Enzymatic Synthesis of Optically Active Lactones via Asymmetric Bioreduction using Ene-Reductases from the Old Yellow Enzyme Family

Engineering of an ene-reductase for producing the key intermediate of antiepileptic drug Brivaracetam

(R)-4-Propyldihydrofuran-2(3H)-one (R-PDFO) is the key chiral intermediate for the antiepileptic drug Brivaracetam. Lacking a simple and economical method to approaching R-PDFO, the production of R-PDFO also remains environmentally unfriendly. Here, we developed a straightforward bioreduction way from easily synthesized 4-propylfuran-2(5H)-one (PFO) using ene-reductases. After screened with 27 ene-reductases, E116 stood out with 25.7% yield and 97% ee (R) as the starting enzyme. To improve the catalytic efficiency of E116, several rounds of directed evolution were first carried out. Through rational design, alanine scanning and random mutagenesis, engineered ene-reductase E116-M3 was obtained, with a 2.63-fold improvement in yields over WT, a 12.6-fold improvement in k_cat/K_m over WT, and stereoselectivity increased to 99% (R). To further improve the yield of R-PDFO, the reaction conditions were then optimized. The catalytic activity of the optimized reaction system was increased again by 2.3 times and the turnover number (TON) of E116-M3 reached 705. Subsequently, whole cells harboring E116-M3 were also shown to have similar capabilities of synthesizing R-PDFO. Finally, E116-M3 was employed in the 50-mL-scale synthesis of R-PDFO under 20 mM of PFO loading to achieve 81% isolated yield and 99% ee. In conclusion, this new approach of engineered ene-reductase catalyzing the asymmetric reduction of PFO could be a green alternative for the efficient synthesis of R-PDFO. KEY POINTS: • An ene-reductase library was first used to screen the bioreduction of PFO. • Rational design contributed to the enhanced R-stereoselectivity of PFO reduction. • E116-M3 was obtained with high activity and stereoselectivity for R-PDFO.

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.

Introduction of Chiral Centers to α- and/or β-Positions of Carbonyl Groups by Biocatalytic Asymmetric Reduction of α,β-Unsaturated Carbonyl Compounds

Biocatalytic asymmetric reductions of acyclic and cyclic α,β-unsaturated carbonyl compounds are favorable protocols for introduction of chiral centers to α- and/or β-positions of the carbonyl groups. Representative biocatalytic reductions of electron deficient olefins are compiled from a synthetic point of view according to compound types from the papers in 2012 to early 2022. Applications to syntheses of some enantiomericaly enriched perfumery ingredients are presented to show the feasibility of the biocatalytic reductions.

The beauty of biocatalysis: sustainable synthesis of ingredients in cosmetics

Covering: 2015 up to July 2021The market for cosmetics is consumer driven and the desire for green, sustainable and natural ingredients is increasing. The use of isolated enzymes and whole-cell organisms to synthesise these products is congruent with these values, especially when combined with the use of renewable, recyclable or waste feedstocks. The literature of biocatalysis for the synthesis of ingredients in cosmetics in the past five years is herein reviewed.

Enzymatic strategies for asymmetric synthesis

Enzymes, at the turn of the 21st century, are gaining a momentum. Especially in the field of synthetic organic chemistry, a broad variety of biocatalysts are being applied in an increasing number of processes running at up to industrial scale. In addition to the advantages of employing enzymes under environmentally friendly reaction conditions, synthetic chemists are recognizing the value of enzymes connected to the exquisite selectivity of these natural (or engineered) catalysts. The use of hydrolases in enantioselective protocols paved the way to the application of enzymes in asymmetric synthesis, in particular in the context of biocatalytic (dynamic) kinetic resolutions. After two decades of impressive development, the field is now mature to propose a panel of catalytically diverse enzymes for (i) stereoselective reactions with prochiral compounds, such as double bond reduction and bond forming reactions, (ii) formal enantioselective replacement of one of two enantiotopic groups of prochiral substrates, as well as (iii) atroposelective reactions with noncentrally chiral compounds. In this review, the major enzymatic strategies broadly applicable in the asymmetric synthesis of optically pure chiral compounds are presented, with a focus on the reactions developed within the past decade.

Non-Conventional Yeasts as Sources of Ene-Reductases for the Bioreduction of Chalcones

Thirteen Non-Conventional Yeasts (NCYs) have been investigated for their ability to reduce activated C=C bonds of chalcones to obtain the corresponding dihydrochalcones. A possible correlation between bioreducing capacity of the NCYs and the substrate structure was estimated. Generally, whole-cells of the NCYs were able to hydrogenate the C=C double bond occurring in (E)-1,3-diphenylprop-2-en-1-one, while worthy bioconversion yields were obtained when the substrate exhibited the presence of a deactivating electron-withdrawing Cl substituent on the B-ring. On the contrary, no conversion was generally found, with a few exceptions, in the presence of an activating electron-donating substituent OH. The bioreduction aptitude of the NCYs was apparently correlated to the logP value: Compounds characterized by a higher logP exhibited a superior aptitude to be reduced by the NCYs than compounds with a lower logP value.

Two new ene-reductases from photosynthetic extremophiles enlarge the panel of old yellow enzymes: CtOYE and GsOYE

Looking for new ene-reductases with uncovered features beneficial for biotechnological applications, by mining genomes of photosynthetic extremophile organisms, we identified two new Old Yellow Enzyme homologues: CtOYE, deriving from the cyanobacterium Chroococcidiopsis thermalis, and GsOYE, from the alga Galdieria sulphuraria. Both enzymes were produced and purified with very good yields and displayed catalytic activity on a broad substrate spectrum by reducing α,β-unsaturated ketones, aldehydes, maleimides and nitroalkenes with good to excellent stereoselectivity. Both enzymes prefer NADPH but demonstrate a good acceptance of NADH as cofactor. CtOYE and GsOYE represent robust biocatalysts showing high thermostability, a wide range of pH optimum and good co-solvent tolerance. High resolution X-ray crystal structures of both enzymes have been determined, revealing conserved features of the classical OYE subfamily as well as unique properties, such as a very long loop entering the active site or an additional C-terminal alpha helix in GsOYE. Not surprisingly, the active site of CtOYE and GsOYE structures revealed high affinity toward anions caught from the mother liquor and trapped in the anion hole where electron-withdrawing groups such as carbonyl group are engaged. Ligands (para-hydroxybenzaldehyde and 2-methyl-cyclopenten-1-one) added on purpose to study complexes of GsOYE were detected in the enzyme catalytic cavity, stacking on top of the FMN cofactor, and support the key role of conserved residues and FMN cofactor in the catalysis.

Catalytic asymmetric synthesis of geminal-dicarboxylates

Stereogenic acetals, spiroacetals and ketals are well-studied stereochemical features that bear two heteroatoms at a common carbon atom. These stereocenters are normally found in cyclic structures while linear (or acyclic) analogues bearing two heteroatoms are rare. Chiral geminal-dicarboxylates are illustrative, there is no current way to access this class of compounds while controlling the stereochemistry at the carbon center bound to two oxygen atoms. Here we report a rhodium-catalysed asymmetric carboxylation of ester-containing allylic bromides to form stereogenic carbon centers bearing two different carboxylates with high yields and enantioselectivities. The products, which are surprisingly stable to a variety of acidic and basic conditions, can be manipulated with no loss of enantiomeric purity as demonstrated by ring closing metathesis reactions to form chiral lactones, which have been extensively used as building blocks in asymmetric synthesis.

Biocatalytic reduction of activated CC-bonds and beyond: emerging trends

The biocatalytic reduction of activated CC-bonds is dominated by ene-reductases from the Old Yellow Enzyme family, which gained broad practical use owing to exquisite stereoselectivity combined with wide substrate scope. Protein diversity is fostered by mining distinct protein classes and by implementing protein engineering techniques. Recent efforts are focusing on expanding the chemical complexity of the product portfolio, either through substrate functionalization or design of multi-step reactions. This review also highlights unusual chemistries catalyzed by ene-reductases and presents emerging methodologies developed to bypass the need of natural nicotinamide cofactors.

Sequential Enzymatic Conversion of α-Angelica Lactone to γ-Valerolactone through Hydride-Independent C=C Bond Isomerization

A case of hydride-independent reaction catalyzed by flavin-dependent ene-reductases from the Old Yellow Enzyme (OYE) family was identified. α-Angelica lactone was isomerized to the conjugated β-isomer in a nicotinamide-free and hydride-independent process. The catalytic cycle of C=C bond isomerization appears to be flavin-independent and to rely solely on a deprotonation-reprotonation sequence through acid-base catalysis. Key residues in the enzyme active site were mutated and provided insight on important mechanistic features. The isomerization of α-angelica lactone by OYE2 in aqueous buffer furnished 6.3 mm β-isomer in 15 min at 30 °C. In presence of nicotinamide adenine dinucleotide (NADH), the latter could be further reduced to γ-valerolactone. This enzymatic tool was successfully applied on semi-preparative scale and constitutes a sustainable process for the valorization of platform chemicals from renewable resources.

Rapid protein immobilization for thin film continuous flow biocatalysis

A versatile enzyme immobilization strategy for thin film continuous flow processing is reported. Here, non-covalent and glutaraldehyde bioconjugation are used to immobilize enzymes on the surfaces of borosilicate reactors. This approach requires only ng of protein per reactor tube, with the stock protein solution readily recycled to sequentially coat >10 reactors. Confining reagents to thin films during immobilization reduced the amount of protein, piranha-cleaning solution, and other reagents by ∼96%. Through this technique, there was no loss of catalytic activity over 10 h processing. The results reported here combines the benefits of thin film flow processing with the mild conditions of biocatalysis.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as sigillin A from Ceratophysella sigillata.