Asymmetric Reduction and Oxidation of Aromatic Ketones and Alcohols Using W110A Secondary Alcohol Dehydrogenase from Thermoanaerobacter ethanolicus

Abstract

An enantioselective asymmetric reduction of phenyl ring-containing prochiral ketones to yield the corresponding optically active secondary alcohols was achieved with W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus (W110A TESADH) in Tris buffer using 2-propanol (30%, v/v) as cosolvent and cosubstrate. This concentration of 2-propanol was crucial not only to enhance the solubility of hydrophobic phenyl ring-containing substrates in the aqueous reaction medium, but also to shift the equilibrium in the reduction direction. The resulting alcohols have S-configuration, in agreement with Prelog's rule, in which the nicotinamide-adenine dinucleotide phosphate (NADPH) cofactor transfers its pro-R hydride to the re face of the ketone. A series of phenyl ring-containing ketones, such as 4-phenyl-2-butanone (1a) and 1-phenyl-1,3-butadione (2a), were reduced with good to excellent yields and high enantioselectivities. On the other hand, 1-phenyl-2-propanone (7a) was reduced with lower ee than 2-butanone derivatives. (R)-Alcohols, the anti-Prelog products, were obtained by enantiospecific oxidation of (S)-alcohols through oxidative kinetic resolution of the rac-alcohols using W110A TESADH in Tris buffer/acetone (90:10, v/v).

Exploiting Enzymatic Dynamic Reductive Kinetic Resolution (DYRKR) in Stereocontrolled Synthesis

“…Thermoanaerobacter ethanolicus secondary ADH (TeSADH) had previously shown synthetic utility due to its ability to stereoselectively operate at elevated temperatures and in an organic solvent-rich milieu (30% v/v). [31] The W110A mutant not only reversed the enantiopreference of Thermoanerobacter ethanolicus ADH (TeSADH), but it also opened up the large pocket of the active site to accommodate bulkier aryl substitutents. Furthermore, this W110A mutant demonstrated the ability to catalyze the DYRKR of α-chloroketones with good diastereo- and excellent enantioselectivity as shown in Scheme 6.…”

Update 1 of: Enantioselective Enzymatic Desymmetrizations in Organic Synthesis

“…The results obtained did confirm this hypothesis since the mutant enzyme was able to reduce a series of such ketones related to the 2-butanone and acetone cores, thus yielding the corresponding Prelog alcohols in high yields and ee's. 155 Moreover, immobilization of this mutant in a xerogel-encapsulated form confers it with a much higher tolerance to organic solvents. 156 Finally, and by following again a size rationale, the I86A mutant also allowed the acceptance of acetophenone derivatives with an inversion of the stereopreference of the enzyme, the anti-Prelog alcohols being now obtained in excellent optical purities.…”

Biocatalysed concurrent production of enantioenriched compounds through parallel interconnected kinetic asymmetric transformations

“…The mixture was shaken at 20ºC 75 and 250 rpm for 6 h. Then, the reaction was stopped by extraction with ethyl acetate (5 x 5 mL) and the organic layer was dried over Na 2 SO 4 . Afterwards, the organic solvents and remanent ketone 4a were removed under reduced pressure and the crude residue was purified using flash chromatography 80 (hexane:CH 2 Cl 2 1:1) obtaining enantiopure (R)-1a (10.1 mg, 81% yield), (S)-2a (11.7 mg, 86% yield), and (R)-3a (7.0 mg, 75% yield). Chemical purity of reaction products was determined by both GC and NMR.…”

“…Since two different catalysts were used, parameters like pH, temperature or cofactor preference must be optimised to find compatible reaction conditions. Another criterion to take into account is that each substrate 80 should be converted by only one of the biocatalysts, since an undesired side reaction would yield a by-product, and more importantly, an incomplete process as result of ineffective cofactor recycling (see below). As previously described, BVMOs catalyse irreversible 85 oxidation processes at the expense of NADPH, 17 while ADHs utilize NAD + and/or NADP + .…”