996, Enantioselective Acylations Catalyzed by Chiral Phosphines. J. Org. Chem. 61 430-431 Vedejs, E., and Rozners, E., 2001, Parallel Kinetic Resolution under Catalytic Conditions A Three-Phase System Allows Selective Reagent Activation Using Two Catalysts. J. Am. Chem. Soc. 123 2428-2429 Qiao, S., and Fu, G. C, 1998, The First Application of a Planar-Chiral Phosphorus Heterocycle in Asymmetric Catalysis Enantioselective Hydrogenation of Dehydroamino Acids, y. Org. Chem. 63 4168-4169. 

The epoxidation of alkenylsilanols parallels that of allylic alcohols in exhibiting good enantioselectivities339. Kinetic resolution of the alkenylsilanol 213 by the Sharpless asymmetric epoxidation has been accomplished, with the rate difference for the oxidation of the enantiomers of 213 being unusually high (>11)340. 

Figure 5.25. Dynamic kinetic resolution for parallel -consecutive asymmetric catalytic reactions.

Recently, a cascade process for the simultaneous preparation of two enantiopure secondary alcohols by the same ADH was investigated [12]. In this work, a kinetic oxidative resolution of different secondary alcohols was coupled with the irreversible asymmetric reduction of selected prochiral activated ketones, that is, a-chloro ketones (Scheme 11.5a). The proposed strategy, named PIKAT (parallel intercoimected kinetic asymmetric transformations), represents an example of redox neutral (or self-sufficient) cascade, with no additional reducing or oxidizing reagents being required. Moreover, the reaction was catalyzed by a single enzyme in the presence of catalytic amounts of the cofactor. As the outcome of the cascade process is a mixture of two different enantioenriched products, substrates were properly selected on the basis of different physical properties. 

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