Chiral Epoxides Research Articles

Intact-cell detection of halohydrin dehalogenase activity and preliminary assessment of marine adaptability

Halohydrin dehalogenase (HHDH) can degrade 1,2,3-trichloropropanol and other halogenated alcohols. These enzymes exhibit stereoselectivity and can be used to produce pure chiral epoxides and β-substituted alcohols of industrial value. The discovery of novel HHDHs from microorganisms in niche environments including the sea requires in-situ analysis of HHDH activity. Owing to its high practical value, HheC was selected as a target enzyme to establish an intact-cell method for detecting HHDH activity based on fluorescence spectroscopy. This approach relied on the principle that HHDH activity can change the pH in a system with weak buffering capacity, and that the fluorescence intensity of certain dyes is influenced by pH. The marine adaptability of the assay was subjected to preliminary investigation. The method is simple, rapid, and highly sensitive, as verified using gas chromatography-mass spectrometry. The detection limit of the method was OD600 (optical density at a 600 nm) of 0.025 for microorganisms containing HHDH which have activity comparable to that of HheC. With some improvements, this method holds potential for in-situ analysis and mining of marine microorganisms containing HHDH.

Total Synthesis and Biological Profiling of Putative (±)-Marinoaziridine B and (±)-N-Methyl Marinoaziridine A.

The total synthesis of two new marine natural products, (±)-marinoaziridine B 7 and (±)-N-methyl marinoaziridine A 8, was accomplished. The (±)-marinoaziridine 7 was prepared in a six-step linear sequence with a 2% overall yield. The key steps in our strategy were the preparation of the chiral epoxide (±)-5 using the Johnson Corey Chaykovsky reaction, followed by the ring-opening reaction and the Staudinger reaction. The N,N-dimethylation of compound (±)-7 gives (±)-N-methyl marinoaziridine A 8. The NMR spectra of synthetized (±)-marinoaziridine B 7 and isolated natural product did not match. The compounds are biologically characterized using relevant in silico, in vitro and in vivo methods. In silico ADMET and bioactivity profiling predicted toxic and neuromodulatory effects. In vitro screening by MTT assay on three cell lines (MCF-7, H-460, HEK293T) showed that both compounds exhibited moderate to strong antiproliferative and cytotoxic effects. Antimicrobial tests on bacterial cultures of Escherichia coli and Staphylococcus aureus demonstrated the dose-dependent inhibition of the growth of both bacteria. In vivo toxicological tests were performed on zebrafish Danio rerio and showed a significant reduction of zebrafish mortality due to N-methylation in (±)-8.

Chiral phosphoric acid-catalyzed asymmetric epoxidation of alkenyl aza-heteroarenes using hydrogen peroxide

The synthesis of chiral α-azaheteroaryl oxiranes via enantioselective catalysis is a formidable challenge due to the required complex stereoselectivity and diverse N-heterocyclic structures. These compounds play a crucial role in developing bioactive molecules, where precise chirality significantly influences biological activity. Here we show that using chiral phosphoric acid as a catalyst, our method efficiently addresses these challenges. This technique not only achieves high enantio- and diastereoselectivity but also demonstrates superior chemo- and stereocontrol during the epoxidation of alkenyl aza-heteroarenes. Our approach leverages a synergistic blend of electrostatic and hydrogen-bonding interactions, enabling the effective activation of both substrates and hydrogen peroxide. The resulting chiral oxiranes exhibit enhanced diversity and functionality, aiding the construction of complex chiral azaaryl compounds with contiguous stereocenters. Kinetic and density functional theory studies elucidate the mechanism, highlighting chiral phosphoric acid’s pivotal role in this intricate enantioselective process.

Thermostabilization and molecular dynamics simulation of NaStyA for highly enantioselective (R)-epoxidation of styrene analogues

Chiral epoxides are one of the most versatile building blocks in organic synthesis, but the asymmetric epoxidation of unfunctionalized terminal olefins remains a challenge. NaStyA is a styrene monooxygenase from Nocardia altamirensis that functions as an enantioselective epoxygenase for terminal olefins. To improve its robustness, we investigated over 50 mutants designed via evolutionary- and energy-based approaches, and successfully identified 18 beneficial mutations, which then led to a combinatorial mutant of TM18 with drastically enhanced thermostability and solvent tolerance while retaining excellent activity and enantioselectivity. Its turnover frequency and half-life at 50 °C were increased to 5- and 148-fold of the parental. In the epoxidation of terminal olefins, TM18 achieved high product yields of over 4-fold of the parental, and excellent enantiomeric excess of 99 % to >99 % ee. The analysis of trajectory data from molecular dynamics simulations further validated that the mutations in TM18 increased its local rigidity, as inferred by RMSF and PCA analyses and through novel salt bridges formation. Furthermore, TM18 displayed reduced residue cross-correlation motions. The combined effects of these mutations result in an overall improvement in the structural and dynamic thermostability of TM18.

Preparation of (S)-epichlorohydrin using a novel halohydrin dehalogenase by selective conformation adjustment.

Chiral epichlorohydrin (ECH) is an attractive intermediate for chiral pharmaceuticals and chemicals preparation. The asymmetric synthesis of chiral ECH using 1,3-dicholoro-2-propanol (1,3-DCP) catalyzed by a haloalcohol dehalogenase (HHDH) was considered as a feasible approach. However, the reverse ring opening reaction caused low optical purity of chiral ECH, thus severely restricts the industrial application of HHDHs. In the present study, a novel selective conformation adjustment strategy was developed with an engineered HheCPS to regulate the kinetic parameters of the forward and reverse reactions, based on site saturation mutation and molecular simulation analysis. The HheCPS mutant E85P was constructed with a markable change in the conformation of (S)-ECH in the substrate pocket and a slight impact on the interaction between 1,3-DCP and the enzyme, which resulted in the kinetic deceleration of the reverse reactions. Compared with HheCPS, the catalytic efficiency (kcat(S)-ECH/Km(S)-ECH) of the reversed reaction dropped to 0.23-fold (from 0.13 to 0.03mM-1s-1), while the catalytic efficiency (kcat(1,3-DCP)/Km(1,3-DCP)) of the forward reaction only reduced from 0.83 to 0.71mM-1s-1. With 40mM 1,3-DCP as substrate, HheCPS E85P catalyzed the synthesis of (S)-ECH with the yield up to 55.35% and the e.e. increased from 92.54 to >99%. Our work provided an effective approach for understanding the stereoselective catalytic mechanism as well as the green manufacturing of chiral epoxides.

Mechanism of the Sharpless Epoxidation Reaction: A DFT Study.

The Sharpless reaction is an enantioselective epoxidation of prochiral allylic alcohols that employs a Ti(IV) catalyst formed from titanium tetra(isopropoxide), Ti(O-i-Pr)4, diethyl tartrate (DET), and the oxidizing agent tert-butyl hydroperoxide. The M06-2X DFT functional with the 6-311+G(d,p) basis set has been employed to model the structures and energetics of the Sharpless epoxidation reaction. The monomeric tetracoordinate titanium(IV) diethyltartrate is thermodynamically strongly favored to dimerize, producing a pentacoordinate catalyst, [Ti(DET)(O-i-Pr)2]2, that is a more reactive chiral epoxidation catalyst. The rapid ligand exchange reactions needed to generate the "loaded" catalyst and to repeat the overall catalytic cycle have been examined and are found to have activation energies that are much lower than the epoxidation barriers. The transition structures and activation energies for the enantioselective epoxidation of allyl alcohol, trans-methyl-allyl alcohol and trans-tert-butyl-allyl alcohol with the "loaded" Sharpless catalyst, [Ti2(DET)2(O-i-Pr)2-(OAllyl)-(OOt-Bu)], are presented. The effect of the C═O···Ti interactions on the activation energies and the significance of the O-C-C═C dihedral angle on the enantioselectivity of the epoxidation reaction are discussed.

Synthesis and examination of alkoxycyanobiphenyl mesogens with a single fluorine atom at specific locations in the tail

ABSTRACT Selective fluorination is a well-established technique to tailor and improve many of the physical properties of liquid crystals. Cyanobiphenyls are among the best-known and understood thermotropic liquid crystals and have served admirably as a testbed for structural modifications that can eventually be extended to other classes of liquid crystals. As such, and expanding on our earlier work, in the present study of the alkoxycyanobiphenyl M series, the individual tail sites of both 4’-(butoxy)[1,1’-biphenyl]-4-carbonitrile (M12) and 4’-(pentyloxy)[1,1’-biphenyl]-4-carbonitrile (M15) were selectively monofluorinated in order to examine the influence on the phase behavior. These fluorinated compounds were synthesized using epoxidation, regioselective epoxide opening and deoxyfluorination chemistry. In all the fluorinated compounds in both series a monotropic nematic phase was found in the cooling cycle. It was observed that as the fluorine atom on the tail moves closer to the core, the compounds exhibited a metastable room temperature nematic mesophase, but recrystallization invariably occurred. Both enantiomers of 4’-(2-fluorobutoxy)[1,1’-biphenyl]-4-carbonitrile were synthesized in high enantiomeric purity by regioselective opening of chiral epoxide intermediates followed by deoxyfluorination, as established by chiral high-performance liquid chromatography, and their phase behaviors were determined. Moreover, the average dipole moments of the synthesized fluorine substituted derivatives were also calculated.

Asymmetric Synthesis of Saturated and Unsaturated Hydroxy Fatty Acids (HFAs) and Study of Their Antiproliferative Activity.

Hydroxy fatty acids (HFAs) constitute a class of lipids, distinguished by the presence of a hydroxyl on a long aliphatic chain. This study aims to expand our insights into HFA bioactivities, while also introducing new methods for asymmetrically synthesizing unsaturated and saturated HFAs. Simultaneously, a procedure previously established by us was adapted to generate new HFA regioisomers. An organocatalytic step was employed for the synthesis of chiral terminal epoxides, which either by alkynylation or by Grignard reagents resulted in unsaturated or saturated chiral secondary alcohols and, ultimately, HFAs. 7-(S)-Hydroxyoleic acid (7SHOA), 7-(S)-hydroxypalmitoleic acid (7SHPOA) and 7-(R)- and (S)-hydroxymargaric acids (7HMAs) were synthesized for the first time and, together with regioisomers of (R)- and (S)-hydroxypalmitic acids (HPAs) and hydroxystearic acids (HSAs), whose biological activity has not been tested so far, were studied for their antiproliferative activities. The unsaturation of the long chain, as well as an odd-numbered (C17) fatty acid chain, led to reduced activity, while the new 6-(S)-HPA regioisomer was identified as exhibiting potent antiproliferative activity in A549 cells. 6SHPA induced acetylation of histone 3 in A549 cells, without affecting acetylated α-tubulin levels, suggesting the selective inhibition of histone deacetylase (HDAC) class I enzymes, and was found to inhibit signal transducer and activator of transcription 3 (STAT3) expression.

Electroenzymatic cascade reaction on a biohybrid boosts the chiral epoxidation reaction

The chiral epoxidation of styrene and its derivatives is an important transformation that has attracted considerable scientific interest in the chemical industry. Herein, we integrate enzymatic catalysis and electrocatalysis to propose a new route for the chiral epoxidation of styrene and its derivatives. Chloroperoxidase (CPO) functionalized with 1-ethyl-3-methylimidazolium bromide (ILEMB) was loaded onto cobalt nitrogen-doped carbon nanotubes (CoN@CNT) to form a biohybrid (CPO-ILEMB/CoN@CNT). H2O2 species were generated in situ through a two-electron oxygen reduction reaction (2e–ORR) at CoN@CNT to initiate the following enzymatic epoxidation of styrene by CPO. CoN@CNT had high electroactivity for the ORR to produce H2O2 at a more positive potential, prohibiting the conversion of FeIII to FeII in the heme of CPO to maintain enzymatic activity. Meanwhile, CoN@CNT could serve as an ideal carrier for the immobilization of CPO-ILEMB. Hence, the coimmobilization of CPO-ILEMB and CoN@CNT could facilitate the diffusion of intermediate H2O2, which achieved 17 times higher efficiency than the equivalent amounts of free CPO-ILEMB in bulk solution for styrene epoxidation. Notably, an enhancement (∼45%) of chiral selectivity for the epoxidation of styrene was achieved.

Organocatalyzed Kinetic Resolution of Racemic Carboxylic Acids

AbstractThe kinetic resolution of racemic compounds is a reliable method to prepare important chiral molecules in highly optically enriched forms. Although methods for the catalytic kinetic resolution of chiral alcohols, amines, and epoxides have been extensively investigated, the kinetic resolution of racemic carboxylic acids has remained under‐developed despite the importance of chiral carboxylic acids. Excellent catalytic approaches for the kinetic resolution of racemic carboxylic acids via esterification using chiral organocatalysts have been reported by several research groups. Furthermore, the organocatalyzed lactonization of alkenyl and alkynyl carboxylic acids has also been adapted to achieve kinetic resolution. Herein, the methods typically used to produce the organocatalyzed kinetic resolution of racemic carboxylic acids are reviewed to elucidate the current status of this research.

Rhodium(I)‐Catalyzed Asymmetric Hydroarylative Cyclization of 1,6‐Diynes to Access Atropisomerically Labile Chiral Dienes

AbstractAxially chiral open‐chained olefins are an underexplored class of atropisomers, whose enantioselective synthesis represents a daunting challenge due to their relatively low racemization barrier. We herein report rhodium(I)‐catalyzed hydroarylative cyclization of 1,6‐diynes with three distinct classes of arenes, enabling highly enantioselective synthesis of a broad range of axially chiral 1,3‐dienes that are conformationally labile (ΔG≠(rac)=26.6–28.0 kcal/mol). The coupling reactions in each category proceeded with excellent enantioselectivity, regioselectivity, and Z/E selectivity under mild reaction conditions. Computational studies of the coupling of quinoline N‐oxide system reveal that the reaction proceeds via initial oxidative cyclization of the 1,6‐diyne to give a rhodacyclic intermediate, followed by σ‐bond metathesis between the arene C−H bond and the Rh−C(vinyl) bond, with subsequent C−C reductive elimination being enantio‐determining and turnover‐limiting. The DFT‐established mechanism is consistent with the experimental studies. The coupled products of quinoline N‐oxides undergo facile visible light‐induced intramolecular oxygen‐atom transfer, affording chiral epoxides with complete axial‐to‐central chirality transfer.

Rhodium(I)-Catalyzed Asymmetric Hydroarylative Cyclization of 1,6-Diynes to Access Atropisomerically Labile Chiral Dienes.

Axially chiral open-chained olefins are an underexplored class of atropisomers, whose enantioselective synthesis represents a daunting challenge due to their relatively low racemization barrier. We herein report rhodium(I)-catalyzed hydroarylative cyclization of 1,6-diynes with three distinct classes of arenes, enabling highly enantioselective synthesis of a broad range of axially chiral 1,3-dienes that are conformationally labile (ΔG≠ (rac)=26.6-28.0 kcal/mol). The coupling reactions in each category proceeded with excellent enantioselectivity, regioselectivity, and Z/E selectivity under mild reaction conditions. Computational studies of the coupling of quinoline N-oxide system reveal that the reaction proceeds via initial oxidative cyclization of the 1,6-diyne to give a rhodacyclic intermediate, followed by σ-bond metathesis between the arene C-H bond and the Rh-C(vinyl) bond, with subsequent C-C reductive elimination being enantio-determining and turnover-limiting. The DFT-established mechanism is consistent with the experimental studies. The coupled products of quinoline N-oxides undergo facile visible light-induced intramolecular oxygen-atom transfer, affording chiral epoxides with complete axial-to-central chirality transfer.

Diastereoselective Access to anti-β-Hydroxy Sulfoxides from ­Chiral Epoxides and Prochiral Sulfenate Anions: Mechanistic ­Insights, Scope, and Limitation

AbstractReported herein is a stereoselective route to anti-β-hydroxy sulfoxides through the reaction of epoxides with sulfenate anions. Extensive experimental/computational studies revealed the dual special roles of MgBr2·OEt2, serving to generate the bromohydrin alkoxide intermediate, which undergoes nucleophilic attack on the prochiral sulfenate in a diastereoselective manner. The present study has opened a general stereoselective synthetic route to anti-β-hydroxy sulfoxides.

Stereoselective total synthesis of Pestalotioprolide C and C7-epi-Pestalotioprolide C

The total synthesis of 14 membered macrolide Pestalotioprolide C and C7-epi- Pestalotioprolide C were accomplished starting from commercially available D-Mannitol by Stereoselective synthetic approach. The key reactions involved are Mitsunobu reaction, Regioselective ring opening of chiral epoxide and ring-closing metathesis reactions (RCM).

Anion-Mediated, Stereospecific Synthesis of Secondary and Tertiary Cyclopropylboronates from Chiral Epoxides and gem-Diborylalkanes.

We report a transition-metal-free deborylative cyclization approach to synthesize enantioenriched secondary and tertiary cyclopropylboronates using γ-phosphate-containing gem-diborylalkanes derived from chiral epoxides and gem-diborylalkanes. Our method enables the synthesis of a broad range of enantioenriched secondary and tertiary cyclopropylboronates in good yields with excellent stereospecificity. We demonstrate the versatility of our approach by performing a gram-scale reaction. We also show that the enantioenriched tertiary cyclopropylboronates can be transformed into a wide array of enantioenriched cyclopropane derivatives in a stereospecific boron-group transformation.

Innentitelbild: Strukturelle und mechanistische Studien zur Substrat‐ und Stereoselektivität der Indol‐Monooxygenase VpIndA1: Neue Wege für biokatalytische Epoxidationen und Sulfoxidationen (Angew. Chem. 17/2023)

A flavin-dependent group E monooxygenase („GEM“) has been characterized by crystal-structure analysis, computational and kinetic studies to reveal substrate binding and mechanistic details for the first time for this class of enzymes. Based on these data, Dirk Tischler, Norbert Sträter et al. (DOI: 10.1002/ange.202300657) redesigned the substrate binding pocket to synthesize chiral epoxides and sulfoxides with high stereoselectivity, a true treasure of molecular gems.

Inside Cover: Structural and Mechanistic Studies on Substrate and Stereoselectivity of the Indole Monooxygenase VpIndA1: New Avenues for Biocatalytic Epoxidations and Sulfoxidations (Angew. Chem. Int. Ed. 17/2023)

A flavin-dependent group E monooxygenase (“GEM”) has been characterized by crystal-structure analysis, computational and kinetic studies to reveal substrate binding and mechanistic details for the first time for this class of enzymes. Based on these data, Dirk Tischler, Norbert Sträter et al. (DOI: 10.1002/anie.202300657) redesigned the substrate binding pocket to synthesize chiral epoxides and sulfoxides with high stereoselectivity, a true treasure of molecular gems.

Asymmetric Bio-epoxidation of Unactivated Alkenes.

Optically active epoxides play important roles in pharmaceutical, agricultural and fine chemical syntheses. There are many chiral medications with pharmacodynamic activity in nature that can be synthesized by chiral epoxides. In recent years, researchers have developed a variety of biocatalysts for the asymmetric epoxidation of alkenes, which use oxygen or hydrogen peroxide as eco-friendly and low-cost oxidants, to provide better chemo-, regio- and stereoselectivity under moderate reaction conditions. In this paper, the advances, opportunities and challenges of the asymmetric epoxidation of unactive alkenes by biocatalyst are reviewed.

Total Synthesis and Biological Evaluation of Seiricuprolide and Pestalotioprolide B

AbstractThe first total syntheses of seiricuprolide and pestalotioprolide B, rare 14‐membered α,β‐unsaturated macrolides embedding a chiral epoxide motif, were achieved in 17 steps with 1.9 % and 1.6 % overall yields, respectively. Our synthesis featured the key Shiina macrolactonization to construct the 14‐membered macrocyclic skeleton, Wittig olefination to generate the (E)‐α,β‐unsaturated ester and selective reduction of advanced chiral propargylic alcohol intermediate to enable the exclusive formation of Z‐ or E‐olefin at C8−C9. Synthetic seiricuprolide and pestalotioprolide B were evaluated for their cytotoxic activity against the HCT116 colon cancer cell line as well as their inhibitory effect on CFTR chloride channel activity in human intestinal epithelial (T84) cells. Preliminary structure–activity relationship suggested that the C5−C6 β‐epoxide moiety suppressed both biological activities.

Total synthesis of (–)-negamycin from a chiral advanced epoxide

A concise route for the total synthesis of (–)-negamycin is described. Key point features (a) Cu-mediated ring-opening reaction of the advanced epoxide with vinyl magnesium chloride leads to the introduction of the carboxyl synthon; (b) Consecutive placement of diamino groups via azides was designed, including an unexpected cleavage of the silyl protecting group; (c) removal of four hydrogenolytically labile groups in one step.