Reductions of keto esters with baker's yeast in organic solvents - a comparison with the results in water

A Novel Baker’s Yeast-Mediated Microwave-Induced Reduction of Racemic 3-Keto-2-Azetidinones: Facile Entry to Optically Active Hydroxy β-Lactam Derivatives

Objective:

Microwave technology, together with enzymatic catalysis is a nature-friendly chemical synthesis method with low wastage of solvent and good yield of the products.

Methods:

Enzymes from various microorganisms can be used in the biochemical processes of a wide range of compounds assisted by microwave irradiation

Results:

In this work, the microwave-induced reaction of α-keto β-lactams by Baker's yeast in organic solvent was conducted to afford optically active cis and trans-α-hydroxy-β-lactams for the first time.

Conclusion:

These hydroxy compounds are the precursors of numerous natural products of medicinal significances.

Chiral Vanadyl(V) Complexes Enable Efficient Asymmetric Reduction of β-Ketoamides: Application toward (S)-Duloxetine

High-valent chiral oxidovanadium(V) complexes derived from 3,5-substituted-N-salicylidene-l-tert-leucine were used as catalysts in asymmetric reduction of N-benzyl-β-ketoamides. Among six different solvents, three different alcohol additives, and two different boranes examined, the use of pinacolborane in tetrahydrofuran (THF) with a t-BuOH additive led to the best results at -20 °C. The corresponding β-hydroxyamides can be furnished with yields up to 92% and an enantiomeric excess (ee) up to 99%. We have successfully extended this catalytic protocol for the synthesis of an (S)-duloxetine precursor.

Baker’s Yeast-Based Organocatalysis: Applications in Organic Synthesis

Background:

Catalyst speeds up any chemical reaction without changing the point of the equilibrium. Catalysis process plays a key role in organic synthesis to produce new organic compounds. Similarly, organocatalysis is a type of chemical catalysis in which the rate of a reaction is accelerated by organic catalysts.

Methods:

Organocatalysts have gained significant utility in organic reactions due to their less of sensitivity towards moisture, readily available, economic, large chiral pool and low toxicity as compared to metal catalysts. Organocatalysts work via both formations of covalent bonds such as enamine and iminium catalysis as well as through non-covalent interactions such as in hydrogen bonding. For example, Bakers’ yeast based organocatalysis is widely used in various organic transformations.

Results:

Baker’s yeast is a fermentation product and used mainly in the preparation of bread dough. It is produced by aerobic fermentation of yeast strain Saccharomyces cerevisiae. Baker's yeast consists of enzymes which can reduce a carbonyl group into a hydroxyl group with high yield and thereby making it suitable for biotransformations in organic synthesis.

Conclusion:

Baker's yeast is widely used as a biocatalyst in various organic reactions such as oxidation, reduction, condensation, hydrolysis, cyclization, etc. because it is readily available, inexpensive and easy to handle.

Enantio- & chemo-selective preparation of enantiomerically enriched aliphatic nitro alcohols using Candida parapsilosis ATCC 7330

Enantiomerically pure β- and γ-nitro alcohols were prepared from their respective nitro ketones by asymmetric reduction using Candida parapsilosis ATCC 7330 under optimized reaction conditions (ee up to >99%; yields up to 76%).

Synthesis of chiral cyanohydrins by recombinant Escherichia coli cells in a micro-aqueous reaction system

Synthesis of chiral cyanohydrins is performed in a monophasic micro-aqueous reaction system using whole recombinant Escherichia coli cells expressing the Arabidopsis thaliana hydroxynitrile lyase (AtHNL). Microscopy studies employing a fusion of AtHNL with a flavin-based fluorescent protein (FbFP) reveal that the cells remain intact in the reaction system.

Enhancing the biocatalytic potential of carbonyl reductase of Candida viswanathii using aqueous-organic solvent system

In the present work, the toxic effect of various solvents with different Log P values was studied on the whole cells of Candida viswanathii. Experiments showed that the lower concentrations of some solvent increased both the activity retention and enzyme activity as compared to the control while this was not the case with higher concentrations of the same solvents. The model compound taken in the present study was 1-acetophenone. The percentage conversion improved from 76 to 94%. Addition of 2-propanol increased the substrate tolerance, giving the conversion of 90% compared to 9% in control at a substrate concentration of 70 mM in 1h. The operational stability increased at higher temperatures with the addition of 2-propanol in the reaction mixture with good conversion (90%) and enantiomeric excess (>99%) at 45 degrees C and 50 degrees C. The effect was also found to be prominent in other tested substrates. In order to further stabilize the cells for long term use in higher concentration of organic solvents, the cells were further immobilized, and were found to have higher activity retention than that of free cells.

Integration of enzyme, strain and reaction engineering to overcome limitations of baker's yeast in the asymmetric reduction of alpha-keto esters

We report on the development of a whole-cell biocatalytic system based on the popular host Saccharomyces cerevisiae that shows programmable performance and good atom economy in the reduction of alpha-keto ester substrates. The NADPH-dependent yeast reductase background was suppressed through the combined effects of overexpression of a biosynthetic NADH-active reductase (xylose reductase from Candida tenuis) to the highest possible level and the use of anaerobic reaction conditions in the presence of an ethanol co-substrate where mainly NADH is recycled. The presented multi-level engineering approach leads to significant improvements in product optical purity along with increases in the efficiency of alpha-keto ester reduction and co-substrate yield (molar ratio of formed alpha-hydroxy ester to consumed ethanol). The corresponding alpha-hydroxy esters were obtained in useful yields (>50%) with purities of > or =99.4% enantiomeric excess. The obtained co-substrate yield reached values of greater than 1.0 with acetate as the only by-product formed.

Application of comparative proteome analysis to reveal influence of cultivation conditions on asymmetric bioreduction of beta-keto ester by Saccharomyces cerevisiae

Industrial bakers' yeast strain Saccharomyces cerevisiae LH1 was selected for asymmetric reduction of ethyl benzoylacetate to (S)-ethyl 3-hydroxy-3-phenylpropionate. Higher reductive efficiency and higher cofactor availability were obtained with the alternation of cultivation condition (mainly growth medium). Compared to the bioreduction by yeast cells grown in malt extract (ME) medium, the concentration of substrate was increased 25-fold (up to 15.6 g/l) in the yeast peptone dextrose (YPD)-grown cells mediated bioreduction with 97.5% of enantioselective excess of (S)-product. The proteomic responses of S. cerevisiae LH1 cells to growth in aerobic batch cultures fed with either YPD or ME medium were examined and compared. Among the relative quantities of 550 protein spots in each gel, changes were shown in the expression level of 102 intracellular proteins when comparing YPD gel to ME gel. Most of the identified proteins were involved in energy metabolism and several cellular molecular biosynthetic pathway and catabolism. For YPD-grown yeast cells, not only enzymes involved in nicotinamide adenine dinucleotide phosphate regeneration, especially 6-phosphogluconate dehydrogenase, but also alcohol dehydrogenase 1 and D: -arabinose 1-dehydrogenase which had been demonstrated activity toward ethyl benzoylacetate to (S)-hydroxy ester were significantly upregulated. These changes provided us insight in the way the yeast cells adapted to a change in cultivation medium and regulated its catalytic efficiency in the bioreduction.

Synthesis of chiral α-hydroxy amides by two sequential enzymatic catalyzed reactions

Enantiomerically pure alpha-hydroxy amides have been prepared from the corresponding alpha-oxo esters by the use of a double sequence reaction involving in a first step the highly enantioselective Saccharomyces cerevisiae bioreduction and then in a second step, the resulting alpha-hydroxy esters followed a non-enantiospecific lipase catalyzed aminolysis with n-butylamine reaction. In the first non-organic solvent process, the moistened baker's yeast reduced seven alpha-oxo esters with high conversions degree (93% for one substrate and >99% for the others) and high enantioselectivities [>99% for all the substrates except for ketopantoyl lactone, which gave 88% of enantiomeric excess (ee)]. At the same way, the isolated resulting chiral alpha-hydroxy esters were subjected to the second Candida antarctica lipase fraction B (CAL-B) catalyzed aminolysis in dioxane conducting to the corresponding chiral alpha-hydroxy amides with high conversions degree, between 88 and 99%. Both processes were carried out at 28-30 degrees C.

Strain engineering for stereoselective bioreduction of dicarbonyl compounds by yeast reductases

Pure chiral molecules are needed in the pharmaceutical and chemical industry as intermediates for the production of drugs or fine chemicals. Microorganisms represent an attractive alternative to chemical synthesis since they have the potential to generate single stereoisomers in high enantiomeric excess (ee). The baker's yeast Saccharomyces cerevisiae can notably reduce dicarbonyl compounds (in particular alpha- and beta-diketones and keto esters) to chiral alcohols with high ee. However, products are formed at a low rate. Moreover, large amounts of co-substrate are required for the regeneration of NADPH that is the preferred co-factor in almost all the known dicarbonyl reductions. Traditionally, better ee, reduction rate and product titre have been achieved via process engineering. The advent of recombinant DNA technology provides an alternative strategy to improve productivity and yield by strain engineering. This review discusses two aspects of strain engineering: (i) the generation of strains with higher reductase activity towards dicarbonyl compounds and (ii) the optimisation of co-substrate utilisation for NADPH cofactor regeneration.

Synthesis of Chiral β3-Aminoxy Peptides

A series of chiral beta(3)-aminoxy acids or amides with various side chains have been synthesized via two different approaches. One is the Arndt-Eistert homologation approach, using chiral alpha-aminoxy acids as starting materials. The other approach, utilizing the enantioselective reduction of beta-keto esters catalyzed by baker's yeast or chiral Ru(II) complexes, produces chiral beta(3)-aminoxy acids with nonproteinaceous side chains. The oligomers of beta(3)-aminoxy acids can be readily prepared using EDCI/HOAt as the coupling reagent.

Applying the Taguchi robust design to the optimization of the asymmetric reduction of ethyl 4-chloro acetoacetate by bakers' yeast

This study examined the characteristics and operational parameters of the asymmetric reduction of ethyl 4-chloro acetoacetate by bakers' yeast in order to produce S-4-chloro-3-hydroxybutyric acid ethyl ester. Eight operational variables were also optimized using the Taguchi method with consideration of the freshness of yeast cells as a noise factor. An L(18) orthogonal array was used to design the experiments. The reaction yield and the product's optical purity were considered as two product quality variables. A desirability function was applied to combine these two qualities as a single objective function. Additionally, the signal-to-noise (SN) ratio was used to estimate the variability in product quality. Optimization was undertaken not only to yield the best performance, but also to minimize the variation in quality. The confirmation experiments indicated that the reaction performance and the robustness of the product quality under the optimized conditions were higher than those obtained in other experiments in this study. Our results further demonstrate that the product's optical purity could be increased to >95% by adjusting the operational level of the main factors.

A two-step enzymatic resolution process for large-scale production of (S)- and (R)-ethyl-3-hydroxybutyrate

An efficient two-step enzymatic process for production of (R)- and (S)-ethyl-3-hydroxybutyrate (HEB), two important chiral intermediates for the pharmaceutical market, was developed and scaled-up to a multikilogram scale. Both enantiomers were obtained at 99% chemical purity and over 96% enantiomeric excess, with a total process yield of 73%. The first reaction involved a solvent-free acetylation of racemic HEB with vinylacetate for the production of (S)-HEB. In the second reaction, (R)-enriched ethyl-3-acetoxybutyrate (AEB) was subjected to alcoholysis with ethanol to derive optically pure (R)-HEB. Immobilized Candida antarctica lipase B (CALB) was employed in both stages, with high productivity and selectivity. The type of butyric acid ester influenced the enantioselectivity of the enzyme. Thus, extending the ester alkyl chain from ethyl to octyl resulted in a decrease in enantiomeric excess, whereas using bulky groups such as benzyl or t-butyl, improved the enantioselectivity of the enzyme. A stirred reactor was found unsuitable for large-scale production due to attrition of the enzyme particles and, therefore, a batchwise loop reactor system was used for bench-scale production. The immobilized enzyme was confined to a column and the reactants were circulated through the enzyme bed until the targeted conversion was reached. The desired products were separated from the reaction mixture in each of the two stages by fractional distillation. The main features of the process are the exclusion of solvent (thus ensuring high process throughput), and the use of the same enzyme for both the acetylation and the alcoholysis steps. Kilogram quantities of (S)-HEB and (R)-HEB were effectively prepared using this unit, which can be easily scaled-up to produce industrial quantities.

Microbial transformations

Although the use of microbial biocatalysts for chemical reactions pre-dates by a considerable margin the application of isolated enzymes for this purpose, considerable progress continues to be made in the use of whole-cell catalysts. The application of whole-cell biocatalysts in oxygenase-catalysed reactions, the reductions of carbonyl and nitro groups, and the hydrolysis of nitriles and epoxides, are all areas of continued development. In addition, the use of microbial transformations in the production of drug metabolites, carbohydrates, and amino acids is expanding. Methodological development such as the application of novel immobilisation techniques or the use of non-aqueous solvents continues to enhance the usefulness of microbial biocatalysts.