Reaction Characteristics of Asymmetric Synthesis of （2S,5S）-2,5- Hexanediol Catalyzed with Baker＇s Yeast Number 6

Baker’s yeast number 6 was selected by screening. It showed good catalytic activity and enantioselec-tivity for asymmetric reduction of 2,5-hexanedione to produce （2S,5S）-2,5-hexanediol. Gas chromatography-mass spectrometry （GC-MS） revealed that the intermediate was （S）-5-hydroxyhexane-2-one. Reduction of 2,5-hexanedione proceeded in a two-step reaction. The hydroxyketone was initially formed, and this intermediate was further re-duced to the diol. Factors influencing the product yield and the enantiomeric excess of the reduction of 2,5-hexandione catalyzed by baker’s yeast number 6 were investigated. Higher concentration （≤100 mmol·L-1） of 2,5-hexandione did not influence 5-hydroxyhexane-2-one production, but 2,5-hexanediol production was inhibited by excess accumulation （〉30 mmol·L-1） of intermediate. The optimal conditions were glucose as the co-substrate at an initial glucose concentration of 20 g·L-1, 34°C, pH 7.0 and cell concentration 60 g·L-1 （cell dry mass）. Under the optimal condition and an initial substrate concentration of 30 mmol·L-1, the yield of 2,5-hexandiol was 78.7% and the enantiomeric excess of （2S,5S）-2,5-hexandiol was 94.4% for 24-h reduction.