Showing papers in "Tetrahedron-asymmetry in 2004"
TL;DR: In this paper, a review of the recent developments in this rapidly growing field demonstrating the versatility of the method in the resolution of racemates is presented, and the main emphasis is on the use of lipases in organic solvents.
Abstract: Lipase-catalyzed kinetic resolution of racemates is considered to be one of the most fascinating topics in asymmetric catalysis. This review focuses on some of the recent developments in this rapidly growing field demonstrating the versatility of the method in the resolution of racemates. The literature search is dated back to the last five years and covers some comprehensive examples. The main emphasis is on the use of lipases in organic solvents.
241 citations
TL;DR: In this paper, the carboxylic acid functionality in the widely used organocatalyst proline with a tetrazolic acid leads to a catalyst with increased reactivity and solvent scope.
Abstract: Replacement of the carboxylic acid functionality in the widely used organocatalyst proline with a tetrazolic acid leads to a catalyst with increased reactivity and solvent scope, as demonstrated in the direct catalytic asymmetric aldol reaction.
215 citations
TL;DR: In this article, a review of the recent developments in the field of asymmetric hydrogenation in the presence of metal catalysts containing monodentate phosphorus ligands is presented.
Abstract: This review reports the recent developments in the field of asymmetric hydrogenation in the presence of metal catalysts containing monodentate phosphorus ligands. Besides monophosphines, that have been used at the origin of asymmetric hydrogenation, it mainly includes the use of monophosphites and monophosphoramidites, which when associated to rhodium precursors have recently led to very efficient enantioselective catalytic systems.
159 citations
TL;DR: In this article, the synthesis and applications in asymmetric catalysis of chiral pyridine Noxide derivatives are reviewed and a review of the main applications of chirality is presented.
Abstract: The synthesis and applications in asymmetric catalysis of chiral pyridine N-oxide derivatives is reviewed.
157 citations
TL;DR: A series of C2-symmetric bis(thiazoline) ligands with a diphenylamine backbone as a linkage between two thiazoline rings were synthesized by the use of the simple reagent phosphorus pentasulfide as mentioned in this paper.
Abstract: A series of novel C2-symmetric bis(thiazoline) ligands with a diphenylamine backbone as a linkage between two thiazoline rings were synthesized by the use of the simple reagent phosphorus pentasulfide. Their application in the catalytic asymmetric Henry reaction of α-keto esters was investigated with comparison to the corresponding bis(oxazoline) ligands. Cu(II)–bis(oxazoline) complexes furnished moderate enantioselectivities (up to 60% ee), while Cu(II)–bis(thiazoline) complexes gave higher enantioselectivities (up to 70% ee) with neat nitromethane. The enantioselectivity was improved when a halogenated solvent, such as CH2Cl2 was used (up to 82% ee), but the yield obtained lower than that in neat reactions.
148 citations
TL;DR: In this article, a new efficient catalytic enantioselective electrophilic fluorination of cyclic and acyclic β-ketoesters is developed, where the use of 1,1, 1,3, 3,3-hexafluoroisopropanol (HFIP) is crucial for achieving high enanti-lectivity.
Abstract: A new efficient catalytic enantioselective electrophilic fluorination of cyclic and acyclic β-ketoesters is developed. As low as 1 mol % of chiral bis(oxazoline)-copper triflate complexes catalyses the fluorination by means of N -fluorobenzenesulfonimide. The use of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) is crucial for achieving high enantioselectivity.
138 citations
TL;DR: In this article, the main results obtained in the field of enantioselective protonations are discussed and a compilation of literature data are given. But the main focus of this paper is on the main performance properties of the enantiomerically enriched protonation.
Abstract: This review deals with the main results obtained in the field of enantioselective protonations. This method allows easy access to various enantiomerically enriched compounds starting from either the corresponding racemic mixture (deracemization process) or various prochiral precursors. Discussions about different factors governing the stereoselectivity as well as a compilation of literature data are given.
132 citations
TL;DR: A review of the recent advances in Rh-catalyzed asymmetric hydroformylation using phosphite ligands is given in this article. But this review is limited to the use of phosphite-based ligands.
Abstract: This review covers the recent advances in Rh-catalyzed asymmetric hydroformylation using phosphite ligands in the most emerging period for this area of research (1996 to February 2004).
120 citations
TL;DR: Laccase-mediated oxidation of the steroid hormone 17β-estradiol 1 in organic solvents or in a biphasic system allowed the isolation of the C–C and C–O dimers 1a–d, which were formed as an equimolar mixture of diastereomeric atropisomers.
Abstract: Laccase-mediated oxidation of the steroid hormone 17β-estradiol 1 in organic solvents or in a biphasic system allowed the isolation of the C–C and C–O dimers 1a–d. Concerning the C–C dimers, the relative ratio of the symmetric 4-4′ 1c and asymmetric 4-2′ 1d products was influenced by the catalyst used. Both 1c and 1d were formed as an equimolar mixture of diastereomeric atropisomers.
105 citations
TL;DR: An enantioselective Strecker-type reaction of imines with Et2AlCN in the presence of chiral additives has been examined in this article, where α-aminonitriles were obtained in good yields with good enanti-lectivities in the reaction of N-benzylidenebenzhydrylamine with BINOL.
Abstract: An enantioselective Strecker-type reaction of imines with Et2AlCN in the presence of chiral additives has been examined. The enantioselectivity varied depending on the substituents of the imino group as well as the chiral additives used. Thus, α-aminonitriles were obtained in good yields with good enantioselectivities in the reaction of N-benzylidenebenzhydrylamine with Et2AlCN and BINOL. The reaction with excess BINOL gave the aminonitrile with reversed configuration.
105 citations
TL;DR: In this article, the synthesis of planar chiral phosphinoimidazolium salts such as (Rp)-3-(4-diphenyl-phosphino[2.2]paracyclophan-12-ylmethyl)-1-(2,6-diisopropylphenyl)imidrazolium bromide (RP)-11c starting from enantiopure 4,12-dibromo[2]
Abstract: The synthesis of planar chiral phosphinoimidazolium salts such as (Rp)-3-(4-diphenyl-phosphino[2.2]paracyclophan-12-ylmethyl)-1-(2,6-diisopropylphenyl)imidazolium bromide (Rp)-11c starting from enantiopure 4,12-dibromo[2.2]paracyclophane (Rp)-6 is reported. After deprotonation of these salts and a subsequent reaction with [Ir(COD)Cl]2, chelating iridium imidazolylidene complexes (Rp)-5a–c are obtained. These complexes catalyze the asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee.
TL;DR: It is demonstrated that the biohydrolysis of four racemic styrene oxide derivatives has been explored, using the (recombinant) Solanum tuberosum epoxide hydrolase showed a marked tendency to operate a so-called enantioconvergent process, thus affording the corresponding (R)-diol in a nearly quantitative yield and good to excellent ee.
Abstract: The biohydrolysis of four racemic styrene oxide derivatives has been explored, using the (recombinant) Solanum tuberosum epoxide hydrolase. Interestingly, this enzyme showed a marked tendency to operate a so-called enantioconvergent process, thus affording the corresponding (R)-diol in a nearly quantitative yield and good to excellent ee. We have demonstrated that this is due to the fact that the (S)-enantiomer of these epoxides was preferably attacked at the (benzylic) more substituted carbon atom, whereas the (R)-epoxide was attacked at the (terminal) less substituted carbon atom. The thus obtained meta- and para-chlorostyrene diol derivatives are important building blocks in the synthesis of various biologically active molecules. A nine cycles repeated batch reactor was performed starting from racemic meta-chlorostyrene oxide and afforded a 100% analytical yield (88% preparative) of the corresponding diol, obtained with ees as high as 97%.
TL;DR: P. putida was found to be the most suitable biocatalyst for further studies as it showed higher reaction rate, lower K m, better growth rate, good yield and ee values and higher stability compared to the other two microorganisms.
Abstract: Several new microorganisms have been isolated with high nitrilase activity against ( RS )-mandelonitrile using the enrichment culture technique. The organisms were cultivated in liquid culture and the enzyme activity was determined at different phases of growth. The organisms having high enzyme titre were further grown and used as catalysts for the transformation of mandelonitrile to mandelic acid. The percentage conversion was checked with RP-HPLC and the enantiomeric excess was determined on a chiral column. Three isolates gave the desired product, ( R )-(−)-mandelic acid with high ee (%) and were identified as Pseudomonas putida , Microbacterium paraoxydans and Microbacterium liquefaciens . All three isolates showed good specific activity (0.33–0.50 U/mg min) with high ee (>93%) and E values. The conversion of racemic mandelonitrile to mandelic acid by these isolates was compared: P. putida was found to be the most suitable biocatalyst for further studies as it showed higher reaction rate ( k Rxn ), lower K m , better growth rate ( μ ), good yield and ee values and higher stability compared to the other two microorganisms.
TL;DR: The first total synthesis of (−)-galipeine was accomplished in seven steps with 54% overall yield from isovanillin based on Ir-catalyzed asymmetric hydrogenation of a quinoline derivative as a key step, with its absolute stereochemistry being established as discussed by the authors.
Abstract: The first total synthesis of (−)-galipeine was accomplished in seven steps with 54% overall yield from isovanillin based on Ir-catalyzed asymmetric hydrogenation of a quinoline derivative as a key step, with its absolute stereochemistry being established.
TL;DR: In this article, the use of chiral auxiliaries in polymer-supported organic synthesis is reviewed, where the auxiliary serves as an element for inducing asymmetry into the synthesis, and it also functions as the linker for attaching the synthesis substrate to the polymer support.
Abstract: The use of chiral auxiliaries in polymer-supported organic synthesis is reviewed. In many of the examples presented, not only does the auxiliary serve as an element for inducing asymmetry into the synthesis, but it also functions as the linker for attaching the synthesis substrate to the polymer support.
TL;DR: In this article, an anti-selective intra-and intermolecular insertions of aromatic sulfamate esters into activated C-H bonds have been achieved via in situ generated phenyliodinanes in the presence of PhI(OAc) 2, MgO, and chiral Rh(II) catalysts.
Abstract: Enantioselective intra- and intermolecular insertions of aromatic sulfamate esters into activated C–H bonds have been achieved via in situ generated phenyliodinanes in the presence of PhI(OAc) 2 , MgO, and chiral Rh(II) catalysts. The optimal results were obtained with [Rh 2 {( S )-nttl} 4 ] and [Rh 2 {( R )-ntv} 4 ] as catalysts with up to 52% ee. In contrast, phenylsulfamates with allylic ortho -substituents reacted via intramolecular aziridination rather than insertion. The intermolecular amidation of indane proceeded with up to 97% yield when the reaction was carried out with the sulfamate in excess.
TL;DR: In this paper, BINOL-derived monodentate phosphites and phosphonites have been reacted with Rh-salts to form three (pre)catalysts, which are in equilibrium; two homo-combinations ML a L a and ML b L b as well as the heterocombination ML a l b.
Abstract: Mixtures of BINOL-derived monodentate phosphites and phosphonites have been reacted with Rh-salts to form three (pre)catalysts, which are in equilibrium; two homo-combinations ML a L a and ML b L b as well as the hetero-combination ML a L b . In these cases in which the latter is more active and more enantioselective than the former, enhanced asymmetric induction results in appropriate transition metal catalyzed reactions. This principle has been extended to include mixtures of BINOL-derived monodentate phosphites, phosphonites and phosphines as ligands in the asymmetric Rh-catalyzed hydrogenation of N -acyl enamines leading to ee values of >97%.
TL;DR: The reaction of different N - tert -butylsulfinyl aldimines with allyl bromides and indium powder in THF at 60°C affords, after hydrolysis with water, the corresponding N- tert -sulfinylamines 3 with high chemical yields and diastereoselectivities as mentioned in this paper.
Abstract: The reaction of different N - tert -butylsulfinyl aldimines 1 with allyl bromides 2 and indium powder in THF at 60 °C affords, after hydrolysis with water, the corresponding N - tert -butylsulfinylamines 3 with high chemical yields and diastereoselectivities.
TL;DR: In this article, alpha-ketoglutarate was obtained through enzymatic oxidation of monosodium glutamate (MSG) catalyzed by L-glutamate dehydrogenase (L-gluDH) coupled with NADH oxidase for the regeneration of NADH back to NAD(+).
Abstract: alpha-Ketoglutarate, employed to treat mild chronic renal insufficiency, was obtained through enzymatic oxidation of monosodium glutamate (MSG) catalyzed by L-glutamate dehydrogenase (L-gluDH) coupled with NADH oxidase for the regeneration of NADH back to NAD(+). The irreversible reduction of molecular oxygen to water by NADH oxidase is demonstrated to drive oxidation of MSG to alpha-ketoglutarate to completion. L-gluDH was found to be inhibited by all three oxidative deamination products, alpha-ketoglutarate, NADH, and ammonia. As the pH in the current system was balanced by sodium, not ammonia, and NADH was recycled to NAD(+), inhibition of L-gluDH by alpha-ketoglutarate is believed to present the biggest challenge to an efficient process. In a batch experiment, we achieved a volumetric productivity of 1 g/(L(.)d). (C) 2004 Elsevier Ltd. All rights reserved. (Less)
TL;DR: An overview of recent synthetic approaches to chiral phospholanes is presented in this article, where some of the more recent applications of phospholane in asymmetric catalysis are discussed.
Abstract: Phospholanes have attracted considerable attention as ligands for asymmetric catalysis. The DuPHOS family of ligands reported by Burk et al. in 1991 has found considerable success as ligands for asymmetric hydrogenations. An overview of recent synthetic approaches to chiral phospholanes is presented. Also included are some of the more recent applications of phospholanes in asymmetric catalysis.
TL;DR: A third generation of Taniaphos 1,5-diphosphine ferrocene ligands with the new and interesting (S Fc,3 S )- and ( R Fc,3 R )-configurations has been prepared in this article.
Abstract: A third generation of Taniaphos 1,5-diphosphine ferrocene ligands with the new and interesting ( S Fc ,3 S )- and ( R Fc ,3 R )-configurations has been prepared. With these ligands, the ruthenium-catalyzed hydrogenation of CO bonds proceeded with high diastereo- and enantioselectivity (up to >99% de and 97% ee). Good results were also obtained for the rhodium-catalyzed hydrogenation of CC (up to 96% ee) and CN bonds (up to 65% ee).
TL;DR: The reductase, which catalyzed the enantioselective reduction of ketoesters was purified to homogeneity from cell extracts of Pichia methanolica SC 13825, and a reaction yield of 98% and an ee of 99% was obtained.
Abstract: The chiral intermediate ( S )-1-(2′-bromo-4′-fluoro phenyl)ethanol 2 was prepared by the enantioselective microbial reduction of 2-bromo-4-fluoro acetophenone 1 . Organisms from genus Candida , Hansenula , Pichia , Rhodotorula , Saccharomyces , Sphingomonas and Baker's yeast reduced 1 to 2 in >90% yield and 99% enantiomeric excess (ee). In an alternative approach, the enantioselective microbial reductions of methyl, ethyl, and tert -butyl 4-(2′-acetyl-5′-fluorophenyl) butanoates 3 , 5 , and 7 , respectively, were demonstrated using strains of Candida and Pichia . Reaction yields of 40–53% and ee's of 90–99% were obtained for the corresponding ( S )-hydroxy esters 4 , 6 , and 8 . The reductase, which catalyzed the enantioselective reduction of ketoesters was purified to homogeneity from cell extracts of Pichia methanolica SC 13825. It was cloned and expressed in Escherichia coli with recombinant cultures used for the enantioselective reduction of keto methyl ester 3 to the corresponding ( S )-hydroxy methyl ester 4 . On a preparative scale, a reaction yield of 98% and an ee of 99% was obtained.
TL;DR: Ferrocenyl oxazoline alcohols were found to be effective in catalyzing the addition reaction of an alkynylzinc reagent to aromatic and aliphatic aldehydes with up to 93% ee of the thus produced chiral propargyl alcohols.
Abstract: Ferrocenyl oxazoline alcohols are found to be effective in catalyzing the addition reaction of an alkynylzinc reagent to aromatic and aliphatic aldehydes with up to 93% ee of the thus produced chiral propargyl alcohols.
TL;DR: An enantioselective irreversible ring opening of an epihalohydrin by N 3 −, combined with racemisation caused by a reversible ring opening by a halide, resulted in an enzymatic dynamic kinetic resolution yielding optically active ( S )-1-azido-3-halo-2-propanol.
Abstract: The haloalcohol dehalogenase from Agrobacterium radiobacter AD1 catalyses the reversible ring closure of vicinal haloalcohols to produce epoxides and halides. In the ring opening of epoxides, nonhalide nucleophiles such as N 3 − are accepted. The enantioselective irreversible ring opening of an epihalohydrin by N 3 − , combined with racemisation caused by a reversible ring opening by a halide, resulted in an enzymatic dynamic kinetic resolution yielding optically active ( S )-1-azido-3-halo-2-propanol. With epichlorohydrin as a substrate, the rate of ring opening by N 3 − was higher than the rate of racemisation, resulting in a mixed kinetic resolution and dynamic kinetic resolution. With epibromohydrin as the substrate, the racemisation rate was higher than the rate of ring opening, resulting in an efficient dynamic kinetic resolution. By optimising the pH of the medium and the concentrations of N 3 − and Br − , the product ( S )-1-azido-3-bromo-2-propanol could be obtained in 84% yield and 94% ee. An ( R )-enantiomer selective ring closure of this bromoalcohol, catalysed by the same enzyme, caused a simultaneously occurring kinetic resolution, yielding when the conversion progressed, an increase in enantiopurity of ( S )-1-azido-3-bromo-2-propanol to >99% ee with a yield of 77%. This compound and the ring-closed product glycidyl azide can be used as chiral synthetic building blocks.
TL;DR: A number of chiral propargylic and allylic alcohols were resolved by lipase-catalyzed kinetic resolution (Novozyme 435) and in some cases the enantiomeric excess was high.
Abstract: A number of chiral propargylic and allylic alcohols were resolved by lipase-catalyzed kinetic resolution (Novozyme 435). In some cases the enantiomeric excess was high (up to >99%).
TL;DR: In this paper, a new category of β-amino alcohols with a bicyclo[3.3.0] octane scaffold has been synthesized and used in the direct asymmetric nitroaldol reaction (Henry reaction).
Abstract: A new category of β-amino alcohols with a bicyclo[3.3.0]octane scaffold has been synthesized and used in the direct asymmetric nitroaldol reaction (Henry reaction). Up to 74% ee was obtained with the addition of nitromethane to relatively bulky aldehydes.
TL;DR: In this paper, tert-Butylphosphinoyl benzene was used as a ligand in the iridium-catalyzed hydrogenation of β-branched dehydroamino esters.
Abstract: Enantiopure secondary phosphine oxides have been tested as ligands in the rhodium- and iridium-catalyzed asymmetric hydrogenation of functionalized olefins. tert-Butylphosphinoyl benzene turned out to be a versatile ligand in the iridium-catalyzed hydrogenation of β-branched dehydroamino esters and in the rhodium-catalyzed hydrogenation of an enol carbamate.
TL;DR: In this article, an asymmetric synthesis of modafinil, a unique CNS stimulant with a reduced abuse liability, is described, which can be used to produce the more water soluble analogue, adrafinil.
Abstract: The asymmetric synthesis of both enantiomers of modafinil, a unique CNS stimulant with a reduced abuse liability, is described. This approach effectively prepares modafinil on a multigram scale in several steps from benzhydrol. The described synthetic route has also been used to produce the more water soluble analogue, adrafinil. X-ray crystallographic analysis on (−)-(diphenylmethanesulfinyl)acetic acid has determined the absolute configuration to be R.
TL;DR: In this article, the in situ prepared Ir(I)/phosphine-imidazoline catalysts were tested in the asymmetric hydrogenation of ketimines in order to evaluate the influence of the electronic parameters of the ligand on the catalytic reaction.
Abstract: Phosphine–imidazoline ligands 8, and the derivatives 16 and 17, which have electron-withdrawing or electron-donating groups in the aminic nitrogen, were synthesized from 2-aryl-imidazolines, which have previously been obtained from dithioesters. The coordination of ligand 8 to Ir(I) was studied and the molecular structure of the [Ir(η4-COD)8]BF4 (COD = 1,5-cyclooctadiene) determined through X-ray diffraction. The in situ prepared Ir(I)/phosphine–imidazoline catalysts were tested in the asymmetric hydrogenation of ketimines in order to evaluate the influence of the electronic parameters of the ligand on the catalytic reaction.
TL;DR: The lipase B from Candida antarctica (CAL-B) displays high enantioselectivity on a broad range of substrates, making it an accepted biocatalyst for asymmetric organic chemistry.
Abstract: The lipase B from Candida antarctica (CAL-B) displays high enantioselectivity on a broad range of substrates, making it an accepted biocatalyst for asymmetric organic chemistry. In recent years, a number of rational and combinatorial protein engineering projects have focused on extending and tailoring CAL-B’s catalytic and physical properties. Beyond generating customized catalysts, these studies have helped to elucidate the enzyme’s structure–function relationship and illuminate its enantioselectivity. Potential directions for future studies, taking into consideration results from engineering efforts on related lipases, are discussed.