Asymmetric Catalysis and Biotransformation of 3'-(Trifluoromethyl)acetophenone for Chiral Synthesis

Asymmetric catalytic addition of ethyl groups to 3'-(Trifluoromethyl)acetophenone catalyzed by ligands derived from trans-1,2-diaminocyclohexane and camphor sulfonyl chloride has been reported. Phenylation of 3'-(Trifluoromethyl)acetophenone in the presence of dihydroxy bis(sulfonamide) ligand (enantioselective catalyst), titanium tetraisopropoxide and diphenylzinc has been investigated. 3'-(Trifluoromethyl)acetophenone has been used in a key step during the preparation of a commercial fungicide.

Effective asymmetric preparation of (R)-1-[3-(trifluoromethyl)phenyl]ethanol

(R)-1-[3-(Trifluoromethyl)phenyl]ethanol ((R)-MTF-PEL) is a key chiral building block for the preparation of neuroprotective compounds like (R)-3-(1-(3-(trifluoromethyl)phenyl)ethoxy)azetidine-1-carboxamide. A chemical method for (R)-MTF-PEL production using [Mn(CO)2(1)]Br as catalyst was reported, with 99% yield and 97% ee value under 0.5 mM 3'-(trifluoromethyl)acetophenone concentration. A metal catalyst was involved in this chemical approach. For (R)-MTF-PEL production, the biocatalytic efficiency is still unsatisfactory. Thus, it is necessary to develop a more efficient whole-cell biotransformation process for its production at high substrate concentration. The recombinant E. coli BL21(DE3)-pET28a(+)-LXCAR-S154Y was acquired in our previous work, which is a recombinant carbonyl reductase variant (LXCAR-S154Y) derived from Leifsoniaxyli HS0904. The cultivation conditions of the recombinant E. coli variant were similar to the literature. After the cultivation, the incubated cells were collected by the centrifugation at 4 ℃ and 9000 rpm, and then used for the 3'-(trifluoromethyl)acetophenone bioreduction. Furthermore, some isolated strains stored in our laboratories were also cultivated by shaken culture and evaluated their abilities for the bioreduction of 3'-(trifluoromethyl)acetophenone. The bioreduction of it was undertaken at 30 °C and 200 rpm in 50 mL Erlenmeyer flask, which containing a certain amount of 3'-(trifluoromethyl)acetophenone, co-substrate and microbial whole cells, either in the PBS buffer system or in a surfactant and NADES containing medium.[1]

The concentrations of the residual 3'-(trifluoromethyl)acetophenone and the resultant (R)-MTF-PEL were quantified by GC (Agilent GC 7820A) with a chiral CP-Chirasil-Dex CB column. Injector and detector temperatures were set as 250 ℃. The initial column temperature was 115 ℃, kept for 2 min, and then increased to 140 ℃ at a rate of 3 ℃/min. The retention times of 3'-(trifluoromethyl)acetophenone, n-dodecane, (R)-MTF-PEL and (S)-MTF-PEL were 2.64 min, 3.94 min, 6.35 min and 6.92 min, respectively. After the completion of the bioreduction, the reaction mixture was extracted twice with an equal volume of EtOAc, and the resultant organic phase was subjected to gas chromatography (GC) analysis. Each trial was performed in triplicate. Details about 3'-(trifluoromethyl)acetophenone concentration, co-substrate concentration, surfactant and NADES content, buffer pH, reaction temperature, cell concentration and reaction time were specified for each case. To calculate the product yield and ee value, the calibration curves of 3'-(trifluoromethyl)acetophenone and (R)-MTF-PEL were firstly established using the corresponding standard of substrate or product with n-dodecane as an internal standard.

In general, the low solubility of hydrophobic substrates decreased the biocatalytic yield owing to the mass-transfer hindrance. The solubility of 3'-(trifluoromethyl)acetophenone was 425.5 mg/L in PBS buffer solution, which hindered the efficient production of (R)-MTF-PEL. To improve the substrate solubility in aqueous buffer system and increase the reaction efficiency, nine surfactants were introduced individually to the reaction medium and their performances were evaluated. To understand the influence of Tween-20 on the reaction, the solubility of 3'-(trifluoromethyl)acetophenone in different media was further examined. The addition of ChCl:Lys (1:1) in the cell-free extract of recombinant E. coli cells can efficiently boost NADH regeneration, which confirmed that ChCl:Lys (1:1) plays a role in promoting cofactor regeneration during 3'-(trifluoromethyl)acetophenone bioreduction catalyzed by recombinant E. coli cells. To further investigate the combined effect of Tween-20 and ChCl:Lys on the reaction, the asymmetric reduction of 3'-(trifluoromethyl)acetophenone were conducted in different reaction system. Although the recombinant E. coli LXCAR-S154Y reduced 3'-(trifluoromethyl)acetophenone with ee > 99.9%, the product yield was only 26.9%. Thus, it was necessary to improve the catalytic system to boost the yield. It was reported that carbohydrates and alcohols as co-substrates can promote coenzyme regeneration, and play an important role in redox biocatalysis.

References

[1]Zhuang W, Liu H, Zhang Y, He J, Wang P. Effective asymmetric preparation of (R)-1-[3-(trifluoromethyl)phenyl]ethanol with recombinant E. coli whole cells in an aqueous Tween-20/natural deep eutectic solvent solution. AMB Express. 2021 Aug 19;11(1):118. doi: 10.1186/s13568-021-01278-6. PMID: 34410519; PMCID: PMC8377109.