Highly Enantioselective Reduction of Ethyl 2-Acyloxy-3-oxobutanoate with Immobilized Baker’s Yeast

Reduction of ethyl 2-acetoxy-3-oxobutanoate (4a) with immobilized baker’s yeast in calcium alginate gel gave a 18 : 19 : 63 mixture of ethyl (2R,3S)- and (2S,3S)-2-acetoxy-3-hydroxybutanoates and ethyl (2S,3S)-2,3-dihydroxybutanoate each with >95% e.e. in 58% yield. A similar treatment of a 2-benzoyloxy analog of 4a afforded a 6 : 94 mixture of ethyl (2R,3S)- and (2S,3S)-2-benzoyloxy-3-hydroxybutanoates (>95% e.e.) in 70% yield. Their absolute configurations were determined by comparison with authentic (2R,3S)- and (2S,3S)-2,3-dihydroxybutanoic acids. Effects of the immobilization and the pH of culture solution on the product ratio are also discussed.