Bioreduction of PEG-acetoacetate 1 is achieved using dry baker's yeast in toluene with a small amount of added water. After detachment from the polymer under basic conditions, the corresponding (S)-(+)-3-hydroxybutanoic acid 3 was esterified with Me3SiCl and ethanol and ethyl (S)-(+)-3-hydroxybutanoate 4 was isolated with 97% e.e. and 70% overall yield.
