Enantioselective Epoxidation Research Articles

Biocatalytic approaches for enantio and diastereoselective synthesis of chiral β-nitroalcohols.

Chiral β-nitroalcohols find significant application in organic synthesis due to the versatile reactivity of hydroxyl and nitro functionalities attached to one or two vicinal asymmetric centers. They are key building blocks of several important pharmaceuticals, bioactive molecules, and fine chemicals. With the growing demand to develop clean and green methods for their synthesis, biocatalytic methods have gained tremendous importance among the existing asymmetric synthesis routes. Over the years, different biocatalytic strategies for the asymmetric synthesis of β-nitroalcohol stereoisomers have been developed. They can be majorly classified as (a) kinetic resolution, (b) dynamic kinetic resolution, (c) Henry reaction, (d) retro-Henry reaction, (e) asymmetric reduction, and (f) enantioselective epoxide ring-opening. This review aims to provide an overview of the above biocatalytic strategies, and their comparison along with future prospects. Essentially, it presents an enzyme-toolbox for the asymmetric synthesis of β-nitroalcohol enantiomers and diastereomers.

Enantioselective Epoxidation of 2,3-Disubstituted Naphthoquinones by a Side Chain Truncated Guanidine-Urea Bifunctional Organocatalyst.

An organocatalytic enantioselective epoxidation of 2,3-disubstituted naphthoquinones with tert-butyl hydroperoxide as an oxidant was developed using a guanidine-urea bifunctional catalyst lacking C2 symmetry, which was designed based upon the insights obtained from the DFT calculation model for our previous C2 symmetric catalyst. The present organocatalytic reaction provides access to a variety of optically active naphthoquinone epoxides bearing aryl and methyl substituents at C2 and C3 in high yields with high enantioselectivities (up to 97:3 er).

Highly Enantioselective Epoxidation of α,β-Unsaturated Ketones Using Amide-Based Cinchona Alkaloids as Hybrid Phase-Transfer Catalysts.

A series of 20 one chiral epoxides were obtained with excellent yields (up to 99%) and enantioselectivities (up to >99% ee) using hybrid amide-based Cinchona alkaloids. Our method is characterized by low catalyst loading (0.5 mol %) and short reaction times. Moreover, the epoxidation process can be carried out in 10 cycles, without further catalyst addition to the reaction mixture. This methodology significantly enhance the scale of the process using very low catalyst loading.

Regenerable copper anode for the Cu(I)-mediated reduction of FAD in the electroenzymatic styrene epoxidation reaction

Styrene monooxygenase (SMO) is a two-component flavoenzyme composed of NADH-dependent flavin reductase (SMOB) and FAD-specific styrene epoxidase (NSMOA) components. The enantioselective styrene epoxidation reaction catalyzed by this enzyme can be streamlined for chemosynthetic applications by substituting NADH and the reductase with an electrode to supply the epoxidase with reducing equivalents required for catalysis. Slow kinetics of adsorption and desorption of FAD from the electrode surface and unproductive side reactions of the reduced flavin with oxygen limit the efficiency of direct electroenzymatic catalysis. In the present work we develop a miniature spectroelectrochemical cell equipped with a copper electrode for the anodic synthesis of Cu(I) chelates of EDTA, glutamate, and citrate as FAD-reducing agents, and a platinum electrode for the electrolytic generation of oxygen. Copper oxidized in the flavin reduction reaction can be reclaimed subsequently as copper metal at the electrode surface. About 80% transformation of styrene is achieved in a single cell cycle of reduction and oxygenation at pH 7 and 25 °C in good agreement with that predicted by numerical simulation. When the cell is operated in two successive cycles, styrene oxide can be synthesized with an electroenzymatic epoxidation activity of 663U/g in 94% yield. This approach to electroenzymatic catalysis shows promise for the quantitative transformation of styrene to styrene oxide and may be applied more generally to other flavoprotein monooxygenases.

Enantioselective Epoxidation by Flavoprotein Monooxygenases Supported by Organic Solvents

Styrene and indole monooxygenases (SMO and IMO) are two-component flavoprotein monooxygenases composed of a nicotinamide adenine dinucleotide (NADH)-dependent flavin adenine dinucleotide (FAD)-reductase (StyB or IndB) and a monooxygenase (StyA or IndA). The latter uses reduced FAD to activate oxygen and to oxygenate the substrate while releasing water. We circumvented the need for the reductase by direct FAD reduction in solution using the NAD(P)H-mimic 1-benzyl-1,4-dihydronicotinamide (BNAH) to fuel monooxygenases without NADH requirement. Herein, we report on the hitherto unknown solvent tolerance for the indole monooxygenase from Gemmobacter nectariphilus DSM15620 (GnIndA) and the styrene monooxygenase from Gordonia rubripertincta CWB2 (GrStyA). These enzymes were shown to convert bulky and rather hydrophobic styrene derivatives in the presence of organic cosolvents. Subsequently, BNAH-driven biotransformation was furthermore optimized with regard to the applied cosolvent and its concentration as well as FAD and BNAH concentration. We herein demonstrate that GnIndA and GrStyA enable selective epoxidations of allylic double bonds (up to 217 mU mg−1) in the presence of organic solvents such as tetrahydrofuran, acetonitrile, or several alcohols. Notably, GnIndA was found to resist methanol concentrations up to 25 vol.%. Furthermore, a diverse substrate preference was determined for both enzymes, making their distinct use very interesting. In general, our results seem representative for many IMOs as was corroborated by in silico mutagenetic studies.

Electrochemical Reduction of FAD for NSMOA‐Catalyzed Epoxidation Reactions

Styrene monooxygenase (SMO) is a two‐component flavoprotein composed of an NADH‐dependent flavin‐reductase (SMOB) and FAD‐dependent epoxidase (NSMOA). The SMO‐catalyzed enantioselective epoxidation of styrene has potential applications in green‐chemical aqueous phase synthesis and environmental remediation. In the present work, we investigate the possibility of eliminating the NADH‐dependence of this two‐component biocatalyst by developing an electrochemical system that can be substituted for SMOB as the source of reduced FAD as the rate‐limiting step in the styrene epoxidation reaction.In this work, a miniature spectroelectrochemical cell was constructed and interfaced with a potentiostat that enabled the study of electrode‐mediated flavin‐reduction reactions by cyclic voltammetry and diode array‐based UV‐vis spectroscopy under anaerobic and aerobic conditions. Different results were obtained depending on the composition of the working electrode in terms of the kinetics of FAD desorption from the electrode surface and the applied electrical potential required to achieve FAD reduction. Glassy carbon and graphene‐based working electrodes are observed to readily establish and remain in electrical communication with FAD, but the kinetics of FAD reduction in bulk solution is limited by the slow kinetics of FAD desorption from the surface of these working electrodes. Under aerobic conditions, oxygen diffuses within and reacts with reduced FAD adsorbed to the electrode surface in a reaction which is rapid compared with the kinetics of the desorption of reduced FAD. The implementation of more hydrophilic working electrodes prepared as composites of graphene and polyethylene glycol and the inclusion of electron carriers with oxidation reduction‐potentials in the range of FAD significantly improves the rate FAD desorption and reduction in bulk solution where the rapid kinetics of association of reduced FAD to the epoxidase (NSMOA) can efficiently out compete undesirable side reactions of reduced FAD with dissolved oxygen. In this work, we address the central challenges associated with the electrochemical reduction preparation and transmission of reduced FAD to styrene monooxygenase and report how this can be achieved efficiently by using a combination of cyclic voltammetry, electron mediators and an electrode material with rapid FAD desorption characteristics.Support or Funding InformationThis research is founded by NIH MBRS‐RISE (R25‐GM059298).Cyclic voltammetry of FAD at the surface of a glassy carbon electrode and saturation plot of the flavin interaction at the electrode surface.Figure 1Absorbance spectra recording the reduction of FAD (Left) and consumption of styrene (right) by UV‐Vis spectroscopy.Figure 2

Computational Design of Enantiocomplementary Epoxide Hydrolases for Asymmetric Synthesis of Aliphatic and Aromatic Diols.

The use of enzymes in preparative biocatalysis often requires tailoring enzyme selectivity by protein engineering. Herein we explore the use of computational library design and molecular dynamics simulations to create variants of limonene epoxide hydrolase that produce enantiomeric diols from meso‐epoxides. Three substrates of different sizes were targeted: cis‐2,3‐butene oxide, cyclopentene oxide, and cis‐stilbene oxide. Most of the 28 designs tested were active and showed the predicted enantioselectivity. Excellent enantioselectivities were obtained for the bulky substrate cis‐stilbene oxide, and enantiocomplementary mutants produced (S,S)‐ and (R,R)‐stilbene diol with >97 % enantiomeric excess. An (R,R)‐selective mutant was used to prepare (R,R)‐stilbene diol with high enantiopurity (98 % conversion into diol, >99 % ee). Some variants displayed higher catalytic rates (k cat) than the original enzyme, but in most cases K M values increased as well. The results demonstrate the feasibility of computational design and screening to engineer enantioselective epoxide hydrolase variants with very limited laboratory screening.

Regio‐ and Enantioselective Substrate‐Directed Epoxidation

Substrate‐directed reactions are well recognized as important reactions to distinguish sterically and electronically similar reactive sites based on polar functional groups to realize regioselective reactions. Sharpless asymmetric epoxidation is a representative substrate‐directed reaction and known as an exceptional reaction to provide useful chiral epoxides from allylic alcohols. Despite the considerable development of the Sharpless asymmetric epoxidation over the last four decades, some difficult and challenging problems remain unsolved. This review describes three fascinating and attractive topics in enantioselective substrate‐directed epoxidation: (1) carboxylic acid‐ or amine derivative‐directed epoxidation, (2) regioselective epoxidation of non‐proximal alkenes, (3) enantioselective epoxidation of bishomoallylic alcohols.

Bisguanidinium-Catalyzed Epoxidation of Allylic and Homoallylic Amines under Phase Transfer Conditions

A highly enantioselective epoxidation reaction of allylic and homoallylic amines has been disclosed using an ion pair catalyst, which consists of chiral cationic bisguanidinium [BG]2+ and an achiral tetraperoxyditungstate anion [W2O2(μ-O)(O2)4]2–. The terminal oxidant is a stoichiometric amount of aqueous hydrogen peroxide, an environmentally benign reagent. Up to 96% enantiomeric excess and 99% yields were achieved for 1,1′-disubstituted and 1,2-disubstituted allylic protected amines and 1,2-disubstituted homoallylic protected amines. The identity of the ion pair catalyst was uncovered using X-ray crystallography and revealed that the achiral tetraperoxyditungstate anion species [W2O2(μ-O)(O2)4]2– is nudged nicely into the central cavity of the chiral dication. The ion pair catalyst was also characterized using infrared (IR) and Raman spectroscopies. The synthesis of (−)-venlafaxine was achieved via this reported methodology to demonstrate its usefulness.

Efficient and Divergent Total Synthesis of (-)-Epicoccin G and (-)-Rostratin A Enabled by Double C(sp3)-H Activation.

Dithiodiketopiperazines are complex polycyclic natural products possessing a variety of interesting biological activities. Despite their interest, relatively few total syntheses have been completed. We herein report the enantioselective, scalable, and divergent total synthesis of two symmetrical pentacyclic dithiodiketopiperazines, (-)-epicoccin G and (-)-rostratin A. A common intermediate was synthesized on a multigram scale from inexpensive, commercially available starting materials using an enantioselective organocatalytic epoxidation and a double C(sp3)-H activation as key steps, with the latter allowing the efficient simultaneous construction of the two five-membered rings. In addition to the cis,cis-fused target (-)-epiccocin G, the more challenging (-)-rostratin A, possessing two trans ring junctions, was obtained for the first time on a 500 mg scale through the optimization of each step and validation on multigram quantities. Both natural products were synthesized with high overall yields (13-20%). This study should facilitate access to this fascinating and yet understudied family of biologically active natural products.

Molybdenum-catalyzed asymmetric anti-dihydroxylation of allylic alcohols

Asymmetric dihydroxylation of alkenes is one of the fundamental reactions in organic synthesis, but the anti-dihydroxylation is much less developed than its syn-variant. Here we report a highly enantio- and diastereoselective anti-dihydroxylation of allylic alcohols by using a chiral molybdenum-bishydroxamic acid complex as catalyst and environmentally benign hydrogen peroxide as oxidant. This reaction enables the construction of the 1,2,3-triol structural unit in high enantio- and diastereocontrol starting from simple allylic alcohol precursors. Our reaction complements the Sharpless dihydroxylation not only in its diastereoselectivity, but also in regiocontrol. The mechanistic studies indicate that this dihydroxylation reaction consists of an initial enantioselective epoxidation and the following in situ regioselective ring opening, both of which are promoted by the molybdenum-catalyst.

Biomimetic Non-Heme Iron-Catalyzed Epoxidation of Challenging Terminal Alkenes Using Aqueous H2O2 as an Environmentally Friendly Oxidant.

Catalysis mediated by iron complexes is emerging as an eco-friendly and inexpensive option in comparison to traditional metal catalysis. The epoxidation of alkenes constitutes an attractive application of iron(III) catalysis, in which terminal olefins are challenging substrates. Herein, we describe our study on the design of biomimetic non-heme ligands for the in situ generation of iron(III) complexes and their evaluation as potential catalysts in epoxidation of terminal olefins. Since it is well-known that active sites of oxidases might involve imidazole fragment of histidine, various simple imidazole derivatives (seven compounds) were initially evaluated in order to find the best reaction conditions and to develop, subsequently, more elaborated amino acid-derived peptide-like chiral ligands (10 derivatives) for enantioselective epoxidations.

High‐Spin and Low‐Spin Perferryl Intermediates in Fe(PDP)‐Catalyzed Epoxidations

AbstractTwo bioinspired hydroxo‐bridged diferric complexes 6 and 7 with N4‐donor ligands of the PDP type (PDP=N,N′‐bis(pyridin‐2‐ylmethyl)‐2,2′‐bipyrrolidine), differing by substituents at the pyridine rings (4‐NMe2 in 7 vs. 3,5‐Me2‐4‐OMe in 6), efficiently catalyze the enantioselective alkene epoxidation with H2O2 and peracetic acid in the presence of a carboxylic acid additive (up to 99 catalyst turnover numbers, TON, toward epoxide, up to 94 % ee). Catalyst systems based on complex 7 display the high‐spin perferryl intermediate 7 aAA (S=3/2, g1, g2=3.69, g3=1.96), whereas catalyst systems based on complex 6 exhibit the low‐spin perferryl intermediate 6 aAA (S=1/2, g1=2.07, g2=2.01, g3=1.96). The S=3/2 and the S=1/2 intermediates directly react with cyclohexene and cyclohexane at low temperatures (−40 °C and −85 °C, respectively). The catalyst systems, exhibiting less reactive intermediate 7 aAA, demonstrate higher enantioselectivity (% ee) in the epoxidation of chalcone. The origin of the unprecedented high‐spin state of the perferryl intermediate is discussed.

Corrigendum: Manganese Catalysts with C1 -Symmetric N4 Ligand for Enantioselective Epoxidation of Olefins.

In this Communication, there is an error in the Table 2, entry 20 on page 6751. In the case of asymmetric epoxidation of α-methylstyrene (2t), the GC with a CP-Chirasil-Dex CB column did not separate both enantiomers of the epoxide (3t). The wrong ee value was provided previously. The experiment was repeated and the ee value was determined by HPLC with an IC column (HPLC-separation conditions: Chiralpack IC, 25 °C, 220 nm, 98/2 hexane/i-PrOH, 1.0 ml min−1; tR=5.32 min, tR [ent]=5.68 min.), 74 % ee should be changed to 11 % ee, and a correct version of the Supporting Information is published online along with this Corrigendum. The yield of epoxide can be reduplicated, and even better (82% isolated yield). The authors apologize for this oversight. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Studying the Reductase‐Chaperone Activity of the Epoxidase Component of Styrene Monooxygenase

Styrene Monooxygenase (SMO) is a two‐component flavoprotein composed of a NADH dependent reductase (SMOB) and FAD specific epoxidase (SMOA) that catalyzes the enantioselective epoxidation of vinyl side‐chains of styrenes. The chemical versatility of SMO makes it a desirable biocatalyst for the environmentally friendly synthesis of pharmaceuticals and for the removal of styrene as a toxic contaminant of terrestrial and aqueous environments. Previous thermodynamic and kinetic studies from our lab and others have demonstrated that we can stabilize the reduced, SMO(FADred), peroxide, SMO(FADOOH) or hydroxyflavin, SMO(FADOH), intermediate states by establishing steady‐state reaction conditions in the presence of NADH alone, NADH and the substrate analog, benzene, or NADH and the substrate, styrene. In addition to its oxygenase function, SMOA has been demonstrated to have a chaperone‐function that stabilizes SMOB and greatly increases the catalytic half‐life of the reductase. The nature of this interaction remains an area of current investigation.In the present work, the protein‐protein interaction of SMOA and SMOB was evaluated by a series of pulldown assays under various conditions of catalytic turnover. In this approach, the N‐terminally histidine‐tagged SMOA (NSMOA) and SMOB protein components were prepared in buffer systems designed to stabilize the aforementioned intermediate states and loaded onto a miniature nickel (Ni2+)‐affinity column. After loading, the column was first washed with buffer containing 10 mM imidazole to remove non‐specifically bound SMOB and then washed with buffer containing 250 mM imidazole to remove SMO bound to the column by the N‐terminal histidine tag of NSMOA. The composition of eluted protein was evaluated by UV spectroscopy and polyacrylamide gel electrophoresis. We find that holoSMOB with oxidized FAD bound (SMOBFADox) passes directly through the column and does not significantly interact with NSMOA immobilized on the Ni2+ column. In the presence of NADH, FADred is efficiently transferred from SMOB to NSMOA bound to the Ni2+‐resin. Under reaction conditions designed to stabilize FADred, FADOOH, and FADOH intermediate states of SMOA, a significant fraction of the resulting apoSMOB is observed to be bound to NSMOA. The fraction of apoSMOB not retained by NSMOA on the column is observed to lose activity and aggregate in the absence of the chaperone‐like activity of SMOA with a half‐life of minutes. These results support a reaction model in which the components of SMO interact weakly with each other when oxidized FAD is bound to SMOB. A stronger binding interaction occurs between apoSMOB and the reduced, peroyiflavin, and hydroyflavin intermediates of NSMOA. These interactions serve to stabilize SMOB in a catalytically active state.Support or Funding InformationNIH GM008574This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Exploiting Designed Oxidase-Peroxygenase Mutual Benefit System for Asymmetric Cascade Reactions.

A unique P450 monooxygenase–peroxygenase mutual benefit system was designed as the core element in the construction of a biocatalytic cascade reaction sequence leading from 3-phenyl propionic acid to (R)-phenyl glycol. In this system, P450 monooxygenase (P450-BM3) and P450 peroxygenase (OleTJE) not only function as catalysts for the crucial initial reactions, they also ensure an internal in situ H2O2 recycle mechanism that avoids its accumulation and thus prevents possible toxic effects. By directed evolution of P450-BM3 as the catalyst in the enantioselective epoxidation of the styrene-intermediate, formed from 3-phenyl propionic acid, and the epoxide hydrolase ANEH for final hydrolytic ring opening, (R)-phenyl glycol and 9 derivatives thereof were synthesized from the respective carboxylic acids in one-pot processes with high enantioselectivity.

Enantioselective Epoxidation of β,β-Disubstituted Enamides with a Manganese Catalyst and Aqueous Hydrogen Peroxide.

Enantioselective epoxidation of β,β-disubstituted enamides with aqueous hydrogen peroxide and a novel manganese catalyst is described. Epoxidation is stereospecific and proceeds fast under mild conditions. Amides are disclosed as key functional groups to enable high enantioselectivity.

Enantioselective Epoxidation of β,γ-Unsaturated Carboxylic Acids by a Cooperative Binuclear Titanium Complex

Enantioselective epoxidation of β,γ-unsaturated carboxylic acids with a cooperative binuclear titanium complex has been developed to provide single diastereomer γ-lactones in high yields and enantioselectivities via intramolecular cyclization. For the success of the reaction, the proper distance between the carboxylic acid and alkene is quite important, offering the regioselective epoxidation of unsaturated carboxylic acids bearing two similar alkenes.

Synthesis of novel phase transfer catalysts derived from proline-mandelic acid/tartaric acid: their evaluation in enantioselective epoxidation and Darzen condensation

Abstract Herein, we have described the synthesis of novel chiral cyclic phase transfer catalysts (PTCs). These catalysts are synthesized from proline, mandelic acid and tartaric acid by using simple synthetic methods with competitive yields. These chiral cyclic phase transfer catalysts have been characterized by \(^{1}\)H NMR and \(^{13}\)C NMR spectroscopy, IR spectroscopy and elemental analysis. The effectiveness of these novel chiral cyclic phase transfer catalysts was evaluated by applying them in the enantioselective epoxide synthesis from \(\upalpha \), \(\upbeta \) unsaturated ketone (chalcone) and Darzens condensation. The obtained results show that the synthesized derivatives of proline, mandelic acid and tartaric acid are effectual as PTCs.Graphical AbstractSYNOPSIS This paper shares the synthesis and characterization of novel chiral phase transfer catalysts from proline, mandelic acid and tartaric acid. The synthesis of these newly explored cyclic derivatives is reported by using a simple method with competitive yields. The effectiveness of these novel chiral cyclic phase transfer catalysts was evaluated by applying them in enantioselective epoxide synthesis from \(\alpha \), \(\beta \) unsaturated chalcone and in Darzen condensation. The obtained results show that the synthesized derivatives of proline, mandelic acid and tartaric acid are effectual as PTCs and useful towards the synthesis of enantioselective epoxide.

Recoverable polystyrene-supported catalysts for Sharpless allylic alcohols epoxidations

In this work, new heterogeneous catalysts intended for enantioselective Sharpless epoxidation were prepared. The catalysts are based on Ti(IV) complexes of cross-linked swellable spherical copolymer beads of styrene with ethyl-(4-vinylbenzyl)-L-tartrate, or with ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate. These catalysts were tested in epoxidation of cinnamyl alcohols. High conversion (up to 99%) and high enantioselectivity (up to 99% ee) were achieved in the case of catalysts based on copolymers of styrene with ethyl-(4-vinylbenzyl)-L-tartrate (5, 20, 50%). Unfortunately, the copolymers lost their enantioselectivity due to the leaching of L-tartrate, caused by alcoholysis of ester bond. This problem has been overcome by replacing the ester bond by a stable keto bond. The prepared catalyst based on the copolymer of styrene with ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate (20%) achieved a similarly high conversion and enantioselectivity as in the previous case (up to 99%, up to 99% ee) and was successfully recycled.