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Enzymatic carbon dioxide fixation is one of the most important metabolic reactions as it allows the capture of inorganic carbon from the atmosphere and its conversion into organic biomass. However, due to the often unfavorable thermodynamics and the difficulties associated with the utilization of CO2, a gaseous substrate that is found in comparatively low concentrations in the atmosphere, such reactions remain challenging for biotechnological applications. Nature has tackled these problems by evolution of dedicated CO2-fixing enzymes, i.e., carboxylases, and embedding them in complex metabolic pathways. Biotechnology employs such carboxylating and decarboxylating enzymes for the carboxylation of aromatic and aliphatic substrates either by embedding them into more complex reaction cascades or by shifting the reaction equilibrium via reaction engineering. This review aims to provide an overview of natural CO2-fixing enzymes and their mechanistic similarities. We also discuss biocatalytic applications of carboxylases and decarboxylases for the synthesis of valuable products and provide a separate summary of strategies to improve the efficiency of such processes. We briefly summarize natural CO2 fixation pathways, provide a roadmap for the design and implementation of artificial carbon fixation pathways, and highlight examples of biocatalytic cascades involving carboxylases. Additionally, we suggest that biochemical utilization of reduced CO2 derivates, such as formate or methanol, represents a suitable alternative to direct use of CO2 and provide several examples. Our discussion closes with a techno-economic perspective on enzymatic CO2 fixation and its potential to reduce CO2 emissions.

ChemBioChem 22(4), 652 - 656 (2020). doi:10.1002/cbic.202000600, Broad substrate tolerance and excellent regioselectivity, as well as independence from sensitive cofactors have established benzoic acid decarboxylases from microbial sources as efficient biocatalysts. Robustness under process conditions makes them particularly attractive for preparative‐scale applications. The divalent metal‐dependent enzymes are capable of catalyzing the reversible non‐oxidative (de)carboxylation of a variety of electron‐rich (hetero)aromatic substrates analogously to the chemical Kolbe‐Schmitt reaction. Elemental mass spectrometry supported by crystal structure elucidation and quantum chemical calculations verified the presence of a catalytically relevant Mg$^{2+}$ complexed in the active site of 2,3‐dihydroxybenoic acid decarboxylase from Aspergillus oryzae (2,3‐DHBD_Ao). This unique example with respect to the nature of the metal is in contrast to mechanistically related decarboxylases, which generally have Zn$^{2+}$ or Mn$^{2+}$ as the catalytically active metal., Published by Wiley-VCH, Weinheim

The concurrent operation of chemical and biocatalytic reactions in one pot is still a challenging task, and, in particular for chemical photocatalysts, examples besides simple cofactor recycling systems are rare. However, especially due to the complementary chemistry that the two fields of catalysis promote, their combination in one pot has the potential to unlock intriguing, unprecedented overall reactivities. Herein we demonstrate a concurrent biocatalytic reduction and photocatalytic oxidation process. Specifically, the enantioselective biocatalytic sulfoxide reduction using (S)-selective methionine sulfoxide reductases was coupled to an unselective light-dependent sulfoxidation. Protochlorophyllide was established as a new green photocatalyst for the sulfoxidation. Overall, a cyclic deracemization process to produce nonracemic sulfoxides was achieved and the target compounds were obtained with excellent conversions (up to 91 %) and superb optical purity (99 % ee).

The utilization of gaseous carbon dioxide instead of bicarbonate would greatly facilitate process development for enzyme catalyzed carboxylations on large scale. As proof-of-concept, 1,3-dihydroxybenzene (resorcinol) was carboxylated in the ortho-position using pressurized CO2 (~30-40 bar) catalyzed by ortho-benzoic acid decarboxylases with up to 68% conversion. Optimization studies revealed tight pH-control and enzyme stability as the most important determinants.

Die Verwendung von CO2 als Kohlenstoffquelle in der Synthese organischer Verbindungen entspricht den aktuellen Bemuhungen fur eine nachhaltigere Produktion von Chemikalien. Wir beschreiben hier die enzymkatalysierte para-Carboxylierung von Catecholen durch eine 3,4-Dihydroxybenzoesaure-Decarboxylase (AroY) aus der UbiD-Enzymfamilie. Die Kristallstrukturen und experimentelle Daten zeigen, dass dieses Enzym ein erst kurzlich entdecktes prenyliertes Flavin als Cofaktor benotigt (prFMN) und oxidative Prozesse zur Bildung der katalytisch aktiven prFMNiminium-Spezies fuhren. Diese Studie beschreibt die In-vitro-Rekonstitution und Aktivierung dieser prFMN-abhangigen Enzyme, welche die direkte Carboxylierung von aromatischen Catecholen unter milden Bedingungen katalysieren. Ein Reaktionsmechanismus fur den reversiblen Carboxylierungsprozess uber ein Intermediat mit einfach kovalent gebundener Chinoidform des Substrats wird vorgeschlagen, der sich von dem 1,3-Cycloadditionsmechanismus in verwandten Enzymen unterscheidet.

The transformation of (abundant) oxygenated biomass-derived building blocks via chemo-enzymatic methods is a valuable concept for accessing useful compounds, as it combines the high selectivity of enzymes and the versatility of chemical catalysts. In this work, we demonstrate a straightforward combination of a phenolic acid decarboxylase (PAD) and palladium on charcoal (Pd/C) that affords the flavor compound 4-ethylguaiacol from ferulic acid. The use of a two-phase system proved to be advantageous in terms of enzyme activity, stability, and volumetric productivity and allows us to carry out the hydrogenation step directly in the organic layer containing exclusively the intermediate, vinylguaiacol. The enzymatic decarboxylation step in the biphasic system afforded 89% conversion of 100 mM (19 g L–1) ferulic acid with an isolated yield of 75%. By extracting 4-vinylguaiacol continuously into the organic phase, conversion was enhanced to 92% using 170 mM (33 g L–1) ferulic acid, which was only possible in the...

The utilization of carbon dioxide as a C1‐building block for the production of valuable chemicals has recently attracted much interest. Whereas chemical CO2 fixation is dominated by C−O and C−N bond forming reactions, the development of novel concepts for the carboxylation of C‐nucleophiles, which leads to the formation of carboxylic acids, is highly desired. Beside transition metal catalysis, biocatalysis has emerged as an attractive method for the highly regioselective (de)carboxylation of electron‐rich (hetero)aromatics, which has been recently further expanded to include conjugated α,β‐unsaturated (acrylic) acid derivatives. Depending on the type of substrate, different classes of enzymes have been explored for (i) the ortho‐carboxylation of phenols catalyzed by metal‐dependent ortho‐benzoic acid decarboxylases and (ii) the side‐chain carboxylation of para‐hydroxystyrenes mediated by metal‐independent phenolic acid decarboxylases. Just recently, the portfolio of bio‐carboxylation reactions was complemented by (iii) the para‐carboxylation of phenols and the decarboxylation of electron‐rich heterocyclic and acrylic acid derivatives mediated by prenylated FMN‐dependent decarboxylases, which is the main focus of this review. Bio(de)carboxylation processes proceed under physiological reaction conditions employing bicarbonate or (pressurized) CO2 when running in the energetically uphill carboxylation direction. Aiming to facilitate the application of these enzymes in preparative‐scale biotransformations, their catalytic mechanism and substrate scope are analyzed in this review.

Fungal ferulic acid decarboxylases (FDCs) belong to the UbiD-family of enzymes and catalyse the reversible (de)carboxylation of cinnamic acid derivatives through the use of a prenylated flavin cofactor. The latter is synthesised by the flavin prenyltransferase UbiX. Herein, we demonstrate the applicability of FDC/UbiX expressing cells for both isolated enzyme and whole-cell biocatalysis. FDCs exhibit high activity with total turnover numbers (TTN) of up to 55000 and turnover frequency (TOF) of up to 370 min−1. Co-solvent compatibility studies revealed FDC's tolerance to some organic solvents up 20 % v/v. Using the in-vitro (de)carboxylase activity of holo-FDC as well as whole-cell biocatalysts, we performed a substrate profiling study of three FDCs, providing insights into structural determinants of activity. FDCs display broad substrate tolerance towards a wide range of acrylic acid derivatives bearing (hetero)cyclic or olefinic substituents at C3 affording conversions of up to >99 %. The synthetic utility of FDCs was demonstrated by a preparative-scale decarboxylation.

Utilization of gaseous carbon dioxide as a C1-building block in the biocatalytic ortho-carboxylation of a phenol., The utilization of gaseous carbon dioxide instead of bicarbonate would greatly facilitate process development for enzyme catalyzed carboxylations on a large scale. As a proof-of-concept, 1,3-dihydroxybenzene (resorcinol) was carboxylated in the ortho-position using pressurized CO2 (∼30–40 bar) catalyzed by ortho-benzoic acid decarboxylases with up to 68% conversion. Optimization studies revealed tight pH-control and enzyme stability as the most important determinants.

The promiscuous regio- and stereoselective hydration of 4-hydroxystyrenes catalyzed by ferulic acid decarboxylase from Enterobacter sp. (FDC_Es) depends on bicarbonate bound in the active site, which serves as a proton relay activating a water molecule for nucleophilic attack on a quinone methide electrophile. This “cofactor” is crucial for achieving improved conversions and high stereoselectivities for (S)-configured benzylic alcohol products. Similar effects were observed with simple aliphatic carboxylic acids as additives. A rational redesign of the active site by replacing the bicarbonate or acetate “cofactor” with a newly introduced side-chain carboxylate from an adjacent amino acid yielded mutants that efficiently acted as C=C hydratases. A single-point mutation of valine 46 to glutamate or aspartate improved the hydration activity by 40% and boosted the stereoselectivity 39-fold in the absence of bicarbonate or acetate.

The catalytic promiscuity of a ferulic acid decarboxylase from Enterobacter sp. (FDC_Es) and phenolic acid decarboxylases (PADs) for the asymmetric conjugate addition of water across the C=C bond of hydroxystyrenes was extended to the N‐, C‐ and S‐nucleophiles methoxyamine, cyanide and propanethiol to furnish the corresponding addition products in up to 91% ee. The products obtained from the biotransformation employing the most suitable enzyme/nucleophile pairs were isolated and characterized after optimizing the reaction conditions. Finally, a mechanistic rationale supported by quantum mechanical calculations for the highly (S)‐selective addition of cyanide is proposed.

Angewandte Chemie / International edition 56(44), 13893 - 13897 (2017). doi:10.1002/anie.201708091, The utilization of CO$-2$ as a carbon source for organic synthesis meets the urgent demand for more sustainability in the production of chemicals. Herein, we report on the enzyme-catalyzed para-carboxylation of catechols, employing 3,4-dihydroxybenzoic acid decarboxylases (AroY) that belong to the UbiD enzyme family. Crystal structures and accompanying solution data confirmed that AroY utilizes the recently discovered prenylated FMN (prFMN) cofactor, and requires oxidative maturation to form the catalytically competent prFMN4$^{iminium}$ species. This study reports on the in vitro reconstitution and activation of a prFMN-dependent enzyme that is capable of directly carboxylating aromatic catechol substrates under ambient conditions. A reaction mechanism for the reversible decarboxylation involving an intermediate with a single covalent bond between a quinoid adduct and cofactor is proposed, which is distinct from the mechanism of prFMN-associated 1,3-dipolar cycloadditions in related enzymes., Published by Wiley-VCH, Weinheim

In order to extend the applicability of the regioselective enzymatic carboxylation of phenols, the substrate scope of o‐benzoic acid (de)carboxylases has been investigated towards complex molecules with an emphasis on flavouring agents and polyphenols possessing antioxidant properties. o‐Hydroxycarboxylic acid products were obtained with perfect regioselectivity, in moderate to excellent yields. The applicability of this method was proven by the regioselective bio‐carboxylation of resveratrol on a preparative scale with 95% yield.

Amination of non-activated aliphatic fatty alcohols to the corresponding primary amines was achieved through a five-enzyme cascade reaction by coupling a long-chain alcohol oxidase from Aspergillus fumigatus (LCAO_Af) with a ω-transaminase from Chromobacterium violaceum (ω-TA_Cv). The alcohol was oxidized at the expense of molecular oxygen to yield the corresponding aldehyde, which was subsequently aminated by the PLP-dependent ω-TA to yield the final primary amine product. The overall cascade was optimized with respect to pH, O2 pressure, substrate concentration, decomposition of H2O2 (derived from alcohol oxidation), NADH regeneration, and biocatalyst ratio. The substrate scope of this concept was investigated under optimized conditions by using terminally functionalized C4-C11 fatty primary alcohols bearing halogen, alkyne, amino, hydroxy, thiol, and nitrile groups.

The asymmetric bioreduction of activated alkenes catalyzed by flavin-dependent enoate reductases from the OYE-family represents a powerful method for the production of optically active compounds. For its preparative-scale application, efficient and economic NADH-recycling is crucial. A novel enzyme-coupled NADH-recycling system is proposed based on the concurrent oxidation of a sacrificial sec-alcohol catalyzed by an alcohol dehydrogenase (ADH-A). Due to the highly favorable position of the equilibrium of ene-reduction versus alcohol-oxidation, the cosubstrate is only required in slight excess.

Dwindling petroleum feedstocks and increased CO(2)-concentrations in the atmosphere currently open the concept of using CO(2) as raw material for the synthesis of well-defined organic compounds. In parallel to recent advances in the chemical CO(2)-fixation, enzymatic (biocatalytic) carboxylation is currently being investigated at an increased pace. On the one hand, this critical review provides a concise overview on highly specific biosynthetic pathways for CO(2)-fixation and, on the other hand, a summary of biodegradation (detoxification) processes involving enzymes which possess relaxed substrate specificities, which allow their application for the regioselective carboxylation of organic substrates to furnish the corresponding carboxylic acids (145 references).

Biocatalytic racemization of aliphatic, (aryl)aliphatic and aromatic α-hydroxycarboxylic acids was achieved via a reversible oxidation-reduction sequence using a pair of stereo-complementary Prelog- and anti-Prelog d - and l -α-hydroxyisocaproate dehydrogenases from Lactobacillus confusus DSM 20196 and Lactobacillus paracasei DSM 20008, resp., overexpressed in Escherichia coli. The mild reaction conditions ensured essential ‘clean’ isomerization, undesired ‘over-oxidation’ of the substrate forming the α-ketoacid could be suppressed by exclusion of O2 and adjustment of the NAD+/NADH-ratio.

Alkenes possessing a C=C double bond adjacent to an aromatic ring were cleaved to yield the corresponding carbonyl compounds by use of molecular oxygen as the sole oxidant and a cell-free extract of the wood-degrading fungus Trametes hirsuta FCC 047 as catalyst. The oxygen pressure required was optimized. Special adapted equipment allowed 96 reactions to be performed in parallel under controlled oxygen pressure. A broad spectrum of aryl-alkenes was successfully converted into the corresponding ketones/aldehydes with excellent chemoselectivity under a controlled oxygen atmosphere (2 bar).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Amination of non-activated aliphatic fatty alcohols to the corresponding primary amines was achieved through a five-enzyme cascade reaction by coupling a long-chain alcohol oxidase from Aspergillus fumigatus (LCAO_Af) with a ω-transaminase from Chromobacterium violaceum (ω-TA_Cv). The alcohol was oxidized at the expense of molecular oxygen to yield the corresponding aldehyde, which was subsequently aminated by the PLP-dependent ω-TA to yield the final primary amine product. The overall cascade was optimized with respect to pH, O2 pressure, substrate concentration, decomposition of H2O2 (derived from alcohol oxidation), NADH regeneration, and biocatalyst ratio. The substrate scope of this concept was investigated under optimized conditions by using terminally functionalized C4–C11 fatty primary alcohols bearing halogen, alkyne, amino, hydroxy, thiol, and nitrile groups.

The biocatalytic racemization of a range of (hetero)aryl- and (di)aryl-aliphatic α-hydroxycarboxylic acids has been achieved by using whole resting cells of Lactobacillus spp. The essentially mild (physiological) reaction conditions ensure the suppression of undesired side reactions, such as elimination, decomposition or condensation. Cofactor/inhibitor studies using a cell-free extract of Lactobacillus paracasei DSM 20207 reveal that the addition of redox cofactors (NAD+/NADH) leads to a distinct increase in the racemization rate, while strong inhibition is observed in the presence of Thio-NAD+, which suggests that the racemization proceeds by an oxidation–reduction sequence rather than involvement of a “racemase” enzyme. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Deracemization of (±)-3-phenyllactic acid ( 1 ) and (±)-2-hydroxy-4-phenylbutanoic acid ( 2 ) was accomplished by lipase-catalysed kinetic resolution coupled to biocatalytic racemization of the non-reacting substrate enantiomers using Lactobacillus paracasei DSM 20008. Cyclic repetition of this sequence led to a single enantiomeric product from the racemate. Access to both enantiomers was achieved by switching between lipase-catalysed acyl-transfer and ester hydrolysis reactions. Both products constitute important building blocks for virus protease- and ACE-inhibitors, respectively.

Microbial decarboxylases, which catalyse the reversible regioselective ortho-carboxylation of phenolic derivatives in anaerobic detoxification pathways, have been studied for their reverse carboxylation activities on electron-rich aromatic substrates. Ortho-hydroxybenzoic acids are important building blocks in the chemical and pharmaceutical industries and are currently produced via the Kolbe-Schmitt process, which requires elevated pressures and temperatures (≥ 5 bar, ≥ 100 °C) and often shows incomplete regioselectivities. In order to resolve bottlenecks in view of preparative-scale applications, we studied the kinetic parameters for 2,6-dihydroxybenzoic acid decarboxylase from Rhizobium sp. in the carboxylation- and decarboxylation-direction using 1,2-dihydroxybenzene (catechol) as starting material. The catalytic properties (K(m), V(max)) are correlated with the overall thermodynamic equilibrium via the Haldane equation, according to a reversible random bi-uni mechanism. The model was subsequently verified by comparing experimental results with simulations. This study provides insights into the catalytic behaviour of a nonoxidative aromatic decarboxylase and reveals key limitations (e.g. substrate oxidation, CO2 pressure, enzyme deactivation, low turnover frequency) in view of the employment of this system as a 'green' alternative to the Kolbe-Schmitt processes.

A single enantiomer of a (stereo)chemically labile allylic-homoallylic alcohol was obtained in 91% e.e. and 96% yield from the racemate by employing a lipase-catalysed kinetic resolution coupled to in situ inversion under carefully controlled (Mitsunobu) conditions in order to suppress side reactions, such as elimination and racemisation. This technique was successfully applied to an enantio-convergent asymmetric total synthesis of the algal fragrance component ( S )-dictyoprolene.

The organic solvent-stable redox-system of Rhodococcus ruber DSM 44541, which allows the efficient oxidation/reduction of sec-alcohols/ketones at the expense of acetone/2-propanol, respectively, as cosubstrate was optimized with respect to a maximum of alcohol dehydrogenase activity during cell growth. Comparison of the fermentation of R. ruber DSM 44541 in shake flasks cultures (1 l flask with 250 ml medium) and in a bioreactor (15 l with 10 l working volume) revealed that the desired organic solvent-stable alcohol dehydrogenase activity reached its maximum during the log phase for the bioreactor. In contrast, in shake flasks the maximum of activity was reached during the stationary phase.

The constitutive epoxide hydrolase activity of Rhodococcus ruber DSM 44540 strongly depends on the status of the cells and appears to be regulated by a catabolic switch: activity peaked when glucose was exhausted and peptone/yeast extract consumption started. The activity-maximum for the kinetic resolution of a 2,2- and the enantioconvergent asymmetric biohydrolysis of a 2,3-disubstituted oxirane coincided. In order to obtain a maximum yield, cells should be harvested after ca. 17 h.

We report on a ‘green’ method for the utilization of carbon dioxide as C1 unit for the regioselective synthesis of (E)‐cinnamic acids via regioselective enzymatic carboxylation of para‐hydroxystyrenes. Phenolic acid decarboxylases from bacterial sources catalyzed the β‐carboxylation of para‐hydroxystyrene derivatives with excellent regio‐ and (E/Z)‐stereoselectivity by exclusively acting at the β‐carbon atom of the C=C side chain to furnish the corresponding (E)‐cinnamic acid derivatives in up to 40% conversion at the expense of bicarbonate as carbon dioxide source. Studies on the substrate scope of this strategy are presented and a catalytic mechanism is proposed based on molecular modelling studies supported by mutagenesis of amino acid residues in the active site. WILEY-VCH

Several decarboxylases are tested for exclusively selective ortho-carboxylation of phenols under ambient reaction conditions using bicarbonate as CO2 source.

The enzymatic carboxylation of electron-rich aromatics, which represents a promising 'green' equivalent to the chemical Kolbe-Schmitt reaction, is thermodynamically disfavored and is therefore impeded by incomplete conversions. Optimization of the reaction conditions, such as pH, temperature, substrate concentration and the use of organic co-solvents and/or ionic liquids allowed to push the conversion in favor of carboxylation by a factor of up to 50%. Careful selection of the type of bicarbonate salt used as CO2 source was crucial to ensure optimal activities. Among two types of carboxylases tested with their natural substrates, benzoic acid decarboxylase from Rhizobium sp. proved to be significantly more stable than phenolic acid decarboxylase from Mycobacterium colombiense; it tolerated reaction temperatures of up to 50 °C and substrate concentrations of up to 100mM and allowed efficient biocatalyst recycling.

The enzymatic carboxylation of phenol and styrene derivatives using (de)carboxylases in carbonate buffer proceeded in a highly regioselective fashion: Benzoic acid (de)carboxylases selectively formed o-hydroxybenzoic acid derivatives, phenolic acid (de)carboxylases selectively acted at the β-carbon atom of styrenes forming (E)-cinnamic acids.

A strategy for the biocatalytic racemization of primary α-chiral amines was developed by employing a pair of stereocomplementary PLP-dependent ω-transaminases. The interconversion of amine enantiomers proceeded through reversible transamination by a prochiral ketone intermediate, either catalyzed by a pair of stereocomplementary ω-transaminases or by a single enzyme possessing low stereoselectivity. To tune the system, the type and concentration of a nonchiral amino acceptor proved to be crucial. Finally, racemization could be achieved by the cross-transamination of two different amines without a requirement for an external amino acceptor. Several synthetically and industrially important amines could be enzymatically racemized under mild reaction conditions.

Nonracemic aryl-substituted alpha-methyldihydrocinnamaldehyde derivatives employed as olfactory principles in perfumes (Lilial, Helional) were obtained via enzymatic reduction of the corresponding cinnamaldehyde precursors using cloned and overexpressed ene-reductases. (R)-Enantiomers were obtained using the old-yellow-enzyme (OYE) homolog YqjM from Bacillus subtilis and 12-oxophytodienoic acid reductase isoenzyme OPR1 from tomato (e.e.(max) 53%), and (S)-aldehydes were furnished in up to 97% e.e. using isoenzyme OPR3, nicotinamide 2-cyclohexene-1-one reductase NCR from Zymomonas mobilis and yeast OYE isoenzymes 1-3 under optimised reaction conditions in the presence of t-butyl methyl ether as the co-solvent. The stereochemical outcome of the reduction of alpha-methylcinnamaldehyde using NCR and OYEs 1-3 [previously reported to be (R)] was unambiguously corrected to be (S).

Alkenes possessing a C=C double bond adjacent to an aromatic ring were cleaved to yield the corresponding carbonyl compounds by use of molecular oxygen as the sole oxidant and a cell-free extract of the wood-degrading fungus Trametes hirsuta FCC 047 as catalyst. The oxygen pressure required was optimized. Special adapted equipment allowed 96 reactions to be performed in parallel under controlled oxygen pressure. A broad spectrum of aryl-alkenes was successfully converted into the corresponding ketones/aldehydes with excellent chemoselectivity under a controlled oxygen atmosphere (2 bar).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

[reaction: see text] Biocatalytic racemization of a range of aliphatic, (aryl)aliphatic, and aromatic alpha-hydroxycarboxylic acids was accomplished by using whole resting cells of a range of Lactobacillus spp. The mild (physiological) reaction conditions ensured an essentially "clean" isomerization in the absence of side reactions, such as elimination or decomposition. Whereas straight-chain aliphatic 2-hydroxycarboxylic acids were racemized with excellent rates (up to 85% relative to lactate), steric hindrance was observed for branched-chain analogues. Good rates were observed for aryl-alkyl derivatives, such as 3-phenyllactic acid (up to 59%) and 4-phenyl-2-hydroxybutanoic acid (up to 47%). In addition, also mandelate and its o-chloro analogue were accepted at a fair rate (45%). This biocatalytic racemization represents an important tool for the deracemization of a number of pharmaceutically important building blocks.

Asymmetric enzyme-catalyzed hydrolysis of methylene-interrupted bis-epoxides 1 a and 1 b catalyzed by bacterial epoxide hydrolases furnished tetrahydrofuran derivatives 2 a and 2 b through a hydrolysis-rearrangement cascade. Whereas racemic bis-oxiranes 1 b-d underwent kinetic resolution with moderate stereoselectivities to yield products with up to 92 % ee and 66 % de: meso-bis-oxirane cis,cis-1 a was transformed into (6R,7R,9S,10S)-2 a in 94 % ee and 89 % de at high conversion (85 %) by Rhodococcus sp. CBS 717.73 as the major product. The reaction sequence resembles a biomimetic reaction cascade and provides an efficient entry into the structural core of annonaceous acetogenins with simultaneous control of four stereocenters.

Organic compounds can be transformed through enzyme-triggered domino (or cascade) reactions via several (inseparable) consecutive steps in an asymmetric fashion to yield nonracemic products. Despite the fact that these sequences often involve the occurrence of highly reactive unstable intermediates, the overall efficiency of these processes can be high, provided that the reaction rates of the individual steps match each other in order to minimize side reactions.

The enzyme catalyzed carboxylation of electron-rich phenol derivatives employing recombinant benzoic acid decarboxylases at the expense of bicarbonate as CO2 source is reported. In contrast to the classic Kolbe–Schmitt reaction, the biocatalytic equivalent proceeded in a highly regioselective fashion exclusively at the ortho-position of the phenolic directing group in up to 80% conversion. Several enzymes were identified, which displayed a remarkably broad substrate scope encompassing alkyl, alkoxy, halo and amino-functionalities. Based on the crystal structure and molecular docking simulations, a mechanistic proposal for 2,6-dihydroxybenzoic acid decarboxylase is presented.