Asymmetric Oxidation Of Sulfides Research Articles

Precise regulation of the substrate selectivity of Baeyer-Villiger monooxygenase to minimize overoxidation of prazole sulfoxides

Baeyer-Villiger monooxygenases (BVMOs) can catalyze the asymmetric oxidation of sulfides to valuable chiral sulfoxides, but the overoxidation of sulfoxides to undesired sulfones limits the synthetic application of BVMOs. This overoxidation is caused by insufficient substrate selectivity of BVMOs, where the desired product sulfoxide can be further oxidized. In this study, a mathematical model was constructed to quantitatively define the substrate selectivity based on the ratio of the specificity constant (kcat/Km) between sulfide and sulfoxide. The substrate selectivity of a pyrmetazole monooxygenase (AcPSMO) was precisely regulated using a structure-guided substrate tunnel engineering approach, which successfully minimized sulfoxide overoxidation. The sulfone content of variant F277L was less than 1% (mol/mol), compared with 65% for the wild-type, in the pyrmetazole oxidation reaction after 24 h. Molecular dynamics simulations and quantum mechanics/molecular mechanics studies showed that the altered H-bonding networks surrounding the flavin hydroperoxide (FADH-OOH) can modulate the mechanism and activity for sulfoxide oxidation. Furthermore, the redesigned mutants of AcPSMO were successfully applied for the controllable synthesis of other chiral prazole sulfoxides.

Enzymatic synthesis of pharmacologically relevant chiral sulfoxides by improved CbBVMO variants.

Baeyer-Villiger monooxygenases (BVMOs) are able to catalyse the asymmetric oxidation of sulfides. This property has made them attractive catalysts for the synthesis of chiral sulfoxide drugs. Here, we have designed and synthesised an exhaustive combinatorial mutant library of the previously identified lansoprazole sulfide monooxygenase CbBVMOV1. From this synthetic combinatorial mutant library, the best mutant, CbBVMOV3, was selected with a specific activity of approximately 1 U mg-1 for lansoprazole sulfoxides. We then optimised the reaction conditions of a two-phase system, achieving the enzymatic asymmetric synthesis of (R)-lansoprazole in a space-time yield of 213 g L-1 d-1 and an enantiomeric excess of >99% (R) with no detectable by-products. In addition, CbBVMOV3 showed higher activity towards other prazole sulfides. These results indicate the potential application of CbBVMO in the chiral sulfoxide drug industry.

Studies on Biosynthesis of Chiral Sulfoxides by Using P450 119 Peroxygenase and Its Mutants

AbstractVarious alkyl‐aryl and heteroaromatic sulfides were catalyzed by P450 119 peroxygenase in the presence of TBHP. P450 119 mutants were used to improve the yield and enantioselectivity of the sulfoxidation. The P450 119 mutation at 153 was found to efficiently improve activity, specificity and enantioselectivity of the sulfoxidation of heteroaromatic sulfides up to 93 % yield and 57 % ee for the first time, meanwhile its thermal stability, kinetic parameters and molecular docking were studied. The experimental results provide a potential method for the biosynthesis of chiral sulfoxides, especially for asymmetric oxidation of heteroaromatic sulfides, by enduring evolution of thermal‐stable P450 119 peroxygenase.

New Catalysts for Asymmetric Oxidation of Prochiral Sulfides Based on Vanadium Complexes with Levopimaric Acid Derivatives

New Catalysts for Asymmetric Oxidation of Prochiral Sulfides Based on Vanadium Complexes with Levopimaric Acid Derivatives

Colorimetric High-Throughput Screening Method for Directed Evolution of Prazole Sulfide Monooxygenase.

Baeyer-Villiger monooxygenases (BVMOs) are important biocatalysts for the enzymatic synthesis of chiral sulfoxides, including chiral sulfoxide-type proton pump inhibitors for the treatment of gastrointestinal diseases. However, native BVMOs are not yet suitable for practical application due to their unsatisfactory activity and thermostability. Although protein engineering approaches can help address these issues, few feasible high-throughput methods are available for the engineering of such enzymes. Herein, a colorimetric detection method to distinguish sulfoxides from sulfides and sulfones was developed for prazole sulfide monooxygenases. Directed evolution enabled by this method has identified a prazole sulfide monooxygenase CbBVMO variant with improved activity and thermostability that catalyzes the asymmetric oxidation of lansoprazole sulfide. A 71.3 % increase in conversion and 6 °C enhancement in the melting point were achieved compared with the wild-type enzyme. This new method is feasible for high-throughput screening of prazole sulfide monooxygenase variants with improved activity, thermostability, and/or substrate specificity.

Engineering of a Baeyer-Villiger monooxygenase reveals key residues for the asymmetric oxidation of omeprazole sulfide.

The structure-guided engineering of a BVMO from Rhodococcus aetherivorans (RaBVMO) was performed for its asymmetric sulfoxidation activity toward omeprazole sulfide. Based on the structural model of RaBVMO, key residues that line the substrate entrance tunnel and the binding pocket were identified, and variants were interrogated with sulfides of varied sizes. The best mutant MT2 (F442A/R337P) was obtained with a specific activity of 2.54 U g-1 and excellent enantioselectivity (≥99%, S) toward omeprazole sulfide, while wild-type RaBVMO exhibited no activity. Further structural analysis reveals that both mutations, F442A and R337P, could render an expanded substrate tunnel and an enlarged substrate binding pocket to enable easier access to the catalytic center for omeprazole sulfide. This work provides valuable guidance for engineering-related BVMOs for improved activity and enantio-preference toward bulky substrates.

Sulfoxide Reductases and Applications in Biocatalytic Preparation of Chiral Sulfoxides: A Mini-Review.

Chiral sulfoxides are valuable organosulfur compounds that have been widely used in medicinal and organic synthesis. Biocatalytic approaches for preparing chiral sulfoxides were developed in the past few years, mainly through asymmetric oxidation of prochiral sulfides. Recently, the application of sulfoxide reductase to prepare chiral sulfoxides through kinetic resolution has emerged as a new method, exhibiting extraordinary catalytic properties. This article reviews the chemical and biological functions of these sulfoxide reductases and highlights their applications in chiral sulfoxide preparation.

Protein engineering of thioether monooxygenase to improve its thermostability for enzymatic synthesis of chiral sulfoxide

• Consensus analysis and random mutagenesis were conducted to engineer a cyclohexanone monooxygenase from the Acinetobacter calcoaceticus (AcCHMO-M6) for improving its thermostability as well as specific activity. • The half-lives of the resultant mutants at 40 °C were increased up to 8.5 h as well as with up to 4-fold enhance in specific activity. • By employing the best mutant AcCHMO-M10, complete conversion (>99% conversion, >99% ee) asymmetric Kagan oxidation of omeprazole sulfide was achieved in 16 h, while the AcCHMO-M6 only afford 39% conversion. Esomeprazole, the S -enantiomer of omeprazole, is the best-selling proton pump inhibitor. In our previous work, a mutant of cyclohexanone monooxygenase from the Acinetobacter calcoaceticus (named Ac CHMO-M6) was successfully obtained through protein engineering which could catalyze the oxidation of omeprazole sulfide. However, its practical application is still hindered by the poor thermostability, especially in the up-scaled reaction process. In this work, site mutagenesis based on consensus analysis and directed evolution were used to engineer this enzyme in order to improve the stability of Ac CHMO-M6. The half-lives of the resultant mutants Ac CHMO-M9 (F29L/R444E) and Ac CHMO-M10 (F29L/R444E/A145S/G430T) at 40 °C were increased from 2.2 h to 8.5 h and 5.9 h respectively, while the corresponding T m values were increased by 7 °C and 5.3 °C in comparison to Ac CHMO-M6. The specific activity of Ac CHMO-M9 was comparable to that of Ac CHMO-M6, and the specific activity of Ac CHMO-M10 was about 4-fold that of Ac CHMO-M6. The Ac CHMO-M10 catalyzed sulfide oxidation reaction reached 100% conversion after 16 hours at 30 °C, in contrast to 39.4% conversion in the case of Ac CHMO-M6. These results show that the potential of this thioether monooxygenase can be significantly improved by protein engineering.

Synthesis and characterization of FeIII/CoIII/CuII complexes with Schiff base ligand and their hybrid proteins, SOD activity and asymmetric catalytic oxidation of sulfides

Three transition metal complexes (FeIII, CoIII, and CuII) were synthesized by hydrothermal method from bis(2-hydroxy-1-naphthalene-formaldehyde) 3,4-diaminobenzoic acid (H3L). They were characterized by infrared spectroscopy, elemental analysis, mass spectrometry. Artificial enzymes were constructed by hybridizing bovine serum albumin (BSA) with the metal complexes and characterized by MALDI-TOF-MS. The interaction of the metal complexes with BSA was studied by Raman spectroscopy, fluorescence spectroscopy, and circular dichroism spectroscopy. All the three metal complexes can cause the intrinsic fluorescence quenching of BSA, which is mainly static quenching. The addition of metal complexes changed the conformation of BSA and decreased the content of α–helix structure. The activity of SOD was measured by xanthine oxidase-NBT method. All metal complexes exhibited certain SOD activity, which increased after the formation of hybrid proteins. Asymmetric oxidation of thioether catalyzed by artificial enzymes was studied. The optimum reaction conditions were obtained by optimizing the reaction conditions. Substrate universality experiments were carried out under the optimum reaction conditions, showing good activity and enantioselectivity, with enantioselectivity up to 81%.

Asymmetric oxidation of sulfides catalyzed by (R)-6,6'-dibromo-BINOL derived titanium complex

An efficient asymmetric oxidation of sulfides was achieved using (R)-6,6'-dibromo-BINOL as chiral ligand in combination with Ti(OiPr)4 using 70% aqueous tertiary butyl hydroperoxide as oxidant. The resulting sulfoxides had high enantiopurities and good yields. A range of aryl alkyl and aryl benzyl sulfides were oxidized to the corresponding sulfoxides with 78–95% ee in 72–80% yields.

Synthesis and Characterization of Fe III/Co III/Cu II Complexes with Schiff Base Liangd and Their Hybrid Proteins, Superoxide Dismutase (SOD) Activity and Asymmetric Catalytic Oxidation of Sulfides

Three transition metal complexes (M = Fe III , Co III , Cu II ) were synthesized by hydrothermal method from bis(2-hydroxy-1-naphthalene-formaldehyde) 3,4-diaminobenzoic acid (H 3 L). They were characterized by infrared spectroscopy, elemental analysis, mass spectrometry. Artificial enzymes were constructed by hybridizing bovine serum albumin (BSA) with the metal complexes and characterized by UV-Vis and MALDI-TOF-MS. The binding of metal complexes with BSA was studied by UV-Vis spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and circular dichroism spectroscopy. The secondary structure changes of BSA caused by the binding of metal complexes were investigated. All the three metal complexes can cause the intrinsic fluorescence quenching of BSA, which is mainly static quenching. The addition of metal complexes changed the conformation of BSA and decreased the content of α–helix structure. The activity of SOD was measured by xanthine oxidase-NBT method. Antioxidant activity tests showed that all metal complexes exhibited certain SOD activity, which increased after the formation of hybrid proteins. Asymmetric oxidation of thioether catalyzed by artificial enzymes was studied. The optimum reaction conditions were obtained by optimizing the reaction conditions. Substrate universality experiments were carried out under the optimum reaction conditions, showing good activity and enantioselectivity, with enantioselectivity up to 81%.

Catalytic asymmetric oxidation of sulfides to sulfoxides using (R)-6,6$$^{\prime }$$-Diphenyl-BINOL as a chiral ligand

The chiral metal complex produced in situ from (R)-6,6 $$^{\prime }$$ -Diphenyl-BINOL and $$\mathrm{Ti}(\mathrm{O}-{} { i}-\mathrm{Pr})_{4}$$ as a catalytic system for asymmetric sulfoxidation of aryl methyl and aryl benzyl sulfides in the presence of 70% aqueous TBHP as the oxidant has been investigated. The influence of variation of reagents mole ratios, temperature, solvent and oxidant was examined, and the optimized conditions were then used to oxidize a number of aryl methyl and aryl benzyl sulfides, producing sulfoxides in high enantiopurities (up to 90% ee) and good yields (up to 81%). (R)-6,6’-Diphenyl Binol in combination with $$\hbox {Ti(O-i-Pr)}_{4}$$ has been developed as an efficient chiral ligand for the asymmetric oxidation of aryl methyl and aryl benzyl sulfides by using 70% aqueous TBHP as an oxidant. High enantioselectivity and good chemical yield have been achieved for various aryl methyl and aryl benzyl sulfides.

Synthesis and characterization of novel chiral imidazolium and pyridinium ionic liquids derived from tartaric acid and 2-oxazolidinone

Novel chiral imidazolium and pyridinium ionic liquids based on tartaric acid and 2-oxazolidinone were designed. Symmetrical dicationic ionic liquids based on tartaric acid have been synthesized and characterized. These chiral ionic liquids were designed by employing very short and simple methods. Incorporation of alkyl halide over tartaric acid and 2-oxazolidinone is an important step. N-methyl imidazole and pyridine were used for preparation of quaternary salts. These ionic liquids have been evaluated for the asymmetric sulfide oxidation. Chiral ionic liquids based on tartaric acid showed superior chiral inducing property as compare to 2-oxazolidinone based chiral ionic liquids.

Bovine serum albumin–vanadium complex hybrid: Synthesis, crystal structure, SOD activities and asymmetric catalytic oxidation of sulfides

Two symmetrical double-sided Schiff base ligands, H4L1 and H4L2, were synthesized by condensation of salicylaldehyde and 2-hydroxy-1-naphthaldehyde with oxalodihydrazine, respectively. The crystal structures of their vanadium complexes were analyzed by single crystal X-ray diffraction. The interaction between the synthesized complexes and BSA was studied by various spectral methods. The binding parameters including quenching constant KSV, binding constant K, and binding mode were obtained using fluorescence quenching method. The study shows that the quenching type was a static quenching process caused by formation of complex. The SOD activities of the complexes were determined by xanthine oxidase-NBT reduction method. The antioxidant capacity of the complexes was determined by the ABTS method. The results indicate that the activity of hybrid protein was more pronounced than that of the corresponding complex. At the same time, the hybrid protein can catalyze asymmetric oxidation of methyl phenyl sulfide.

Anionic oxide‑vanadium Schiff base amino acid complexes as potent inhibitors and as effective catalysts for sulfides oxidation: Experimental studies complemented with quantum chemical calculations

Three anionic oxide‑vanadium Schiff base N-salicylidene amino acid complexes are derived from the reaction of sodium salicylaldehyde-5-sulfonate with phenylalanine (VO-1), alanine (VO-2) or glycine (VO-3), followed by mixing with vanadyl sulfate monohydrate. Inhibition effect of VO-1, VO-2 and VO-3 is investigated for the corrosion of carbon steel in chloride acid solution. From the electrochemical measurements, VO-complexes protect carbon steel form corrosion, yielding maximum inhibition efficiency up to 94.7% in the presence of 1.0mM VO-1 inhibitor. All VO-complexes act as mixed-type inhibitors. The SEM and EDX investigations provided the appearance of an inhibitor layer encasement the steel surface. Catalytic efficiency of VO-complexes is measured in the symmetric and asymmetric oxidation of sulfides by using an aqueous H2O2. All complexes show high catalytic potentials towards sulfides oxidation. VO-1, VO-2 and VO-3 are optimized at B3LYP/GEN and B3LYP/LANL2DZ levels of theory in gas and aqueous phases. Theoretical results agree with the experimental reactivity of the VO-complexes, as corrosion inhibitors and catalysts for sulfides oxidation.

The Use Haloperoxidases in Organic Synthesis: Selected Reactions of Oxidation, Epoxydation and Sulfoxidation

<p>Haloperoxidases are ubiquitous metalloenzymes that catalyse a variety of enantioselective oxygen-transfer reactions with hydrogen peroxide or alkylperoxides. Haloperoxidases are enzymes which catalyze the reaction of oxidation, epoxidation and sulfoxidation by hydrogen peroxide. These enzymes usually contain the FeHeme moiety or vanadium as an essential constituent at their active site, however, a few haloperoxidases which lack a metal cofactor are known. This review will examine the reactivity of the different haloperoxidases, particularly the mechanism of oxidation by hydrogen peroxide, and the mechanism of oxidation and sulfoxidation, including the newly reported regioselectivity and enantioselectivity of the haloperoxidases. The structure of chloroperoxidase, the vanadium active site and the role of critical amino acid side chains for catalysis and functional biomimetic systems, with specific relevance to the mechanism of the haloperoxidase enzymes. Advances have recently been made in using them to prepare, under controlled conditions, chiral organic molecules that are valuable for the synthesis of a wide range of useful compounds. The application of biocatalytic methods in asymmetric organic synthesis is of great interest as an alternative to chemical procedures employing chiral auxiliaries. Asymmetric oxidation of prochiral sulfides to yield optically active sulfoxides has been performed by many different techniques yielding varying enantiomeric excess values. Oxygenated metabolites are compounds that are commonly found in nature and they are produced by many different organisms. The oxygen atom is incorporated into organic compounds by enzyme-catalyzed reactions with oxygen ions as the oxygen source. For over 40 years haloperoxidases were thought to be responsible for the incorporation of mainly halogen atoms into organic molecules. However, haloperoxidases lack substrate specificity and regioselectivity, and the connection of haloperoxidases with the in vivo formation of oxygenated as well as halometabolites has been demonstrated. Recently, molecular genetic investigations showed that, at least in bacteria, fungi, and other organisms a different class of halogenases is involved in halo- and oxygenated metabolite formation. These halogenases were found to require FADH2, which can be produced from FAD and NADH by unspecific flavin reductases. The FADH<sub>2</sub>-dependent halogenases and haloperoxidases show substrate specificity and regioselectivity, and their genes have been detected in many halometabolite-producing organisms, suggesting that this type of halogenating enzymes constitutes the major source for halo- and oxygenated metabolite formation in bacteria and also in other organisms. Distribution of haloperoxidases in nature also is demonstrated in this brief review.</p>

Asymmetric Oxidation of a Dihydrothienopyrimidine

A robust catalytic asymmetric oxidation of prochiral sulfide 1 has been achieved by the use of a chiral titanium–hydrobenzoin complex. Reproducible reaction conditions were identified by screening reaction parameters and DOE optimization. The enantioselectivity was found to be mostly influenced by the amount of water and the time the catalyst was aged before addition of the sulfide.

An efficient silica supported Chitosan@vanadium catalyst for asymmetric sulfoxidation and its application in the synthesis of esomeprazole

A new type of silica supported Chitosan@vanadium complex was used as a highly active heterogeneous catalyst for asymmetric oxidation of aryl alkyl sulfides. With the economic aqueous H2O2 (30%) as the oxidant, the oxidation products were obtained in high yields (up to 95%) with good enantioselectivities (up to 68% ee). It is noted that the marketed drug Nexium (first proton-pump inhibitor, esomeprazole) could be synthesized easily by the newly developed asymmetric sulfoxidation reaction. In addition, the highly active catalyst can be reused five times without losing its catalytic activity.

Enantioselective synthesis of sulfoxide using an SBA-15 supported vanadia catalyst: a computational elucidation using a QM/MM approach.

Metal catalyzed asymmetric oxidation of prochiral sulfides is one of the prevailing strategies to produce enantiopure sulfoxides. Keeping in view the reported reactivity of peroxo vanadium complexes towards asymmetric oxidation reactions, this study explores the reactivity of vanadia represented as a VO4 cluster with CH3-S-Ph through DFT computations. The mechanism of the oxidation of sulfides to sulfoxides with unsupported VO4 is thoroughly investigated. The chiral centre in the VO4 cluster is introduced by grafting it on an SBA-15 support and two conformers of the supported cluster are thus obtained. The study was extended to locate transition states for the reaction of each conformer with CH3-S-Ph. The large enantiomeric excess obtained from the energy difference of the transition states confirms the formation of enantiopure sulfoxide. Analysis of the computational results provides a rational explanation for the observed enantioselectivity, which is remarkable. The optical stability as well as asymmetry of chiral sulfoxides obtained by the current approach has been further confirmed by locating the planar transition state, through which conversion from one enantiomer to another takes place. The calculations suggest that transition between the two enantiomers of sulfoxide is hampered by sufficiently high inversion barriers.

New bis(oxazoline)–vanadyl complexes, supported by electrostatic interaction in Laponite clay, as heterogeneous catalysts for asymmetric oxidation of methyl phenyl sulfide

Chiral bis(oxazoline) ligands are used for the first time to promote the enantioselective vanadium-catalyzed oxidation of sulfides with alkyl hydroperoxides. Several bis(oxazoline)-VO complexes have been prepared and supported by cation exchange in Laponite clay. The substituent in the oxazoline ring, and the type of hydroperoxide are relevant parameters that control the activity and selectivity of the resulting catalysts. Tert-butyl hydroperoxide is more reactive but less enantioselective than cumenehydroperoxide, both with the neat and supported VO-bis(oxazoline) complexes. Activities and enantioselectivities obtained with the heterogeneous catalysts are always lower than in solution, and in general better and more consistent results are obtained with box(iPr) ligand which seems to be also the best chiral auxiliary in homogeneous phase, leading to a modest but significant enantioselectivity of 20% ee in heterogeneous phase and of 28% in homogeneous phase. The recovered of the best catalyst shows a decrease in the catalytic activity and in the enantioselectivity, in agreement with some decomplexation of the chiral ligand, whereas the supported vanadium species remain stable and recoverable.