Enolates continued racemization

For an asymmetric reaction to be really useful enantiomeric excesses typically above 90% are needed and preferably even higher. The recent use of a chiral proton donor attached to polystyrene achieves this and perhaps equally important offers a methodology far superior to the nonsupported analogue [93]. In this work (D)-mandelic acid has been bound to chloromethylated polystyrene via its carboxylic acid. This species was then employed as a proton donor to the silyl enol ether derived from racemic mandelic acid to reform specifically one optical isomer of man-delic acid. Enantiomeric excesses up to 94% have been achieved. The chiral polymer has been recycled satisfactorily and one could speculate that a continuous process could be established converting racemic acid into one pure enantiomer using the corresponding polymer-bound enantiomer as the mediator. 

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