Chemical and Enzymatic Synthetic Methods for Asymmetric Oxidation of the C–C Double Bond

A number of synthetically useful methods for asymmetric oxidation of the C–C double bond are briefly reviewed. This includes chemical asymmetric epoxidation, such as Sharpless, Julia, and Jacobsen epoxidation, asymmetric cis-dihydroxylation of olefins, monooxygenase-catalyzed epoxidation, dioxygenase-catalyzed cis-dihydroxylation of aromatics, and trans-dihydroxylation of C–C double bond catalyzed by a monooxygenase and an epoxide hydrolase. The catalytic system, substrate range, enantioselectivity, synthetic application, and scope and limitation of each method are described.     

Enantioselectivity in the Rabbit Liver Microsomal Biotransformation of Styrene and Phenylpropenes

The rabbit liver microsomal P-450 catalyzed oxidation of styrene (1a) and isomeric phenylpropenes, trans-1-phenylpropene (1b), cis-1-phenylpropene (1c) and 3-phenylpropene (1d), was investigated and the enantioselectivity of the epoxidation of the olefinic double bond was determined by checking the enantiomeric excesses of the corresponding first formed epoxides (2). These enantiomeric excesses were always modest, ranging between 7% of (1S,2S)-(2b) and 22% of (1R,2R)-(2c). In the case of (1d) a nonenantioselective hydroxylation at the benzylic-allylic C(3) was also oberved. The ratio between this hydroxylation and olefin epoxidation of (Id) was 1:2.     

Lanthanide induced shift as a tool for isomer assignment in esters of unsaturated fatty acids

Addition of Yb(fod)₃ to methyl oleate (cis) and methyl elaidate (trans) shifts the carboxylic lines of their ¹³C-NMR spectra to different extents; that of the cis isomer less than that of the trans isomer, as is to be expected.

On the same theoretical ground it can be anticipated that the opposite will occur for C-17: an effect that has been confirmed experimentally. The method is thus proposed as a means of aiding in the assignment of the cis and trans configuration in esters of fatty acids with one double bond.     

Stereochemical analysis of prenyltransferase reactions leading to (Z)- and (E)-polyprenyl chains

A feasible method was developed to determine the stereochemical direction of the C-C bond formation with respect to the face of the double bond of isopentenyl diphosphate in the prenyltransferase reactions. This method was applied to the reactions of undecaprenyl diphosphate synthase and heptaprenyl diphosphate synthase, which catalyze (Z)-prenyl chain elongation and (E)-prenyl chain elongation, respectively. In both cases, the C-C bond formation was found to take place at the si face of the double bond with elimination of one of the hydrogens of C-2 in a syn fashion.     

Palladium-catalyzed seven-membered N,O-heterocycle ring formation

3-Methyl-5-methylene-1,3-benzoxazepin-2-one (3) has been prepared starting from o-iodophenyl N-allyl-N-methylcarbamate in the presence of tetrakis(triphenylphosphine) palladium(0) as catalyst. The X-ray structure of the intermediate palladium complex (2), resulting from oxidative addition of the aromatic iodide to palladium, is reported. It consists of a square planar arrangement of aryl, iodide and two mutually cis triphenylphosphine ligands. The double bond being not coordinated to palladium, its insertion to generate the seven-membered ring almost quantitatively requires the use of thallium acetate in dimethylacetamide, otherwise intermolecular reactions predominate, leading to carbon-carbon bond formation between the aryl and double bond carbon of two and three substrate molecules, respectively, with formation, in the latter case, of a 24-membered ring.     

Biotransformation of ent-13-epi-manoyl oxides difunctionalized at C-3 and C-12 by filamentous fungi

Biotransformation of ent-3beta,12alpha-dihydroxy-13-epi-manoyl oxide with Fusarium moniliforme gave the regioselective oxidation of the hydroxyl group at C-3 and the ent-7beta-hydroxylation. The action of Gliocladium roseum in the 3,12-diketoderivative originated monohydroxylations at C-1 and C-7, both by the ent-beta face, while Rhizopus nigricans produced hydroxylation at C-7 or C-18, epoxidation of the double bond, reduction of the keto group at C-3, and combined actions as biohydroxylation at C-2/epoxidation of the double bond and hydroxylation at C-7/reduction of the keto group at C-3. In the ent-3-hydroxy-12-keto epimers, G. roseum originated monohydroxylations at C-1 and C-7 and R. nigricans originated the oxidation at C-3 as a major transformation, epoxidation of double bond and hydroxylation at C-2. Finally, in the ent-3beta-hydroxy epimer R. nigricans also originated minor hydroxylations at C-1, C-6, C-7 and C-20 and F. moniliforme produced an hydroxylation at C-7 and a dihydroxylation at C-7/C-11.     

Effect of cholesterol on the bond ordering in hydrated unsaturated lipid bilayers

Molecular dynamics computer simulations of hydrated bilayers of unsaturated phosphatidylcholines in which double bonds are in the states: 18:0/18:1(n-9)cis (PC), 18:0/18:2(n-6)cis (PC), 18:0/18:3(n-3)cis (PC), 18:0/20:4(n-6)cis (PC), and 18:0/22:6(n-3)cis in the presence of cholesterol (40 mol%) and its absence have been performed. The simulation have been performed at 303 K and 1 atm, under the conditions corresponding to the experimentally observed liquid-crystalline state of the bilayer from phosphatidylcholine. The C-C and C-H bond order parameter profiles with respect to the bilayer normal and the C-C bond orientation distribution functions have been calculated. The widths of the functions and positions of their maxima have been determined. The dependence of these characteristics on the type of the bond, the degree of unsaturation of the chain, the presence of cholesterol in the bilayer, and the bond order parameters have been analyzed.     

Molecular Dynamics Investigation of Bond Ordering of Unsaturated Lipids in Monolayers

Molecular dynamics simulations of three model lipid monolayers of 2,3-diacyl-D-glycerolipids, that contained stearoyl (18:0) in the position 3 and oleoyl (18:9cis), linoleoyl (18:26cis), or linolenoyl (18:33cis) in the position 2, have been carried out. The simulation systems consisted of 24 lipid molecules arranged in a rectangular simulation cell, with periodic boundary conditions in the surface plane. 1 nanosecond simulations were performed at T = 295 K. C-C and C-H bond order parameter profiles and the bond orientation distributions about the monolayer normal have been calculated. The relation of the distributions to the order parameters was analyzed in terms of maxima and widths of the distributions. The cis double bond order parameter is found to be higher than those of adjacent single C-C bonds. The widths of the two distributions of C-H bonds of the cis double bond segment in di- and triunsaturated molecules are much smaller than that obtained for methylene group located between the double bonds. The bond orientation distribution function widths depend on both the segment location in the chain and the segment chemical structure.     

Structural requirements of cholesterol for binding to Vibrio cholerae hemolysin

Cholesterol is necessary for the conversion of Vibrio cholerae hemolysin (VCH) monomers into oligomers in liposome membranes. Using different sterols, we determined the stereochemical structures of the VCH-binding active groups present in cholesterol. The VCH monomers are bound to cholesterol, diosgenin, campesterol, and ergosterol, which have a hydroxyl group at position C-3 (3betaOH) in the A ring and a C-C double bond between positions C-5 and C-6 (C-C Delta(5)) in the B ring. They are not bound to epicholesterol and dihydrocholesterol, which form a covalent link with a 3alphaOH group and a C-C single bond between positions C-5 and C-6, respectively. This result suggests that the 3betaOH group and the C-CDelta(5) bond in cholesterol are required for VCH monomer binding. We further examined VCH oligomer binding to cholesterol. However, this oligomer did not bind to cholesterol, suggesting that the disappearance of the cholesterol-binding potential of the VCH oligomer might be a result of the conformational change caused by the conversion of the monomer into the oligomer. VCH oligomer formation was observed in liposomes containing sterols with the 3betaOH group and the C-C Delta(5) bond, and it correlated with the binding affinity of the monomer to each sterol. Therefore, it seems likely that monomer binding to membrane sterol leads to the assembly of the monomer. However, since oligomer formation was induced by liposomes containing either epicholesterol or dihydrocholesterol, the 3betaOH group and the C-C Delta(5) bond were not essential for conversion into the oligomer.     

Squalene monooxygenase - a target for hypercholesterolemic therapy

Squalene monooxygenase catalyzes the epoxidation of C-C double bond of squalene to yield 2,3-oxidosqualene, the key step of sterol biosynthesis pathways in eukaryotes. Sterols are essential compounds of these organisms and squalene epoxidation is an important regulatory point in their synthesis. Squalene monooxygenase downregulation in vertebrates and fungi decreases synthesis of cholesterol and ergosterol, respectively, which makes squalene monooxygenase a potent and attractive target of hypercholesterolemia and antifungal therapies. Currently some fungal squalene monooxygenase inhibitors (terbinafine, naftifine, butenafine) are in clinical use, whereas mammalian enzymes' inhibitors are still under investigation. Research on new squalene monooxygenase inhibitors is important due to the prevalence of hypercholesterolemia and the lack of both sufficient and safe remedies. In this paper we (i) review data on activity and the structure of squalene monooxygenase, (ii) present its inhibitors, (iii) compare current strategies of lowering cholesterol level in blood with some of the most promising strategies, (iv) underline advantages of squalene monooxygenase as a target for hypercholesterolemia therapy, and (v) discuss safety concerns about hypercholesterolemia therapy based on inhibition of cellular cholesterol biosynthesis and potential usage of squalene monooxygenase inhibitors in clinical practice. After many years of use of statins there is some clinical evidence for their adverse effects and only partial effectiveness. Currently they are drugs of choice but are used with many restrictions, especially in case of children, elderly patients and women of childbearing potential. Certainly, for the next few years, statins will continue to be a suitable tool for cost-effective cardiovascular prevention; however research on new hypolipidemic drugs is highly desirable. We suggest that squalene monooxygenase inhibitors could become the hypocholesterolemic agents of the future.     

Synthesis, crystal structure and catalytic activity of a novel Mo(VI)–oxazoline complex in highly efficient oxidation of sulfides to sulfoxides by urea hydrogen peroxide

Novel molybdenum complex, cis-[MoO₂(phox)₂] has been synthesized and characterized by IR, ¹H NMR, elemental analyses (CHN), and X-ray molecular structure determination methods. This complex was found to be an efficient, selective catalyst for the oxidation of various sulfides to sulfoxides with urea hydrogen peroxide (UHP) in excellent yields (100% for diallylsulfide) and short reaction times (20 min) at room temperature. The catalytic system oxidizes diallylsulfide chemoselectively to its corresponding sulfoxide without any over oxidation in double bond.     

First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins by aqueous 30% H₂O₂ when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-MeIm) in aprotic solvent. Epoxides were the predominant products with trace amounts of allylic oxidation products, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide product, demonstrating that neither hydroperoxy radical (HOO·) nor oxoiron(IV) porphyrin [(TMP)Fe^IV=O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(TPFPP)Cl] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H₂O₂. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H₂O₂ in the absence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O–O bond cleavage of an iron(III) hydroperoxide porphyrin (or H₂O₂–iron(III) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O–O bond cleavage.     

A Synthetic Approach to Carbocyclic Sinefungin

Abstract

An approach to an asymmetric synthesis of carbocyclic sinefungin (cSF) 2 is proposed. The sequence, which uses an original radical based chemistry for C-C bond formation, led to the immediate precursor 18 of the protected desired compound. While the overall yield is modest, it is noticeable that only a limited number of steps are needed to obtain the target compound.     

On the identity and reactivity patterns of the "second oxidant" of the T252A mutant of cytochrome P450cam in the oxidation of 5-methylenenylcamphor

Density functional calculations show that in the absence of Compound I, the primary oxidant species of P450, the precursor species, Compound 0 (FeOOH), can effect double bond activation of 5-methylenylcamphor (1). The mechanism is initiated by homolytic cleavage of the O–O bond and formation of an OH radical bound to the Compound II species by hydrogen bonding interactions. Subsequently, the so-formed OH radical can either activate the double bond of 1 or attack the meso position of the heme en route to heme degradation. The calculations show that double bond activation is preferred over attack on the heme. Past the double bond activation, the intermediate can either lead to epoxidation or to a glycol formation. The glycol formation is predicted to be preferred, although in the P450_cam pocket the competition may be closer. Therefore, in the absence of Compound I, Compound 0 will be capable of epoxidizing double bonds. Previous studies [E. Derat, D. Kumar, H. Hirao, S. Shaik, J. Am. Chem. Soc. 128 (2006) 473–484] showed that in the case of a substrate that can undergo only C–H activation, the bound OH prefers heme degradation over hydrogen abstraction. Since the epoxidation barrier for Compound I is much smaller than that of Compound 0 (12.8 vs. 18.9 kcal/mol), when Compound I is present in the cycle, Compound 0 will be silent. As such, our mechanism explains lucidly why T252A P450_cam can epoxidize olefins like 5-methylenylcamphor but is ineffective in camphor hydroxylation [S. Jin, T.M. Makris, T. A. Bryson, S.G. Sligar, J.H. Dawson, J. Am. Chem. Soc. 125 (2003) 3406–3407]. Our calculations show that the glycol formation is a marker reaction of Compound 0 with 5-methylenylcamphor. If this product can be found in T252A P450_cam or in similar mutants of other P450 isozymes, this will constitute a more definitive proof for the action of Cpd 0 in P450 enzymes.     

Selective aliphatic and aromatic carbon-hydrogen bond activation catalysed by mutants of cytochrome p450cam

The substrate range of the haem monooxygenase cytochrome P450_cam (CYP101) has been broadened by site-directed mutagenesis. The hydroxylation selectivity of five mutants at the 96 position towards a range of substrates has been used to investigate P450_cam -substrate molecular recognition. The substrates contained aromatic and activated and unactivated aliphatic C---H bonds, as well as reactive functional groups. Diphenylmethane, diphenylether, diphenylamine, and 1,1-di-phenylethylene were all hydroxylated regiospecifically at the para position, with no attack at the amine or the olefinic double bond. With benzylcyclohexane the activated benzylic and tertiary C---H bonds were not attacked, and the reactions catalysed by the Y96G and Y96A mutants were highly diastereoselective, with 4-trans-benzylcyclohexanol constituting 90% of the products. 1-Phenyl-1-cyclohexylethylene was oxidised predominantly at the 4-position of the cyclohexane ring without attack at the olefinic double bond, and approximately equal amounts of cis- and trans-4-phenylethenylcyclohexanol were formed. These results show that P450_cam can be engineered to oxidise C---H bonds without attacking more reactive functional groups.     

Epoxidation of unsaturated fatty acids by a soluble cytochrome P-450-dependent system from Bacillus megaterium

In previous publications from this laboratory we have described a soluble, partially purified cytochrome P-450-dependent monooxygenase complex that, in the presence of NADPH and O2, catalyzes the monohydroxylation of long chain fatty acids, alcohols, and amides at the omega -1, omega -2, and omega -3 positions. We have now found that this preparation catalyzes the epoxidation as well as the hydroxylation of palmitoleic acid and a variety of other monounsaturated fatty acids. The experimental results reported here strongly support the concept that both hydroxylation and epoxidation are catalyzed by an identical cytochrome P-450 complex utilizing the same active and binding sites. Furthermore, for saturating levels of these substrates, the rate-limiting step in oxygenation does not appear to involve substrate structure. Thus, although the position and geometry of the double bond may dramatically affect the rate of epoxidation relative to hydroxylation, the combined rate of substrate oxygenation is essentially a constant independent of this ratio. Finally, we propose and present evidence for an enzyme-substrate binding model that involves polar binding of the carboxyl terminus and strong hydrophobic binding and sequestering of the terminal methyl group of the fatty acid. The three methylene carbons adjacent to the methyl group are positioned in a set geometry around the active site but the midchain region of a monounsaturated fatty acid is relatively free to interact or bind loosely with the enzyme surface in a variety of conformations. Depending on fatty acid structure, one or more of these conformations can bring the unsaturated center close enough to the active site to permit epoxidation of the double bond.     

Platinum allenylidene complexes and formation of platinum and palladium acetonitrile alkynyl complexes

The platinum(0) complex [Pt(PPh₃)₄] reacts with brominated propargylic amides and esters in benzene by oxidative addition to give trans-[Br(PPh₃)₂Pt-CC-C(O)R] complexes whereas no reaction occurs when halogenated solvents (CH₂Cl₂, CHCl₃) are used. The cis-ligands PPh₃ can be replaced by P(ⁱPr)₃ and the bromide by trifluoroacetate. O-Alkylation of those trans-[X(PR′₃)₂Pt-CC-C(O)R] complexes (X = Br, CF₃COO; R′ = Ph, ⁱPr) derived from propargylic amides with MeOTf or [Me₃O]BF₄ in CH₂Cl₂ gives the first cationic monoallenylidene complexes of platinum, trans-[X(PR′₃)₂PtCCC(OMe)NR₂]⁺Y⁻ (Y = OTf, BF₄). In contrast, trans-[Br(PPh₃)₂Pt-CC-C(O)OMenthyl] derived from a propargylic ester does not react with MeOTf in CH₂Cl₂. However, in acetonitrile instead of O-methylation the substitution of acetonitrile for the bromide ligand to yield the cationic acetonitrile alkynyl platinum complex trans-[MeCN(PPh₃)₂Pt-CC-C(O)OMenthyl]⁺OTf⁻ is observed. The related palladium complexes trans-[X(PR′₃)₂Pd-CC-C(O)OR] (X = Br, CF₃COO; R′ = Ph, ⁱPr, R = menthyl, Et) react with MeOTf or [Et₃O]BF₄ analogously affording trans-[MeCN(PR′₃)₂Pd-CC-C(O)OR]⁺Y⁻ (Y = OTf, BF₄).     

Preparative fractionation of triglyceride mixtures according to acyl carbon number, using hydroxyalkoxypropyl Sephadex

The present paper describes the use of hydroxyalkoxypropyl Sephadex in a liquid chromatography system. When the column is held at 40 degrees C, and when elution is made with a linear gradient of two solvents, an excellent separation of saturated triglycerides in the region C(9)-C(56) is obtained in 24 hr, even with sample loads as high as 0.5 g/cm(2) of column. Triglycerides containing unsaturated fatty acids are eluted more rapidly than their saturated homologs, one C-C double bond being equivalent to -1.42 fatty acid carbon atoms.     

Xenobiotic oxidation by cytochrome P-450-enriched extracts of Streptomyces griseus

Crude extracts of Streptomyces griseus grown on soybean flour-enriched medium contain high levels of cytochrome P-450. The cytochrome P-450-enriched fractions, obtained by ammonium sulfate fractionation (30-50% saturation), catalyze the NADPH-dependent oxidation of a variety of xenobiotics when complemented with both spinach ferredoxin:NADP+ oxidoreductase and spinach ferredoxin. Reactions observed are aromatic, benzylic and alicyclic hydroxylations, O-dealkylation, non-aromatic double bond epoxidation, N-oxidation and N-acetylation.     

A double helical dicopper(II) complex with biacetyl bis(benzoylhydrazone)

A dinuclear double helicate of copper(II) with the Schiff base biacetyl bis(benzoylhydrazone) (H₂babh) is reported. The solid state physical properties of the dicopper(II) species are compared with that of its mononuclear precursor. The O,N,N,O-donor babh²⁻ shows an unusual bridging coordination mode in [Cu₂(μ-babh)₂]. Each metal center in the complex is in very similar tetrahedrally distorted square-planar N₂O₂ coordination sphere assembled by the two halves of the two babh²⁻. To accommodate the metal centers, two halves of each ligand are twisted along the (CH₃)C-C(CH₃) bond.