Keto enzymatic reduction

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]. 

From the very successful developments of the alcohol dehydrogenase technology for production of secondary alcohols and enzymatic reductive amination of keto-acids for production of amino acids, it is expected that we will also soon see applications for other enzymatic redox chemistries for example, reduction of unsaturated carbonyl compounds with... 

Formation of the identical sugars of the D-series, 6-deoxymannose (rhamnose) and 6-deoxytalose, seems to proceed by a different pathway. According to Winkler and Markowitz (13), GDP-6-deoxy-D-mannose is first converted to GDP-6-deoxy-D-lyxo-4-hexulose. This 4-keto intermediate is the direct precursor for the unspecific enzymatic reduction leading to GDP-6-deoxy-D-mannose and GDP-6-deoxy-D-talose. For a pyridine-nucleotide requiring enzyme, the transformation seems to be unusual because of its lack of stereospecificity. However, closer examination and evaluation of properties of the different 4-keto-intermediate reductases must await availability of more highly purified enzyme preparations. 

M. Kottenhahn, M. Schwarm, and M.-R. Kuia, Enzymatic reduction of a-keto... 

R)-2-Hydroxy-4-phenylbutyric acid was produced continuously in an enzyme membrane reactor by enzymatic reductive animation of the a-keto acid with d-lactate dehydrogenase coupled with formate dehydrogenase (FDH) for regeneration of NADH. Reactor performance data matched a kinetic reactor model (Schmidt, 1992). 

In the use of whole cells severe problems may arise from strain specific activity of intracellular enzymes reacting with the product. For instance, the organism may use the substrate or the product as the carbon source or intracellular esterase activities may influence the yield of hydroxy esters formed by enzymatic reduction of keto esters. These problems may be avoided using isolated enzymes. These potential side reactions also define the purification grade of a technical enzyme sample because the complete separation of the disturbing activities must be ensured. 

ENANTIOSELECTIVE ENZYMATIC REDUCTION OF N,N-DIMETHYL-3-KETO-3-(2-THIENYL)-1-PROPANAMINE... 

In the course of mechanistic investigations covering these enzymatic reductions, labeling experiments were carried out with biologically produced, selectively deuterated NADPH-mole-cules 4-(/ )-[4- H]NADPH and 4-(5)-[4-2H] NADPH [11], The formation of hydroxy derivatives of opposite stereochemistry is caused by the ketoreductase domains KRl and KR2 from the protein DEBS 1 of the erythromycin poly-ketide-synthase. However, both domains have a preference for the 4-pro-(5)-hydride of the NADPH molecule. Probably the binding of the cofactor in KR domains takes place in an identical manner, whereas the individual y9-keto-acylthioester building blocks in the domains KR 1 and KR 2 of DEBS 1 capture a different orientation relative to the cofactor [11]. 

Kula M-R. Enzymatic reduction of a-keto acids leading to L-amino acids, d- or L-hydroxy acids. J. Biotechnol. 1997 53 ... 

Pandit has provided evidence for the Lewis acid catalysis postulated to operate in these reduction reactions. The reduction of various cinnamoylpyridines by 1,4-dihydropyridine derivatives to the corresponding saturated ketones is catalyzed by zinc or magnesium cations. The reduction rate was fastest in the case of 2-cinnamoylpyridine, in which the metal ion can complex simultaneously to both the nitrogen and oxygen sites (Scheme 78). This example is regarded as a model of Lewis acid catalysis of the NADH-dependent enzymatic reduction of A -3-keto steroids. 

In a different approach, instead of using a production enzyme together with an NADH-regenerating enzyme, baker s yeast was used to take over both objectives. Thus 3-keto esters were electrochemically reduced to give the optically active 3-hydroxy esters in the presence of baker s yeast and NAD" " using a viologen as redox catalyst to shuttle the electrons from the cathode to the yeast cells which then catalyze the NADH formation and the enzymatic reduction. In such an approach, usually the permeation in and out of the yeast cells is a limiting factor [54]. 

The selective enzymatic reduction of 2-substituted / -keto esters or /S-diketones is more interesting than that of unsubstituted compounds since two stereogenic centers can be introduced into the molecule in one step. The observation that baker s yeast reduction of these compounds results in predominantly one of the four possible diastereomeric products has been explained by a keto- enol equilibration of the enantiomeric / -dicarbonyls with the simultaneous removal of one of these substrates by an asymmetric reduction49. 

Nonenolizable, prostereogcnic 2,2-disubstituted 1,3-cycloalkanediones are good substrates for microbiological and enzymatic reductions. By analogy to the reductions of 2-substituted /J-keto esters (see Section 2.3.6.1.2.). two stcreogenic centers are introduced into the molecule in the course of the reaction. 

The optically active carbinols 6 (77 85% ee) of the compounds listed in the table above are only obtained in small amounts (3-9% yield). However, the situation changes if the perfluoroalkyl group is an immediate substituent at the carbon-carbon double bond. In this case the keto group is preferably reduced affording the corresponding unsaturated optically active alcohol 7 which is further converted into the saturated optically active alcohol 8 after prolonged incubation 52. Since the carbon-carbon double bond bears, in this case, a substituent other than H, a second center of asymmetry is created during the double-bond reduction. The ratios for the two observed diastereomers (d.r. see table) are a measure of the enantiospecificity of the enzymatic reductions. 

The preparation of 2 by enzymatic reductive amination provides the single enantiomer of the amino acid acetal by a shorter route than the previously published eight-step synthesis of racemic ally sine ethylene acetal. " Unlike the previously described route, the enzymatic route does not reqnire addition and removal of protecting groups, and therefore gives better atom economy. The synthesis of keto acid 1 and enzymatic reductive amination to 2 as described proved to be suitable for the preparation of the large qnantities of the vasopeptidase inhibitor needed for clinical trials. 

Another enzymatic approach to obtain the desired stereochemistry in the side chain is shown in Scheme 17.13. The keto ester precursor 10 of the side chain ethyl ester can be reduced to the hydroxy (2R,3S) ester 11 using either of the yeasts Hansenula polymorpha SC 13865 or Hansemla fabianii SC 13894. Screening a variety of strains from our culture collection revealed many other strains that could carry out the reduction reaction, but the best yields and ee s using whole cells were obtained with the two strains of Hansenula. Of four possible reduction products, the desired product 11 is obtained with 95 to 99% ee and 80 to 90% yield. Because of rapid ketone/enol tautomerism, the enzymatic reduction can work as a dynamic resolution and fix the stereochemistry at both the 2- and 3-positions. 

Protease or lipase enzymes were useful for the regioselective aminoacylation of lobucavir. Lipase was also used for resolution of a synthon for the paclitaxel side chain. The paclitaxel side-chain ester was also prepared by reduction of a keto ester precursor. Enzymatic reduction of ketones to chiral alcohols is another reaction that has been widely applicable. C14-deacylase, ClO-deacety-lase, and C7-xylosidase were identified from microorganisms isolated from soil samples and were useful for converting complex mixtures of taxanes found in yew extracts primarily to 10-deacetyl baccatin III, a precursor for the semisynthesis of paclitaxel and analogs. 

In general, the initial metabolic attack converts the 15-hydroxyl of the prostaglandin into a keto group [240,251,282] (Fig. 9), which considerably reduces the biological activity of the compound [10, 283-287]. This reaction is followed by enzymatic reduction of the A double bond which further reduces the potency of the metabolites. In fact, the products of these two reactions, the 15-keto-13,14-dihydro prostaglandins (Fig. 9), are biologically essentially inactive [10, 283-287]. 

The keto group of methyl acetoacetate may also be reduced selectively with sodium borohydride. Describe how the product of this reaction would differ from the product of the enzymatic reduction of methyl acetoacetate with baker s yeast. 

The first task was to prepare the chiral sulfoxide. The synthesis began with the conversion of methyl propionate (144) to keto-sulfide 145. Enzymatic reduction of the ketone using Baker s Yeast gave 146 with decent enantiose-lectivity. A directed oxidation of the sulfide provided an unequal mixture of sulfoxides 147 and 148 (and presumably minor amounts of material derived from the 4-5% of ent- 46 present in the starting material) from which 148 could be isolated in 50% yield. Dehydration of the alcohol provided 149 (along with some of the Z isomer). Notice that Mori decided to place the alcohol beta to the sulfoxide in the precursor of 149. There might be a number of reasons for this, but one is that it facilitated the elimination reaction (dehydration) because of the electron-withdrawing properties of the sulfoxide. 

Krix, G., Bommarius, A.S., Drauz, K., Kottenhahn, M., Schwann, M., and Kula, M.R. (1997) Enzymatic reduction of a-keto acids leading to L-amino adds, D- or L-hydroxy adds. /. Biotechnol., 53, 29—39. 

Bariotaki, A., Kalaitzakis, D., and Smonou, 1. (2012) Enzymatic reductions for the regio- and stereoselective synthesis of hydroxy-keto esters and dihydroxy esters. Org. Lett., 14,1792-1795. 

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