Enantioselective formation of an α, β-epoxy alcohol by reaction of methyl 13(S)-hydroperoxy-9(Z), 11(E)-octadecadienoate with titanium isopropoxide

Methyl 11(R), 12(R)-epoxy-13(S)-hydroxy-9(Z)-octadecenoate (threo isomer) was generated from linoleic acid by the sequential action of an enzyme and two chemical reagents. Linoleic acid was treated with lipoxygenase to yield its corresponding hydroperoxide [13(S)-hydroperoxy-9(Z), 11(E)-octadecadienoic acid]. After methylation with CH₂N₂, the hydroperoxide was treated with titanium (IV) isopropoxide [Ti(O-i-Pr)₄] at 5°C for 1 h. The products were separated by normal-phase high-performance liquid chromatography and characterized with gas chromatography-mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Approximately 30% of the product was methyl 13(S)-hydroxy-9(Z), 11(E)-octadecadienoate. Over 60% of the isolated product was methyl 11(R), 12(R)-epoxy-13(S)-hydroxy-9(Z)-octadecenoate. After quenching Ti(O-i-Pr)₄ with water, the spent catalyst could be removed from the fatty products by partitioning between CH₂Cl₂ and water. These results demonstrate that Ti(O-i-Pr)₄ selectively promotes the formation of an α-epoxide with the threo configuration. It was critically important to start with dry methyl 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoate because the presence of small amounts of water in the reaction medium resulted in the complete hydrolysis of epoxy alcohol to trihydroxy products.     

Biosynthesis of Oxylipins by Rhizoctonia solani with Allene Oxide and Oleate 8S,9S‐Diol Synthase Activities

Oxylipin biosynthesis by fungi is catalyzed by both the lipoxygenase (LOX) family and the linoleate diol synthase (LDS) family of the peroxidase‐cyclooxygenase superfamily. Rhizoctonia solani, a pathogenic fungus, infects staple crops such as potato and rice. The genome predicts three genes with 9–13 introns, which code for tentative dioxygenase (DOX)–cytochrome P450 fusion enzymes of the LDS family, and one gene, which might code for a 13‐LOX. The objective was to determine whether mycelia or nitrogen powder of mycelia oxidized unsaturated C₁₈ fatty acids to LDS‐ or LOX‐related metabolites. Mycelia converted 18:2n‐6 to 8R‐hydroxy‐9Z,12Z‐octadecadienoic acid and to an α‐ketol, 9S‐hydroxy‐10‐oxo‐12Z‐octadecenoic acid. In addition to these metabolites, nitrogen powder of mycelia oxidized 18:2n‐6 to 9S‐hydroperoxy‐10E, 12Z‐octadecadienoic, and 13S‐hydroperoxy‐9Z,11E‐octadecadienoic acids; the latter was likely formed by the predicted 13‐LOX. 18:1n‐9 was transformed into 8S‐hydroperoxy‐9Z‐octadecenoic and into 8S,9S‐dihydroxy‐10E‐octadecenoic acids, indicating the expression of 8,9‐diol synthase. The allene oxide, 9S(10)epoxy‐10,12Z‐octadecadienoic acid, is unstable and decomposes rapidly to the α‐ketol above, indicating biosynthesis by 9S‐DOX‐allene oxide synthase. This allene oxide and α‐ketol are also formed by potato stolons, which illustrates catalytic similarities between the plant host and fungal pathogen.     

Enantioselective conversion of linoleate hydroperoxide to an α, β-epoxy alcohol by niobium ethoxide

Niobium (V) ethoxide [Nb(OC₂H₅)₅] catalyzed the rearrangement of methyl 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoate (Me-HPODE) to epoxy hydroxy isomers. At low temperature (5°C) in aprotic solvent, Me-HPODE was converted to the diastereomeric α, β-epoxy alcohols, methyl 11(R,S),12(R,S)-epoxy-13(S)-hydroxy-9(Z)octadecenoate. These products are referred to as oxylipids and structurally resemble those obtained from the vanadium- and epoxygenase-catalyzed rearrangement of Me-HPODE but are distinct from products obtained from ferrous iron-, hematin-, and hemoglobin-catalyzed rearrangements. Because the product of the niobium-catalyzed rearrangement of Me-HPODE was predominantly the erythro diastereomer, the rearrangement is distinguished from that produced by a titanium catalyst, in which the threo diastereomer [methyl 11(R), 12(R)-epoxy-13(S)-hydroxy-9(Z)-octadecenoate] predominates, and from that produced by a vanadium catalyst, in which both diastereomers are produced in equal proportion. The synthesis of alcohol epoxide by Nb(OC₂H₅)₅ was inhibited by traces of water, but inclusion of molecular sieves in the reaction medium did not improve yield, as the alcohol epoxide rearranged to ketonic materials.     

Synthesis and Physicochemical Properties of Partially and Fully Epoxidized Methyl Linoleate Derived from Jatropha curcas Oil

Partial epoxidation of methyl linoleate was carried out at room temperature (30 °C) using a methyltrioxorhenium catalyst in the presence of pyridine and urea‐hydrogen peroxide. Full epoxidation of methyl linoleate was carried out using the Prilezhaev method. The reactions were monitored using the oxirane oxygen content value. The products from partial and full epoxidation were analyzed using GC‐FID, FTIR, NMR and GC–MS. Methyl 9,10‐epoxy‐12Z‐octadecenoate and methyl 12,13‐epoxy‐9Z‐octadecenoate were obtained as the major products from partial epoxidation, with a percent yield of 46 %. The product from full epoxidation afforded 97 % yield with methyl 9,10‐12,13‐diepoxyoctadecanoate as the major component. Physicochemical properties such as kinematic viscosity, viscosity index, crystallization temperature and oxidative stability were examined. Fully epoxidized methyl linoleate exhibits superior kinematic viscosity and oxidative stability due to the complete conversion of double bonds to epoxy groups. Partially epoxidized methyl linoleate exhibits intermediate kinematic viscosity, viscosity index, crystallization temperature and oxidative stability.     

Epoxidation of a conjugated linoleic acid isomer

Epoxidation of methyl (9Z, 11E)‐octadecadienoate ( 1 ) with various epoxidizing agents viz. m‐chloroperoxybenzoic acid, dimethyl dioxirane, methyltrioxorhenium/hydrogen peroxide, potassium peroxomonosulfate (Oxone, 2KHSO₅ · KHSO₄ · K₂SO₄)/tetrahydrothiopyran‐4‐one, and Novozyme 435/hydrogen peroxide is described. The reactions furnished the corresponding mono‐epoxy [methyl (11, 12E)‐epoxy‐(9Z)‐octadecenoate ( 2 ) and methyl (9, 10Z)‐epoxy‐(11E)‐octadecenoate ( 3 )] and a mixture of diastereomers of syn‐ and anti‐diepoxy‐stearate [methyl (9, 10Z;11, 12E)‐diepoxystearate ( 4a‐4d )], which were identified by a combination of spectroscopic and spectrometric analyses.     

Cerebrosides from a Far‐Eastern Glass Sponge Aulosaccus sp.

Nine new cerebrosides 1a–d , 2a , 2b , 3a–c were found in the extract of a Far‐Eastern glass sponge Aulosaccus sp. (class Hexactinellida). These β‐d ‐glucopyranosyl‐(1 → 1)‐ceramides contain sphingoid bases (2S,3S,4R,11Z)‐2‐aminoeicos‐11‐ene‐1,3,4‐triol (in 1a – d ), (2S,3S,4R,13Z)‐2‐aminoeicos‐13‐ene‐1,3,4‐triol (in 2a , b ) and (2S,3S,4R,13S*,14R*)‐2‐amino‐13,14‐methylene‐eicosane‐1,3,4‐triol (in 3a – c ), which are N‐acylated by (2R,15Z)‐2‐hydroxydocos‐15‐enoic (in 1a , 2a , 3a ), (2R,16Z)‐2‐hydroxytricos‐16‐enoic (in 1b , 2b , 3b ), (2R,17Z)‐2‐hydroxytetracos‐17‐enoic (in 1d ) and (2R)‐2‐hydroxydocosanoic (in 1c , 3c ) acids. The monoenoic and cyclopropane‐containing sphingoid bases of compounds 1a–d , 2a , 2b , 3a–c have not been found previously in any sphingolipids. The structures of the cerebrosides were elucidated on the basis of ¹H‐, ¹³C‐NMR spectroscopy, mass spectrometry, optical rotation data and chemical transformations. A simplified method for the assignment of the absolute configuration of 2‐hydroxy fatty acids by GC analysis of their (2R)‐ and (2S)‐oct‐2‐yl esters was proposed.     

Mechanism of linoleic acid hydroperoxide reaction with alkali

Treatment of (13S,9Z,11E)-13-hydroperoxy-9,11-octadecadienoic acid (13S-HPODE) with strong alkali resulted in the formation of about 75% of the corresponding hydroxy acid, (13S,9Z,11E)-13-hydroxyl-9,11-octadecadienoic acid (13S-HPODE), and the remaining 25% of products was a mixture of several oxidized fatty acids, the majority of which was formed from (9Z,11R,S,12S,R)-13-oxo-11, 12-epoxy-9-octadecenoic acid by Favorskii rearrangement (Gardner, H.W.,et al. (1993)Lipids 28, 487–495). In the present work, isotope experiments were completed in order to get further information about the initial steps of the alkali-promoted decomposition of 13S-HPODE.1. Reaction of [hydroperoxy-¹⁸O₂]13S-HPODE with 5 M KOH resulted in the formation of [hydroxy-¹⁸O]13S-HPODE and [epoxy-¹⁸O](9Z,11R,S,12S,R)-13-oxo-11, 12-epoxy-9-octadecenoic acid;2. treatment of a mixture of [U-¹⁴C]13S-HPODE and [hydroperoxy-¹⁸O₂]13S-HPODE with KOH and analysis of the reaction product by radio-TLC showed that 13S-HPODE was stable under the reaction conditions and did not serve as precursor of other products;3. reaction of a mixture of [U-¹⁴C]13-oxo-9,11-octadecadienoic acid (13-OODE) and [hydroperoxy-¹⁸O₂]13S-HPODE with KOH resulted in the formation of [U-¹⁴C-epoxy-¹⁸O](9Z,11R,S,12S,R)-13-oxo-11,12-epoxy-9-octadecenoic acid;4. treatment of a mixture of [hydroperoxy-¹⁸O₂] 13S-HPODE and [carboxyl-¹⁸O₁]13S-HPODE with KOH afforded (9Z,11R,S,12S,R)-13-oxo-11,12-epoxy-9-octadecenoic acid having an¹⁸O-labeling pattern which was in agreement with its formation by intermolecular epoxidation. It was concluded that (9Z,11R,S,12S,R)-13-oxo-11, 12-epoxy-9-octadecenoic acid is formed from 13S-HPODE by a sequence involving initial dehydration into the α,β-unsaturated ketone, 13-OODE, followed by epoxidation of the Δ¹¹ double bond of this compound by the peroxyl anion of a second molecule of 13S-HPODE. Rapid conversion of hydroperoxides by alkali appreared to require the presence of an α,β-unsaturated ketone intermediate as an oxygen acceptor. This was supported by experiments with a saturated hydroperoxide, methyl 12-hydroperoxyoctadecanoate, which was found to be much more resistant to alkali-promoted conversion than 13S-HPODE.     

Regio- and stereochemical analysis of trihydroxyoctadecenoic acids derived from linoleic acid 9- and 13-hydroperoxides

The methyl esters of 9S,10S,13R-trihydroxy-11E-octadecenoic acid, 9S,10R,13S-trihydroxy-11E-octadecenoic acid, and 9S,10R,13R-trihydroxy-11E-octadecenoic acid were prepared from 9S-hydroperoxy-10E,12Z-octadecadienoic acidvia the epoxy alcohols methyl 10R,11R-epoxy-9S-hydroxy-12Z-octadecenoate and methyl 10S,11S-epoxy-9S-hydroxy-12Z-octadecenoate. The trihydroxyesters, as well as four stereoisomeric methyl 9,12,13-trihydroxy-10E-octadecenoates earlier prepared [Hamberg, M.,Chem. Phys. Lipids 43, 55–67 (1987)], were characterized by thin-layer chromatography, gas-liquid chromatography, mass spectrometry, and by chemical degradation. Availability of these chemically defined trihydroxyoctadecenoates made it possible to design a method for regio- and stereochemical analysis of 9,10,13- and 9,12,13-trihydroxyoctadecenoic acids obtained from various sources. Application of the method revealed that the mixture of 9,10,13- and 9,12,13-trihydroxyoctadecenoic acids formed during autoxidation of linoleic acid in aqueous medium contained comparable amounts of the sixteen possible regio- and stereoisomers. Furthermore, hydrolysis of the allylic epoxy alcohol, methyl 9S,10R-epoxy-13S-hydroxy-11E-octadecenoate, yielded a major trihydroxyoctadecenoate,i.e., methyl 9S,10S,13S-trihydroxyl-11E-octadecenoate, together with smaller amounts of methyl 9S,10R,13S-trihydroxy-11E-octadecenoate, methyl 9S,12R,13S-trihydroxy-10E-octadecenoate, and methyl 9S,12S,13S-trihydroxy-10E-octadecenoate.     

Structural Analysis of the Minor Cerebrosides from a Glass Sponge Aulosaccus sp.

The minor cerebrosides from a Far‐Eastern glass sponge Aulosaccus sp. were analyzed as constituents of some multi‐component RP‐HPLC fractions. The structures of eighteen new and one known cerebrosides were elucidated on the basis of NMR spectroscopy, mass spectrometry, optical rotation data and chemical transformations. These β‐D‐glucopyranosyl‐(1→1)‐ceramides contain sphingoid bases N‐acylated with straight‐chain (2R)‐2‐hydroxy fatty acids, namely, (2S,3S,4R,11Z)‐2‐aminoeicos‐11‐ene‐1,3,4‐triol, acylated with 15E‐22:1, 16Z‐21:1, 15Z‐21:1, 15Z‐20:1, 15E‐20:1, 19:0, 18:0 acids, (2S,3S,4R)‐2‐amino‐13‐methyltetradecane‐1,3,4‐triol—with 19Z‐26:1, 16Z‐23:1, 23:0, 22:0 acids, (2S,3S,4R)‐2‐amino‐14‐methylpentadecane‐1,3,4‐triol—with 16Z‐23:1, 16E‐23:1, 15Z‐22:1, 22:0 acids, (2S,3S,4R)‐2‐amino‐14‐methylhexadecane‐1,3,4‐triol, linked to 16Z‐23:1, 15Z‐22:1 acids, (2S,3S,4R)‐2‐amino‐9‐methylhexadecane‐1,3,4‐triol—to 16Z‐23:1 acid, and (2S,3S,4R)‐2‐aminohexadecane‐1,3,4‐triol, attached to 15Z‐22:1 acid. The 13‐methyl and 9‐methyl‐branched trihydroxy sphingoid base backbones (C₁₅ and C₁₇, respectively) have not been found previously in sphingolipids. The ceramide parts, containing other backbones, present new variants of N‐acylation of the marine sphingoid bases with the 2‐hydroxy fatty acids. The combination of the instrumental and chemical methods used in this study improved the efficiency of the structural analysis of such complex cerebroside mixtures that gave more detailed information on glycosphingolipid metabolism of the organism.     

Synthesis of poly(methylphenylsiloxane) rich in syndiotacticity by Rh‐catalysed stereoselective cross‐dehydrocoupling polymerization of optically active 1,3‐dimethyl‐1,3‐diphenyldisiloxane derivatives

Cross‐dehydrocoupling reactions of (R)‐methyl(1‐naphthyl)phenylsilane (>99%ee) with (S)‐methyl(1‐naphthyl)phenylsilanol (>99% ee) proceeded with 82–99% retention of configuration of chiral silicon centres in the presence of various Rh‐catalysts. Cross‐dehydrocoupling polymerization of 1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxanediol with 1,3‐dihydro‐1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxane gave poly(methylphenylsiloxane) of moderate molecular weight in toluene at 60 °C in the presence of [RhCl(cod)]₂ (5.0 mol%) and triethylamine (1.0 equivalent). Assignment of the triad signals of the resulting polymer was made by ¹H NMR spectroscopy of the methyl proton (I = 0.04, H = 0.09 and S = 0.14 ppm) and ¹³C NMR spectroscopy of the ipso carbon of the phenyl group (S = 136.7, H = 136.9, and I = 137.1 ppm). Although the reaction of optically pure (S,S)‐1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxanediol with 1,3‐dihydro‐1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxane [(S,S):(S,R):(R,R)] = 84:16:0] gave a poly(methylphenylsiloxane) of rather low molecular weight, its triad tacticity was found to be rich in syndiotacticity (S:H:I = 60:32:8) by ¹³C NMR spectroscopy. © 2001 Society of Chemical Industry     

Metabolic profile of linoleic acid in stored apples: Formation of 13(R)-hydroxy-9(Z),11(E)-octadecadienoic acid

During our ongoing project on the biosynthesis of R-(+)-octane-1,3-diol the metabolism of linoleic acid was investigated in stored apples after injection of [1-¹⁴C]-, [9,10,12,13-³H]-, ¹³C₁₈- and unlabeled substrates. After different incubation periods the products were analyzed by gas chromatography-mass spectroscopy (MS), high-performance liquid chromatography-MS/MS, and HPLC-radiodetection. Water-soluble compounds and CO₂ were the major products whereas 13(R)-hydroxy- and 13-keto-9(Z),11(E)-octadecadienoic acid, 9(S)-hydroxy-and 9-keto-10(E),12(Z)-octadecadienoic acid, and the stereoisomers of the 9,10,13- and 9,12,13-trihydroxyoctadecenoic acids were identified as the major metabolites found in the diethyl ether extracts. Hydroperoxides were not detected. The ratio of 9/13-hydroxy- and 9/13-keto-octadecadienoic acid was 1∶4 and 1∶10, respectively. Chiral phase HPLC of the methyl ester derivatives showed enantiomeric excesses of 75% (R) and 65% (S) for 13-hydroxy-9(Z),11(E)-octadecadienoic acid and 9-hydroxy-10(E),12(Z)-octadecadienoic acid, respectively. Enzymatically active homogenates from apples were able to convert unlabeled linoleic acid into the metabolites. Radiotracer experiments showed that the transformation products of linoleic acid were converted into (R)-octane-1,3-diol. 13(R)-Hydroxy-9(Z), 11(E)-octadecadienoic acid is probably formed in stored apples from 13-hydroperoxy-9(Z),11(E)-octadecadienoic acid. It is possible that the S-enantiomer of the hydroperoxide is primarily degraded by enzymatic side reactions, resulting in an enrichment of the R-enantiomer and thus leading to the formation of 13(R)-hydroxy-9(Z),11(E)-octadecadienoic acid.     

A New Class of 3′‐Sulfonyl BINAPHOS Ligands: Modulation of Activity and Selectivity in Asymmetric Palladium‐Catalysed Hydrophosphorylation of Styrene

Palladium‐catalysed monophosphorylation of (R)‐2,2′‐bisperfluoroalkanesulfonates of BINOL (R^F=CF₃ or C₄F₉) by a diaryl phosphinate [Ar₂P(O)H] followed by phosphine oxide reduction (Cl₃SiH) then lithium diisopropylamide‐mediated anionic thia‐Fries rearrangement furnishes enantiomerically‐pure (R)‐2′‐diarylphosphino‐2′‐hydroxy‐3′‐perfluoralkanesulfonyl‐1,1′‐binaphthalenes [(R)‐ 8ab and (R)‐ 8g–j ], which can be further diversified by Grignard reagent (RMgX)‐mediated CF₃‐displacement [→(R)‐ 8c–f ]. Coupling of (R)‐ 8a–j with (S)‐1,1′‐binaphthalene‐2,2′‐dioxychlorophosphine (S)‐ 9 generates 3′‐sulfonyl BINAPHOS ligands (R,S)‐ 10a–j in good yields (43–82%). These new ligands are of utlility in the asymmetric hydrophosphonylation of styrene ( 1 ) by 4,4,5,5‐tetramethyl‐1,3,2‐dioxaphospholane 2‐oxide ( 2 ), for which a combination of the chiral ligands with either [Pd(Cp)(allyl)] or [Pd(allyl)(MeCN)₂]⁺/NaCH(CO₂Me)₂ proves to be a convenient and active pre‐catalyst system. A combination of an electron‐rich phosphine moiety and an electron‐deficient 3′‐sulfone moiety provides the best enantioselectivity to date for this process, affording the branched 2‐phenethenephosphonate, (−)‐iso‐ 3 , in up to 74% ee with ligand (R,S)‐ 10i , where Ar=p‐anisyl and the 3′‐SO₂R group is triflone.     

Improved conversion of 6‐endo‐tosyloxybicyclo[2.2.2]octan‐2‐ones into 6‐exo‐acetoxy and 6‐exo‐benzoyloxybicyclo[2.2.2]octan‐2‐ones

The reaction conditions for the conversion of 6‐endo‐tosyloxybicyclo[2.2.2]octan‐2‐one ( 7b ) into 6‐exo‐acetoxy ( 8b ) and 6‐exo‐benzoyloxybicyclo[2.2.2]octan‐2‐one ( 8a ), respectively, were improved. Thus known 6‐endo‐tosyloxy‐bicyclo[2.2.2]octan‐2‐ones (+)‐(1RS,6SR,8SR,11RS)‐11‐[(4‐toluenesulfonyl)oxy]tricyclo[6.2.2.0^1,6]dodecan‐9‐one ( 1a ), 13‐methyl‐15‐oxo‐9β,13b‐ethano‐9β‐podocarpan‐12β‐yl‐4‐toluenesulfonate ( 3a ), and methyl (13R)‐16‐oxo‐13‐[(4‐tolylsulfonyl)oxy]‐17‐noratisan‐18‐oate ( 5 ), were converted,in comparable yields, as previously recorded, but much shorter times, into (+)‐(1RS,6SR,8SR,11SR)‐11‐(benzoyloxy) tricyclo[6.2.2.0^1,6]dodecan‐9‐one ( 2 ), 13‐methyl‐15‐oxo‐9β,13β‐ethano‐9β‐podocarpan‐12α‐yl benzoate ( 4 ), and methyl (13S)‐13‐(benzoyloxy)‐16‐oxo‐17‐noratisan‐18‐oate ( 6 ), respectively.     

An epoxy alcohol synthase pathway in higher plants: Biosynthesis of antifungal trihydroxy oxylipins in leaves of potato

[1-¹⁴C]Linoleic acid was incubated with a whole homogenate preparation of potato leaves (Solanum tuberosum 1., var. Bintje). The methyl-esterified product was subjected to straight-phase high-performance liquid chromatography and was found to contain four major radioactive oxidation products, i.e., the epoxy alcohols methyl 10(S), 11(S)-epoxy-9(S)-hydroxy-12(Z)-octadecenoate (14% of the recovered radioactivity) and methyl 12(R), 13(S)-epoxy-9(S)-hydroxy-10(E)-octadecenoate (14%), and the trihydroxy derivatives methyl 9(S), 10(S), 11(R)-trihydroxy-12(Z)-octadecenoate (18%) and methyl 9(S), 12(S), 13(S)-trihydroxy-10(E)-octadecenoate (30%). The structures and stereochemical configurations of these oxylipins were determined by chemical and spectral methods using the authentic compounds as references. Incubations performed in the presence of glutathione peroxidase revealed that lipoxygenase activity of potato leaves generated the 9- and 13-hydroperoxides of linoleic acid in a ratio of 95∶5. Separate incubations of these hydroperoxides showed that linoleic acid 9(S)-hydroperoxide was metabolized into epoxy alcohols by particle-bound epoxy alcohol synthase activity, whereas the 13-hydroperoxide was metabolized into α- and γ-ketols by a particle-bound allene oxide synthase. It was concluded that the main pathway of linoleic acid metabolism in potato leaves involved 9-lipoxygenase-catalyzed oxygenation into linoleic acid 9(S)-hydroperoxide followed by rapid conversion of this hydroperoxide into epoxy alcohols and a slower, epoxide hydrolase-catalyzed conversion of the epoxy alcohols into trihydroxyoctadecenoates. Trihydroxy derivatives of linoleic and linolenic acids have previously been reported to be growth-inhibitory to plant-pathogenic fungi, and a role of the new pathway of linoleic acid oxidation in defense reactions against pathogens is conceivable.     

A Tandem Enzymatic sp2‐C‐Methylation Process: Coupling in Situ S‐Adenosyl‐l‐Methionine Formation with Methyl Transfer

A one‐pot, two‐step biocatalytic platform for the regiospecfic C‐methylation and C‐ethylation of aromatic substrates is described. The tandem process utilises SalL (Salinospora tropica) for in situ synthesis of S‐adenosyl‐l ‐methionine (SAM), followed by alkylation of aromatic substrates by the C‐methyltransferase NovO (Streptomyces spheroides). The application of this methodology is demonstrated for the regiospecific labelling of aromatic substrates by the transfer of methyl, ethyl and isotopically labelled ¹³CH_3, ¹³CD₃ and CD₃ groups from their corresponding SAM analogues formed in situ.     

Synthesis and Evaluation of Novel Acyclic Nucleoside Phosphonates as Inhibitors of Plasmodium falciparum and Human 6‐Oxopurine Phosphoribosyltransferases

Acyclic nucleoside phosphonates (ANPs) are a promising class of antimalarial therapeutic drug leads that exhibit a wide variety of K_i values for Plasmodium falciparum (Pf) and human hypoxanthine‐guanine‐(xanthine) phosphoribosyltransferases [HG(X)PRTs]. A novel series of ANPs, analogues of previously reported 2‐(phosphonoethoxy)ethyl (PEE) and (R,S)‐3‐hydroxy‐2‐(phosphonomethoxy)propyl (HPMP) derivatives, were designed and synthesized to evaluate their ability to act as inhibitors of these enzymes and to extend our ongoing antimalarial structure–activity relationship studies. In this series, (S)‐3‐hydroxy‐2‐(phosphonoethoxy)propyl (HPEP), (S)‐2‐(phosphonomethoxy)propanoic acid (CPME), or (S)‐2‐(phosphonoethoxy)propanoic acid (CPEE) are the acyclic moieties. Of this group, (S)‐3‐hydroxy‐2‐(phosphonoethoxy)propylguanine (HPEPG) exhibits the highest potency for PfHGXPRT, with a K_i value of 0.1 μM and a K_i value for human HGPRT of 0.6 μM . The crystal structures of HPEPG and HPEPHx (where Hx=hypoxanthine) in complex with human HGPRT were obtained, showing specific interactions with active site residues. Prodrugs for the HPEP and CPEE analogues were synthesized and tested for in vitro antimalarial activity. The lowest IC₅₀ value (22 μM ) in a chloroquine‐resistant strain was observed for the bis‐amidate prodrug of HPEPG.     

Enzymes in Organic Chemistry, 11:[1] Hydrolase‐Catalyzed Resolution of α‐ and β‐Hydroxyphosphonates and Synthesis of Chiral,Non‐Racemic β‐Aminophosphonic Acids

The enantioselective acylation of racemic diisopropyl α‐ and β‐hydroxyphosphonates by hydrolases in t‐butyl methyl ether with isopropenyl acetate as acyl donor is limited by the narrow substrate specificity of the enzymes. High enantiomeric excesses (up to 99%) were obtained for the acetates of (S)‐diisopropyl 1‐hydroxy‐(2‐thienyl)methyl‐, 1‐hydroxyethyl‐ and 1‐hydroxyhexylphosphonate and (R)‐diisopropyl 2‐hydroxypropylphosphonate. The hydrolysis of a variety of β‐chloroacetoxyphosphonates by the lipase from Candida cylindracea and protease subtilisin in a biphasic system gives (S)‐β‐hydroxyphosphonates (ee 51–92%) enantioselectively. (S)‐2‐Phenyl‐2‐hydroxyethyl‐ and (S)‐3‐methyl‐2‐hydroxybutylphosphonates (ee 96% and 99%, respectively) were transformed into (R)‐2‐aminophosphonic acids of the same ee.     

Design of gem‐Difluoro‐bis‐Tetrahydrofuran as P2 Ligand for HIV‐1 Protease Inhibitors to Improve Brain Penetration: Synthesis,X‐ray Studies,and Biological Evaluation

The structure‐based design, synthesis, biological evaluation, and X‐ray structural studies of fluorine‐containing HIV‐1 protease inhibitors are described. The synthesis of both enantiomers of the gem‐difluoro‐bis‐THF ligands was carried out in a stereoselective manner using a Reformatskii–Claisen reaction as the key step. Optically active ligands were converted into protease inhibitors. Two of these inhibitors, (3R,3aS,6aS)‐4,4‐difluorohexahydrofuro[2,3‐b]furan‐3‐yl(2S,3R)‐3‐hydroxy‐4‐((N‐isobutyl‐4‐methoxyphenyl)sulfonamido)‐1‐phenylbutan‐2‐yl) carbamate ( 3 ) and (3R,3aS,6aS)‐4,4‐difluorohexahydrofuro[2,3‐b]furan‐3‐yl(2S,3R)‐3‐hydroxy‐4‐((N‐isobutyl‐4‐aminophenyl)sulfonamido)phenylbutan‐2‐yl) carbamate ( 4 ), exhibited HIV‐1 protease inhibitory K_i values in the picomolar range. Both 3 and 4 showed very potent antiviral activity, with respective EC₅₀ values of 0.8 and 3.1 nM against the laboratory strain HIV‐1_LAI. The two inhibitors exhibited better lipophilicity profiles than darunavir, and also showed much improved blood–brain barrier permeability in an in vitro model. A high‐resolution X‐ray structure of inhibitor 4 in complex with HIV‐1 protease was determined, revealing that the fluorinated ligand makes extensive interactions with the S2 subsite of HIV‐1 protease, including hydrogen bonding interactions with the protease backbone atoms. Moreover, both fluorine atoms on the bis‐THF ligand formed strong interactions with the flap Gly 48 carbonyl oxygen atom.     

Bis(3,5‐dimethylphenyl)‐(S)‐pyrrolidin‐2‐ylmethanol: an Improved Organocatalyst for the Asymmetric Epoxidation of α,β‐Enones

The asymmetric epoxidation of α,β‐enones by the readily available bis(3,5‐dimethylphenyl)‐(S)‐pyrrolidin‐2‐ylmethanol and tert‐butyl hydroperoxide (TBHP) is described. Stereoelectronic substitution on the aryl moiety of diaryl‐2‐pyrrolidinemethanols was found to significantly affect the efficiency with respect to the previously reported (S)‐diphenyl‐2‐pyrrolidinemethanol. Improved reactivity and enantioselectivity were achieved with bis(3,5‐dimethylphenyl)‐(S)‐pyrrolidin‐2‐ylmethanol at reduced catalyst loading (20 mol %) with ees up to 94% for chalcone epoxides under mild reaction conditions, whereas (S)‐diphenyl‐2‐pyrrolidinemethanol afforded a maximum ee of 80%. Interestingly, the methodology is applicable to the epoxidation of more challenging aliphatic or enolizable enones with good control of the asymmetric induction (up to 87% ee).     

Asymmetric 1,3‐Dipolar Cycloaddition Reaction between α,β‐Unsaturated Aldehydes and Nitrones Catalyzed by Well‐Defined Iridium or Rhodium Catalysts

Reaction of the complexes (S_M,R_C)‐[(η⁵‐C₅Me₅)M{(R)‐Prophos}(H₂O)](SbF₆)₂ (M=Rh, Ir) with α,β‐unsaturated aldehydes diastereoselectively gave complexes (S_M,R_C)‐[(η⁵‐C₅Me₅)M{(R)‐Prophos}(enal)](SbF₆)₂ which have been fully characterized, including an X‐ray molecular structure determination of the complex (S_Rh,R_C)‐[(η⁵‐C₅Me₅)Rh{(R)‐Prophos}(trans‐2‐methyl‐2‐pentenal)](SbF₆)₂. These enal complexes efficiently catalyze the enantioselective 1,3‐dipolar cycloaddition of the nitrones N‐benzylideneaniline N‐oxide and 3,4‐dihydroisoquinoline N‐oxide to the corresponding enals. Reactions occur with excellent regioselectivity, perfect endo selectivity and with enantiomeric excesses up to 94 %. The absolute configuration of the adduct 5‐methyl‐2,3‐diphenylisoxazolidine‐4‐carboxaldehyde was determined through its (R)‐(−)‐α‐methylbenzylamine derivative.