Biocatalyst ketone reduction

In summary, ketoreductases have emerged as valuable catalysts for asymmetric ketone reductions and are preparing to enter the mainstream of synthetic chemistry of chiral alcohols. These biocatalysts are used in three forms wild-type whole-cell microorganism, recombinant... 

Biocatalysis is still an emerging field hence, some transformations are more established than others.Panke et alP have performed a survey of patent applications in the area of biocatalysis granted between the years 2000 and 2004. They found that although hydrolases, which perform hydrolyses and esterifications, still command widespread attention and remain the most utilized class of enzyme (Figure 1.5), significant focus has turned towards the use of biocatalysts with different activities and in particular alcohol dehydrogenases (ADHs) - also known as ketoreductases (KREDs) - used for asymmetric ketone reduction. 

Microbial reduction has been recognized for decades as a laboratory method of preparing alcohols from ketones with exquisite enantioselectivity. The baker s yeast system represents one of the better known examples of biocatalysis, taught on many undergraduate chemistry courses. Numerous other microorganisms also produce the ADH enzymes (KREDs) responsible for asymmetric ketone reduction, and so suitable biocatalysts have traditionally been identified by extensive microbial screening. Homann et have... 

The chemoenzymatic synthesis of chiral alcohols is a field of major interest within biocatalytic asymmetric conversions. A convenient access to secondary highly enan-tiomerically enriched alcohols is the usage of alcohol dehydrogenases (ADHs) (ketoreductases) for the stereoselective reduction of prochiral ketones. Here, as in many other cases in asymmetric catalysis, enzymes are not always only an alternative to chemical possibilities, but are rather complementary. Albeit biocatalysts might sometimes seem to be more environmentally friendly, asymmetric ketone reduction... 

Yeast reductions have provided the synthetic organic chemist with highly versatile methods to prepare chiral alcohols from prochiral ketones of which Saccharomyces cerevisiae (baker s yeast) is the most commonly used biocatalyst. In addition to prochiral ketone reductions, hundreds of... 

Bakers s yeast is the most widely employed biocatalyst for asymmetric ketone reductions [43]. A group at Roussel Uclaf described an industrial application of bakers yeast mediated ketone reduction for the synthesis of trimegestone [44], a progestomimetic compound for the treatment of postmenopausal diseases. The key step of the nine-step synthesis is a chemo-, regio- and stereoselective bakers yeast reduction of the triketone 5 (Scheme 4.9). This transformation could not be performed efficiently using nonenzymatic methods. 

Geotrichum candidum is a well-known catalyst for asymmetric ketone reductions, and different cell-disintegration methods were used to liberate inuacellular CREDs from these cells. Solid shear (bead milling) proved best. Next, various solid supports were examined for the immobilization of the purified enzyme. The CRED was immobilized on silica with an optimized protein loading of 4 mg/g support. Cross-linking vwth glutaraldehyde rendered the preparation more stable and suitable for use in consecutive batches. This immobilized enzyme was found to be a highly efficient biocatalyst formulation for the production of S(-)-l-(10-naphthyl)ethanol [49]. 

Besides wild-type strains, more recently the use of recombinant whole cells has gained increasing popularity for application in asymmetric ketone reduction. When overexpressing the ADFi only, in situ cofactor recycling based on a "substrate-coupled approach" represents a favorite approach as demonstrated in an early contribution by the Itoh group [86] utilizing a recombinant ADH from a Corynebacterium overexpressed in E. coli. This concept has been also applied by Daicel researchers in the presence of an E. coli catalyst with recombinant ADH from Candida parapsilosis. This biocatalyst catalyzes the reduction of p-ketoester 28 at a 36.6 g/1 substrate loading and fimiished the alcohol (R)-29 in 95.2% yield and with 99%ee (Scheme 23.12) [87]. 

Figure 8.7 Reduction of ketone with photosynthetic biocatalyst using lightenergy [6b,c].

Dynamic kinetic resolution of racemic ketones proceeds through asymmetric reduction when the substrate does racemize and the product does not under the applied experimental conditions. Dynamic kinetic resolution of a-alkyl P-keto ester has been performed through enzymatic reduction. One isomer, out of the four possible products for the unselective reduction (Figure 8.38), can be selectively synthesized using biocatalyst, and by changing the biocatalyst or conditions, all of the isomers can be selectively synthesized [29]. 

Indeed, recent research on the use of a cyanobacterium as a biocatalyst has opened up this area asymmetric reduction of ketones by a cyanobacteria, Syne-chococcus elongates PCC 7942, with the aid of light energy proceeded smoothly... 

The reduction of several ketones, which were transformed by the wild-type lyophilized cells of Rhodococcus ruber DSM 44541 with moderate stereoselectivity, was reinvestigated employing lyophilized cells of Escherichia coli containing the overexpressed alcohol dehydrogenase (ADH- A ) from Rhodococcus ruber DSM 44541. The recombinant whole-cell biocatalyst significantly increased the activity and enantioselectivity [41]. For example, the enantiomeric excess of (R)-2-chloro-l-phenylethanol increased from 43 to >99%. This study clearly demonstrated the advantages of the recombinant whole cell biocatalysts over the wild-type whole cells. 

Nakamura, K., Yamanaka, R., Matsuda, T. and Harada, T. (2003) Recent developments in asymmetric reduction of ketones with biocatalysts. Tetrahedron Asymmetry, 14 (18), 2659—2681. 

Carballeira, J.D., Alvarez, E., Campillo,M,etal. (2004)DiplogelasinosporagrovesiilNll 171018, anew whole cell biocatalyst for the stereoselective reduction of ketones. Tetrahedron Asymmetry, 15 (6), 951-962. 

Based on the (/ )-specific ADH from L. kefir, a recombinant E. coli strain was constructed as a whole-cell biocatalyst, and co-expressed GDH was used for regeneration of NADPH [157]. These designer cells were applied for the reduction of 4-fluoroacetophenone to the corresponding optically active (/ )-4-fluorophe-nylethan-l-ol at 0.5 M educt concentration [158]. After a reaction time of 23 h, a conversion of >95% has been achieved, and the purified isolated chiral alcohol showed an ee value of >99% (87% yield). (S)-p-Halohydrins were obtained with this whole-cell catalyst by means of an enantioselective reduction of the corresponding ketones with both high conversions of >95% and enantioselectivities of >99% (Fig. 40). Base-induced cyclization of the [S-halohydrin led to enantiomeri-cally pure (S)-epoxides in high yield and enantiomeric purity (>99% ee) [159]. 

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