Showing papers in "Tetrahedron-asymmetry in 1997"
465 citations
TL;DR: An overview of methods available for asymmetric preparation of a range of 1H- aziridines and their N-substituted analogues can be found in this article,.
Abstract: The report contains an overview of methods available for asymmetric preparation of a range of 1H- aziridines and their N-substituted analogues
356 citations
TL;DR: In this article, the enantioselective addition of diethylzinc to aldehydes was conveniently achieved by using a catalyst prepared in situ by mixing titanium tetraisopropoxide with S - or R -binaphthol.
Abstract: The enantioselective addition of diethylzinc to aldehydes was conveniently achieved by using a catalyst prepared in situ by mixing titanium tetraisopropoxide with S - or R -binaphthol. Optical yields as high as 95.6% were obtained.
166 citations
TL;DR: The [RuCl2-(S)-BINAP]2NEt3 catalyst precursor dissolved in 1-n-butyl-3-methylimidazolium tetrafluoroborate molten salt is able to hydrogenate 2-arylacrylic acids (aryl=Ph or 6-MeO-naphthyl) with enantioselectivities similar or higher than those obtained in homogeneous media as mentioned in this paper.
Abstract: The [RuCl2-(S)-BINAP]2NEt3 catalyst precursor dissolved in 1-n-butyl-3-methylimidazolium tetrafluoroborate molten salt is able to hydrogenate 2-arylacrylic acids (aryl=Ph or 6-MeO-naphthyl) with enantioselectivities similar or higher than those obtained in homogeneous media Moreover, the hydrogenated products can be quantitatively separated from the reaction mixture and the recovered ionic liquid catalyst solution can be reused several times without any significant changes in the catalytic activity or selectivity
151 citations
TL;DR: Aryl, alicyclic, and aliphatic epoxides were obtained by a strain of Rhodotorula glutinis as mentioned in this paper, and homochiral vicinal diols were formed from several methyl substituted aryl-ryl and meso-epoxides.
Abstract: Enantioselective epoxide hydrolysis by yeasts has been demonstrated for the hydrolysis of several aryl, alicyclic and aliphatic epoxides by a strain of Rhodotorula glutinis. High enantioselectivity was obtained in the hydrolysis of methyl substituted arylryl and aliphatic epoxides whereas selectivity towards terminal epoxides in all cases was lower. Homochiral vicinal diols were formed from several methyl substituted epoxides and also from meso epoxides. Kinetic resolution of trans-1-phenyl-1,2-epoxypropane was studied in more detail.
130 citations
TL;DR: In this article, a new strategy for asymmetric induction termed the Prochiral Auxiliary approach is introduced, where reaction of acylating agents with prochiral N -methyl- o - tert -butyl aniline provide anilides that are axially chiral by virtue of restricted rotation about the NAr bond.
Abstract: A new strategy for asymmetric induction termed the ‘prochiral auxiliary’ approach is introduced. Reactions of acylating agents with prochiral N -methyl- o - tert -butyl aniline provide anilides that are axially chiral by virtue of restricted rotation about the NAr bond. Rotamer populations about the amide bond (E/Z) were studied by 1 H NMR. Several pairs of enantiomeric o - tert -butyl anilides were separated by chiral chromatography and barriers about the NAr bond were measured by thermal racemization. Related o -(1-(trialkylsilyloxy)-1-methylethyl) anilides were also studied.
98 citations
TL;DR: In this article, a dimeric form 1 of Jacobsen's catalyst was synthesized for better steric occlusion in a polydimethylsiloxane membrane, which showed that leaching could be avoided only if low solubility was combined with low swelling or in the case of complete insolubility.
Abstract: A dimeric form 1 of Jacobsen's catalyst was synthesized for better steric occlusion in a polydimethylsiloxane membrane. In homogeneous conditions, the dimer is about as active and enantioselective as Jacobsen's catalyst itself. The relationship between leaching of the complex out of the membrane on one hand and the solubility of the complex and the swelling of the membrane in the solvent used on the other, showed that leaching could be avoided only if low solubility was combined with low swelling or in the case of complete insolubility. As the dimer is less soluble and larger than the monomeric form, this form leaches less. The yields and enantioselectivities of the heterogenised system are comparable to those of the homogeneous monomer.
95 citations
TL;DR: In this article, a large scale application was proposed based on the separation of the diastereomeric salt (+)-1/(+)-CSA and the sulfoximine (−)-1.
Abstract: Racemic S-methyl-S-phenylsulfoximine (±)-1 can be resolved with only 0.6 equivalents of (+)-camphorsulfonic acid by avoiding any recrystallization to give (+)-1 of ⩾99% ee in 80% yield and (−)-1 of 97–99% ee in 74% yield. This procedure, which is based on the separation of the diastereomeric salt (+)-1/(+)-CSA and the sulfoximine (−)-1, should be suited for a large scale application.
93 citations
TL;DR: A brief survey of nonlinear effects in asymmetric reactions can be found in this paper, with a focus on catalysis and autocatalysis applications in the context of catalysis.
Abstract: A brief survey discussing the conceptual considerations of nonlinear effects in asymmetric reactions as well as their recent applications in catalysis and autocatalysis
93 citations
TL;DR: In this article, the synthesis of non-racemic chiral hypervalent iodine reagents was described together with the first application of these reagents in asymmetric functionalizations of ketones and alkenes.
Abstract: The synthesis of non-racemic chiral hypervalent iodine reagents is described together with the first application of these reagents in asymmetric functionalizations of ketones and alkenes.
90 citations
TL;DR: In this article, the effect of several anions on the enantioselectivity of the Pd-catalyzed substitution reaction of 1,3-diphenylprop-2-enylethyl carbonate 1 with benzylamine was investigated.
Abstract: The effect of several anions on the enantioselectivity of the Pd-catalyzed substitution reaction of 1,3-diphenylprop-2-enylethyl carbonate 1 with benzylamine, utilizing chiral ferrocenyl ligands such as 1-[(R)-1-[(S)-2-(diphenylphosphino)-ferrocenyl]ethyl]-3-(1-adamantyl)-1H-pyrazole3a, was investigated. Small hard anions such as F− and BH4−, added in co-catalytic amounts to the catalyst, were found to enhance the ee to >99.5%. On the other hand, non-coordinating anions, e.g. PF6−, had a detrimental effect on the enantioselectivity (
TL;DR: In this article, X-Ray diffraction, IR measurements and catalytic hydrogenation of various substituted acrylic acids in apolar solvents, as well as molecular modelling provide new insight into the nature of the cinchonidine-substrate interaction and a general rule to predict the major enantiomer.
Abstract: X-Ray diffraction, IR measurements and catalytic hydrogenation of various substituted acrylic acids in apolar solvents, as well as molecular modelling provide new insight into the nature of the cinchonidine-substrate interaction and a general rule to predict the major enantiomer.
TL;DR: A two-enzyme system consisting of a cyclohexanone mono-oxygenase from Acinetobacter NCIMB 9871 and a protein-engineered formate dehydrogenase for the regeneration of the cofactor NADPH was used for the synthesis of chiral e-lactones.
Abstract: A two-enzyme system consisting of a cyclohexanone mono-oxygenase from Acinetobacter NCIMB 9871 and a protein-engineered formate dehydrogenase for the regeneration of the cofactor NADPH was used for the synthesis of chiral e-lactones. 4-Methylcyclohexanone was used as the model substrate yielding ( S )-(−)-5-methyl-oxepane-2-one with high chemical and enantiomeric purity. Syntheses were carried out in a repetitive-batch reactor with integrated bubble-free aerationby means of a thin-walled silicon tube.
TL;DR: In this article, atropisomeric naphthamides have been used as model compounds to investigate solute interconversion phenomena in free solution and during liquid chromatography on a chiral stationary phase.
Abstract: Chiral atropisomeric naphthamides have been used as model compounds to investigate solute interconversion phenomena in free solution and during liquid chromatography on a chiral stationary phase; energy barriers measured in the mobile phase (thermal racemization of enriched samples) are 0.3–1.3 Kcal/mol lower than those measured in the stationary phase (computer simulation).
TL;DR: In this paper, the use of Ti(H 8 -BINOL) as a catalyst for the diethylzinc addition to aldehydes has been studied, and high e.g., 98.5% were obtained for the chiral alcohol products.
Abstract: The use of Ti(H 8 -BINOL) (H 8 -BINOL=5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-bi-2-naphthol) as a catalyst for the diethylzinc addition to aldehydes has been studied, and high e.e.s (up to 98.5%) were obtained for the chiral alcohol products. The results were significantly better than those obtained with the corresponding Ti(BINOL) catalyst (BINOL=1,1′-bi-2-naphthol).
TL;DR: The asymmetric conjugate addition of diethyl zinc to cyclohexen-2-one occurs with 0.5% copper (II) triflate and 1 % chiral phosphite.
Abstract: The asymmetric conjugate addition of diethyl zinc to cyclohexen-2-one occurs with 0.5% copper (II) triflate and 1 % chiral phosphite. Cyclic phosphites derived from various tartrates gave a moderate enantiomeric excess. The nature of the exocyclic substituent of the dioxaphospholane ring is important, but the chiral induction is imposed by the tartrate framework.
TL;DR: In this article, Triazolinylidene and Rhodium(COD) complexes with an axis of chirality were used as catalysts in an asymmetric hydrosilylation reaction of methyl ketones.
Abstract: (Triazolinylidene)rhodium(COD) complexes 1–3 containing an axis of chirality have been prepared with de =91–97%. The application of these complexes as catalysts in an asymmetric hydrosilylation reaction of methyl ketones 4–7 has been investigated, resulting in an ee of up to 44%. A non-linear temperature effect (‘principle of isoinversion’) has been examined.
TL;DR: In this article, a series of chiral diphenylphosphino(oxazolinyl)ferrocene ligands 4-8 were used for the Pd-catalyzed asymmetric allylic alkylation.
Abstract: Pd-Catalyzed asymmetric allylic alkylations have been studied using a series of chiral diphenylphosphino(oxazolinyl)ferrocene ligands 4-8. With these ligands, up to 99% and 63% ee are Obtained for rac-(E)-l,3-diphenylprop-2-enyl-l-acetate and rac-(E)-l,3- dimethylprop-2-enyl-l-acetate, respectively, with dimethyl malonate as the nucleophile. Some of the ligands 4-6 generate corresponding Pd-catalysts more reactive than those formed from other known ligands. A noticeable influence of the planar chirality on the enantioselectivity is observed. Different coordination modes can be suggested for the distinguishable enantioselectivity and reactivity patterns observed in the reactions with the ligands 4-8. (~) 1997 Elsevier Science Ltd The Pd-catalyzed asymmetric allylic substitution reaction I has been demonstrated to be useful in the syntheses of valuable small molecules and complex natural products. 2 A variety of chiral ligands have been studied in the Pd-catalyzed allylic substitutions. 3 Much effort has been made to elucidate crucial factors for the enantioselection of the reaction. 4 With a few exceptions, 2g one set of chiral ligands that produces high selectivity for acyclic allylic acetates, usually shows greatly decreased selectivity for cyclic substrates, and vice versa. Furthermore, for given acetates, the steric bulkiness of the acetate dramatically alters the enantioselectivity.3p Therefore, the search for the best ligand for the catalysis may be dependent on the type and steric bulkiness of allylic substrates. A recent introduction of 'hetero-chelates' such as (phosphinoaryl)oxazoline ligands 1 opened up a path to the development of other kinds of hetero-chelates. 3p-r These N,P-chelates, a hybrid of the previously known oxazoline and phosphine ligands, are demonstrated to be efficient in the Pd-catalyzed allylic substitution and other catalytic reactions. 5 Of particular interest is that these ligands generate more reactive Pd-catalysts than most of the known N,N- and P,P-chelates, seemingly owing to the 'trans effect'.6 Recently we and others have developed a highly diastereoselective lithiation of oxazolinylfer-
TL;DR: A review of O -glycosyl α-amino acid structures representing most of the glycosylation sites of known O-glycoproteins found in nature, their preparation by chemical methods and application as building blocks for either solution or solid-phase synthesis of Oglycopeptides is presented in this paper.
Abstract: A review of O -glycosyl α-amino acid structures representing most of the glycosylation sites of known O -glycoproteins found in nature, their preparation by chemical methods and application as building blocks for either solution or solid-phase synthesis of O -glycopeptides is presented.
TL;DR: In this paper, whole bacterial cells containing stereospecific amidases were used for the kinetic resolution of racemic piperazine-2-carboxamide and piperidine-2 carboxamide.
Abstract: Whole bacterial cells containing stereospecific amidases were used for the kinetic resolution of racemic piperazine-2-carboxamide and piperidine-2-carboxamide to (S)- and (R)-piperazine-2-carboxylic acid, and (S)-piperidine-2-carboxylic acid, respectively. (S)-Piperazinecarboxylic acid dihydrochloride produced with the biocatalysts Klebsiella terrigena DSM 9174 had an ee value of 99.4% (41% yield), and the ee value of (R)-piperazinecarboxylic acid dihydrochloride obtained with Burkholderia sp. DSM 9925 was 99.0% (22% yield). Using Pseudomonas fluorescens DSM 9924 (S)-piperazine-2-carboxylic acid with an ee value of 97.3% (20% yield) was isolated.
TL;DR: In this article, optically active amino alcohols have been obtained from CeCl 3 -activated (+)-camphor and (−)-fenchone and N-functionalized organolithium compounds, which catalyzes the addition of diethylzinc to benzaldehyde in high yields and enantioselectivities up to 64%.
Abstract: New optically active aminoalcohols have been prepared from CeCl 3 -activated (+)-camphor and (−)-fenchone and N-functionalized organolithium compounds. The aminoalcohols obtained catalyze the addition of diethylzinc to benzaldehyde in high yields and enantioselectivities up to 64%.
TL;DR: Chiral cobalt-diamine complexes have been prepared and tested in catalytic reduction of β-ketoesters and ketones with molecular hydrogen or hydride transfer reduction (HTR) as mentioned in this paper.
Abstract: Chiral cobalt-diamine complexes have been prepared and tested in catalytic reduction of β-ketoesters and ketones with molecular hydrogen or hydride transfer reduction (HTR). Modest to high conversions, but low e.e.s were obtained in the first case, whereas encouraging e.e.s (up to 58%) but low conversions were observed in the reduction of acetophenone by HTR. Synthesis of chiral nitrogen tetradentate ligands for Co, Ir and Rh are described. Furthermore, (1R,2R)-(−)-N-tosyl-1,2-ethanediamine proved to be a particularly efficient ligand for iridium in the hydride transfer reduction of acetophenone (87% conversion, 92% e.e.).
TL;DR: In this article, the performance of the copper-catalyzed asymmetric aziridination of olefins is highly dependent on the properties of the nitrene precursor, and preliminary results show significant improvements of...
Abstract: The performance of the copper-catalyzed asymmetric aziridination of olefins is highly dependent on the properties of the nitrene precursor. Our preliminary results show significant improvements of ...
TL;DR: In this paper, enantioselective Michael additions of α-nitroesters 2a-d with α,β-unsaturated ketones were carried out in the presence of a catalytic amount of chiral Al-Li-(R,R′)-2,2′-dihydroxy-1,1′-binaphthyl (AlLiBINOL) complex prepared in situ from LiAlH 4 and 2.45 equiv.
Abstract: Enantioselective Michael additions of α-nitroesters 2a-d with α,β-unsaturated ketones were carried out in the presence of a catalytic amount of chiral Al-Li-(R,R′)-2,2′-dihydroxy-1,1′-binaphthyl (‘AlLiBINOL’) complex prepared in situ from LiAlH 4 and 2.45 equiv. of (R,R′)-BINOL. The enantioselectivity of the Michael addition proved to be extremely temperature dependent: Michael adduct 4a showed 7% e.e when the reaction was performed at RT, whereas 72% e.e. of the opposite enantiomer of 4a was found when the 1,4-addition was performed at −23°C. Solvent variation showed that tetrahydrofuran gave the highest selectivity (up to 80% e.e.), whereas the highest enantioselectivity for the opposite enantiomer was found in methylene chloride (up to 25%). X-Ray structure analysis of the AlLi 3 BINOL 3 complex 6 in combination with 27 Al NMR studies showed that ‘AlLiBINOL’ is a mixture of aluminium complexes in solution.
TL;DR: In this paper, the reaction of functionalized diorganozincs with chlorodiorganophosphines provides polyfunctional phosphines in good yields, especially attractive is the hydroboration/boron-zinc exchange sequence which allows the conversion of functionalised olefins into poly functional phosphines.
Abstract: The reaction of functionalized diorganozincs with chlorodiorganophosphines provides polyfunctional phosphines in good yields. Especially attractive is the hydroboration/boron-zinc exchange sequence which allows the conversion of functionalized olefins into polyfunctional phosphines in a one-pot procedure. Several new chiral phosphines have been prepared starting from readily available chiral olefins (terpenes) and their efficiency in asymmetric hydrogenation reactions has been evaluated.
TL;DR: In this article, 3-Chlorostyrene oxide was prepared in >99% ee employing the (salen)Co-catalyzed hydrolytic kinetic resolution (HKR) reaction.
Abstract: 3-Chlorostyrene oxide was prepared in >99% ee employing the (salen)Co-catalyzed hydrolytic kinetic resolution (HKR) reaction. The HKR was performed successfully on both racemic and enantiomerically enriched epoxides, the latter was obtained via (salen)Mn-catalyzed asymmetric epoxidation reactions.
TL;DR: The enantioselective hydrogenation of several α-keto esters with a set of four representative neutral homogeneous rhodium-amidophosphine-phosphinite catalysts has been investigated in this paper.
Abstract: The enantioselective hydrogenation of several α-keto esters ( 3a-f, 5a-j ), α-keto amides ( 7a-e ) and isatine derivatives ( 9a-d ) with a set of four representative neutral homogeneous rhodium-amidophosphine-phosphinite catalysts has been investigated. Trifluoroacetato-Rh-AMPP catalytic precursors promoted the rapid, efficient synthesis of aliphatic α-hydroxy esters 4a-f in moderate to high enantioselectivities (66–95% ee), in contrast to most aromatic α-hydroxy esters 6a-j (8–81% ee). Best enantioselectivities for α-hydroxy amides 8a-e (85–95% ee) and dioxindoles 10a-d (80–94% ee) were obtained with chloro-Rh-AMPP precursors. It is proposed that, contrary to α-keto amides, α-keto esters do not chelate onto the rhodium center and that, in such circumstances, the asymmetric induction is mainly controlled by the steric hindrance around the CO function.
TL;DR: In this article, an asymmetrically trimethylsilylcyanated benzaldehyde was used to obtain a high level of efficiency using chiral (salen)Ti(IV) catalysts.
Abstract: Benzaldehyde has been asymmetrically trimethylsilylcyanated, using chiral (salen)Ti(IV) catalysts 1a and 1b, 2a and 2b, 3a and 3b prepared in situ from titanium tetraisopropoxide and the Schiff's bases derived from (R)- or (S)-4-hydroxy-5-formyl[2.2]paracyclophane [(R)- and (S)-FHPC] and diaminoethane (EDA) (catalysts 1a or 1b); (R)- or (S)-FHPC and 1,3-diaminopropane (PDA) (catalysts 2a or 2b); (R)- or (S)-FHPC and (1R,2R)-1,2-diaminocyclohexane [(1R,2R)-CHDA] (catalysts 3a and 3b). Surprisingly, 1a and 1b were found to be the most efficient catalysts whereas both enantiomers of 2 were completely inactive. The maximum e.e. of 84% (S) at −78°C was obtained in the reaction catalysed by 1b at 5–10 mol%. The catalyst could be precipitated with hexane from the reaction mixture and reused.