The Study Of Asymmetric Reduction Of Prochiral Aromatic Ketone

The biocatalysis and chiral synthesis are attracting more and more attention nowadays because of the escalating concerns on different pharmacological effects of different optical isomers and the limitations on the registration and application of racemic drugs. Synthesis methods with high product yield and optical purity, especially biotransformation has become increasingly popular due to its advantages such as mild reaction condition and high selectivity.Chiral aromatic alcohols were important building blocks for chiral pharmaceuticals. The asymmetric reduction of 4'-chloroacetophenone to its corresponding chiral alcohol in the aqueous phase was chosen as the model reaction in this study. Candida krusei SW 2026, a strain selected from our strain collection showed high enantioselectivity towards a series of halogenated aromatic ketones. The optimum fermentation conditions were: 50 g/L glycerol, 20 g/L corn steep liquor, 4 g/L KH2PO4, 1.5 g/L MgSO4·7H2O, initial pH of 6.0, 30 oC, rotation speed of 200 r/min, and fermentation time of 48 h. After the optimization, the yield of (S)-4'-chloro-α-phenylethyl alcohol reached 88.56 %, and the e.e. value was around 87 %.The optimum reactions were: 200 g(wet cell weight)/L, pH of 6.0, temperature 30℃. 5 % glucose as co-substrate, and 5 g/L substrate concentration. When Candida crusei SW 2026 cells were pretreated by 2 % CTAB for 20 min at 4℃, the reaction was finished at 16 h, which was 8 h earlier than cells without CTAB treatment. Also, our study showed that product e.e. value was increased from 85.07 % to 97.19 % when 3 g/L allyl alcohol was added as selective inhibitor into the reaction system, along with the decrease in yield from 83.67 % to 52.53 %.The asymmetric reduction of another aromatic ketone substrate with diaryl in the chemical structure, (4-chlorophenyl)-(pyrid-2-yl)-methanone, was also investigated in this work. A HPLC method was established to determine the concentration and e.e. value of the product: HPLC (Dionex P680 ), column: Daicel Chiralcel OB-H column(5μm, 250mm×4.6mm), mobile phase hexane/iso-propanol(95:5, V/V), flow rate 0.4 mL/min, column temperature 38℃, sample size 5μL, detection wave length 254 nm.Our screening result showed that Candida albicans SW 2022 was able to reduce (4-chlorophenyl)-(pyrid-2- yl)-methanone to its corresponding alcohol. The reaction conditions were optimized as follows: temperature 30℃, pH 6.0, rotation speed 180 r/min. In addition, 5 % glucose was added as cosubstrate, and DMSO, a cosolvent, was added to enhance the efficiency of the reaction. In a 50 mL reaction system containing 6 g wet cells, 0.25 g substrate, the yield was improved from 85.32 % to 97.75 % when 0.25 g DMSO was added after 48 h of reaction. when 2 g/L allyl alcohol was added as selective inhibitor, a slight increase in e.e. value to 35 % was observed with compromised yield. When 10 mmol/L Na2SO3 was added to reduce dissolved oxygen, e.e. value was increased to 35%, while the yield remained at around 96 %.