Showing papers in "Tetrahedron-asymmetry in 1998"
TL;DR: The present review is intended to focus on the recent developments of bis(oxazoline) ligand–metal catalyzed asymmetric reactions and their applications in organic synthesis.
Abstract: The development of methodologies for efficient asymmetric synthesis is one of the most important areas of synthetic organic chemistry.1 The syntheses of biologically relevant natural and unnatural organic molecules in optically pure form are of central interest in medicinal chemistry and related disciplines. Variations in the stereochemistry of molecular probes for a target enzyme or receptor sites very often display dramatic differences in their binding properties and biological activities. For meaningful biological studies it is important, if not mandatory, to synthesize such agents in enantiomerically pure form. Recent advances in molecular biology and modern instrumentation techniques have led to a better understanding of many complex human diseases at the molecular level. Concurrent with these remarkable achievements have come new challenges and opportunities for asymmetric synthesis. Thus, from the design of enzyme inhibitors to the synthesis of receptor agonists or antagonists and bioactive natural products, asymmetric synthesis is of fundamental significance in biology and medicine.
The advances in asymmetric synthesis have now reached the point that many organic molecules can be prepared with near complete enantioselectivity. This technology is particularly sophisticated in the generation of new stereogenic centers in the presence of existing chiral centers. A number of asymmetric catalysts or so called ‘abiological catalysts’ are approaching an efficiency and selectivity comparable to enzymes such as in the asymmetric hydrogenation of dehydroamino acids utilizing chiral bisphosphine–rhodium complexes,2 asymmetric isomerization of allylic amines with rhodium(I)–BINAP complexes,3 asymmetric epoxidation of allylic alcohols,4 asymmetric epoxidation of unfunctionalized olefins,5 asymmetric reductions with chiral oxazaborolidenes6 and asymmetric dihydroxylation reactions.7 The advantage of abiological catalysts, however, is the availability of either enantiomer of the catalyst which enables one to synthesize either enantiomer of the target molecule. Today there is enormous emphasis on the design and development of efficient chiral catalysts for enantioselective synthesis and this field has become one of the most intense areas of organic chemical research.
In recent years, C2-symmetric chiral bis(oxazoline) ligand–metal complexes have received a great deal of attention through their use in various catalytic process.1c The bis(oxazoline) ligands are structurally related to C2-symmetric semicorrins pioneered by Pfaltz and co-workers.8a–c The inception of bis(oxazoline) ligands, however, added a new dimension in terms of flexibility in ligand design, convenient synthesis and availability of ligands in both enantiomeric forms. Since the early 1990s, many impressive enantioselective carbon–carbon bond forming reactions, aziridination reactions, hydrosilylations, oxidations and reductions have been recorded using bis(oxazoline)–metal complexes. The present review is intended to focus on the recent developments of bis(oxazoline) ligand–metal catalyzed asymmetric reactions and their applications in organic synthesis. The authors do not intend to provide an exhaustive review of this area since earlier developments have been reviewed by Pfaltz8a–c and Bolm.9 Applications of mono- and tris(oxazoline) ligand–metal complex catalyzed reactions are not included in this review.
615 citations
401 citations
332 citations
271 citations
143 citations
TL;DR: In this article, the SOCl2 promoted condensation of BP with 2-aminobenzophenone (BPB) was used to synthesize Ni(II) complexes of Schiff's bases derived from BPB and amino acids using an improved procedure involving the use of KOH as a base and MeOH as solvent.
Abstract: (S)-N-Benzylproline (BP) was obtained by the reaction of (S)-proline and benzylchloride in high chemical yield (89%). (S)-2-[N-(N′-Benzylprolyl)amino]benzophenone (BPB) was synthesized in amounts greater than 100 g by the SOCl2 promoted condensation of BP with 2-aminobenzophenone (yield 82%). Ni(II) complexes of Schiff's bases derived from BPB and amino acids were prepared by an improved procedure involving the use of KOH as a base and MeOH as solvent (yield 90–91%).
129 citations
TL;DR: Chiral β-amino alcohols were afforded by desymmetric ring-opening of meso-epoxides with anilines catalyzed by chiral Yb triflate complex in up to 801% ee.
Abstract: Chiral β-amino alcohols were afforded by desymmetric ring-opening of meso -epoxides with anilines catalyzed by chiral Yb triflate complex in up to 801% ee
104 citations
TL;DR: The copper-II catalyzed enantioselective 1,4-addition reactions of diethylzinc to cyclic enones in the presence of novel phosphorus amidite ligands, easily prepared from α,α,α-α-β-β, β-tetraphenyl-2,2′-dimethyl-1,3-dioxolane-4,5-dimethanol (TADDOL) derivatives, resulted in e.g. up to 71% for cyclohexenone and up to 62% for
Abstract: The copper(II) catalyzed enantioselective 1,4-addition reactions of diethylzinc to cyclic enones in the presence of novel phosphorus amidite ligands, easily prepared from α,α,α′,α′-tetraphenyl-2,2′-dimethyl-1,3-dioxolane-4,5-dimethanol (TADDOL) derivatives, resulted in e.e.s up to 71% for cyclohexenone and up to 62% for cyclopentenone. A remarkable enhancement of enantioselectivity was observed upon the addition of powdered molecular sieves to the reaction mixture.
98 citations
TL;DR: In this article, the use of main chain chiral polymer catalysts has been studied and three classes of synthetic main-chain chiral polymers are discussed: helical polymers represented by polypeptides, polymers with flexible chiral chains such as polyesters and polyamides, and polymers of rigid and sterically regular chiral chain represented by chiral conjugated polybinaphthyls.
Abstract: Some recent developments in the use of main chain chiral polymer catalysts are summarized. These polymers are different from the traditional polymer catalysts that are prepared by anchoring monomeric chiral catalysts to an achiral polymer backbone. Three classes of synthetic main chain chiral polymers are discussed including: (1) helical polymers represented by polypeptides; (2) polymers with flexible chiral chains such as polyesters and polyamides; and (3) polymers of rigid and sterically regular chiral chains represented by chiral conjugated polybinaphthyls. Some of these polymer catalysts have shown high enantioselectivity in asymmetric organic transformations.
94 citations
TL;DR: In this paper, the recombinant epoxide hydrolase from Agrobacterium radiobacter AD1 was used to obtain enantiomerically pure epoxides by means of a kinetic resolution.
Abstract: The recombinant epoxide hydrolase from Agrobacterium radiobacter AD1 was used to obtain enantiomerically pure epoxides by means of a kinetic resolution. Epoxides such as styrene oxide and various derivatives thereof and phenyl glycidyl ether were obtained in high enantiomeric excess and in reasonable yield. The enantioselectivity (E-value) of the resolution was calculated from progress curves for styrene oxide (E=16.2) and para-chlorostyrene oxide (E=32.2).
91 citations
TL;DR: Only catalytic amounts of chiral selenium reagents are necessary to achieve a sequence of methoxyselenenylation and oxidative β-hydride elimination of alkenes.
Abstract: Only catalytic amounts of chiral selenium reagents are necessary to achieve a sequence of methoxyselenenylation and oxidative β-hydride elimination of alkenes. Performing this reaction with peroxodisulfates and various metal salts led to enantiomeric excesses of up to 75%.
TL;DR: In this article, an enantiospecific synthesis of unnatural unsaturated and saturated α-amino acids based on a Wittig type reaction is described, where the versatile synthetic intermediates, l -glutamic and l -aspartic acid semialdehydes, are obtained from the corresponding N, N -di-Boc-diesters, by the selective reduction of the ω-ester with DIBAL ® under controlled conditions.
Abstract: The enantiospecific synthesis of unnatural unsaturated and saturated α-amino acids based on a Wittig type reaction is described. The versatile synthetic intermediates, l -glutamic and l -aspartic acid semialdehydes, are obtained from the corresponding N , N -di-Boc-diesters, by the selective reduction of the ω-ester with DIBAL ® under controlled conditions. The semialdehydes are chemically stable for a prolonged time and react with various phosphorous ylides, under controlled conditions, to produce the enantiomerically pure unsaturated α-amino acids in high yields. The method is equally applicable to homologated diesters obtained by the presented methodology providing unsaturated amino acids with variable unsaturated positions and geometries. The corresponding saturated products can be obtained by simple hydrogenation.
TL;DR: In this paper, polymer enlarged oxazaborolidines that are homogeneously soluble were used as catalysts for the enantioselective reduction of ketones in a continuously operated membrane reactor equipped with a nanofiltration membrane.
Abstract: Polymer enlarged oxazaborolidines that are homogeneously soluble were used as catalysts for the enantioselective reduction of ketones in a continuously operated membrane reactor equipped with a nanofiltration membrane. The amount of chiral product per mole of catalyst could be enhanced from 10 up to 560 moles (≡0.18 mol% catalyst). The chiral alcohols are obtained in good to excellent ee and space–time yield (up to 99% ee and 1.4 kg/[Lreactor volume×day)].
TL;DR: In this article, symmetric phosphino-oxazolines, which possess both phosphine and oxazoline moieties, were prepared from racemic binaphthol and enantiomerically pure (S )-(+)-2-amino-3-methyl-1-butanol in high yields.
Abstract: C 1 -Symmetric phosphino-oxazolines, ( S,S )- and ( S,R )-2-[4-(isopropyl)oxazol-2-yl]-2′-diphenylphosphino-1,1′-binaphthyl, which possess both phosphine and oxazoline moieties, were prepared from racemic binaphthol and enantiomerically pure ( S )-(+)-2-amino-3-methyl-1-butanol in high yields. Reaction of 1,3-diphenyl-2-propenyl acetate with dimethyl sodiomalonate in the presence of 2 mol% of palladium catalysts bearing the new chiral ligands proceeded with high enantioselectivity to give allylic alkylation products of up to 91% ee.
TL;DR: A chiral (salen)Ti(IV) complex was an efficient catalyst in the asymmetric ring opening of meso-epoxides with thiols in high yields and good energy as discussed by the authors.
Abstract: A chiral (salen)Ti(IV) complex was an efficient catalyst in the asymmetric ring opening of meso -epoxides 1 with thiols 2 in high yields and good ee.
TL;DR: A detailed protocol for the resolution of the Betti base [i.e. 1-(α-aminobenzyl)-2-naphthol] was investigated and the absolute configuration of the two isomers was established by means of an X-ray diffractometric study as discussed by the authors.
Abstract: A detailed protocol for the resolution of the Betti base [i.e. 1-(α-aminobenzyl)-2-naphthol] was investigated and the absolute configuration of the two isomers was established by means of an X-ray diffractometric study. A series of optically active derivatives was also prepared. The list includes the N-benzyl- and the N,N,O-trimethyl derivatives. The N,N-dimethyl derivative was prepared in racemic form by means of the Betti reaction and was resolved into the two enantiomers with an extremely easy and efficient procedure. The O-methyl derivative was prepared in a racemic form and subjected to resolution. The configuration of each base was determined by correlation with the configuration of the Betti amine.
TL;DR: The asymmetric 1,4-addition of diethylzinc to nitroolefins is efficiently catalyzed by copper(I) complexes with BINOL based phosphoramidite ligands as discussed by the authors.
Abstract: The asymmetric 1,4-addition of diethylzinc to nitroolefins is efficiently catalyzed by copper(I) complexes with BINOL based phosphoramidite ligands.
TL;DR: In this paper, the first visualization of chiral molecules oriented in a polypeptide liquid crystalline system (PBLG) using proton-decoupled natural abundance deuterium NMR was reported.
Abstract: We report the first visualization of chiral molecules oriented in a polypeptide liquid crystalline system (PBLG) using proton-decoupled natural abundance deuterium NMR. The chiral discrimination is observed through measurements of the quadrupolar splitting differences and we demonstrate that the sensitivity of natural abundance deuterium NMR is sufficient to measure the differential ordering effects (DOEs) without the need for isotopic enrichment. The feasibility and the potential of this novel method were investigated using a 5.87 T spectrometer (proton frequency 250 MHz). Several examples of chiral discrimination are presented and particular emphasis is given to demonstrate the potential of this approach.
TL;DR: A polymer bound transfer hydrogenation catalyst has been developed based on Noyori's (1S,2S)- or (1R,2R)-N-(p-tolylsulfonyl)-1,2-diphenylethylenediamine as mentioned in this paper.
Abstract: A polymer bound transfer hydrogenation catalyst has been developed based on Noyori's (1S,2S)- or (1R,2R)-N-(p-tolylsulfonyl)-1,2-diphenylethylenediamine. The ruthenium catalysed reduction of acetophenone was examined and the activity of the catalyst was found to be dependent on the type of polymer used. The catalyst was found to be reusable and retained high ee's when HCO2H:Et3N was used as the hydrogen donor.
TL;DR: In this paper, the authors have devised new equations as well as a new method allowing for the total determination of the regioselectivity occurring during biohydrolysis of a racemic epoxide by an epoxide hydrolase.
Abstract: In the course of this work we have devised new equations as well as a new method allowing for the total determination of the regioselectivity occurring during biohydrolysis of a racemic epoxide by an epoxide hydrolase. This determination is achievable by simply studying the racemic epoxide as a substrate. The results showed that, depending on the enantioselectivity (E value) and the regioselectivity involved, the absolute configuration as well as the enantiopurity of the residual epoxide and of the formed diol appear to be highly variable. For a specific enzyme/substrate couple, the yield and enantiopurity of the less reactive (remaining) epoxide—and thus the possibility to prepare it in enantiopure form—exclusively depend upon the enzyme enantioselectivity. On the other hand, the ee of the formed diol (eep) depends upon the enantioselectivity and on the regioselectivity of the oxirane ring opening. A theoretical analysis based on the material balance, as well as several practical examples, are provided to illustrate the various possibilities of such biohydrolyses.
TL;DR: The kinetic resolutions of both (±)-N-(benzyloxycarbonyl-2-(hydroxymethyl)piperidine [(±)- 5 ] and (±]-N-( tert -butoxy carbonyl)-2-(hexyloxycaronyl) piperidine [6] catalyzed by the enzyme acylase I from Aspergillus species (AA-I) afforded the chiral building blocks (S )- 5 and ( S )- 6, respectively; which were used for the syntheses of the title natural products and
Abstract: The kinetic resolutions of both (±)-N-(benzyloxycarbonyl-2-(hydroxymethyl)piperidine [(±)- 5 ] and (±)-N-( tert -butoxycarbonyl)-2-(hydroxymethyl)piperidine [(±)- 6 ] catalyzed by the enzyme acylase I from Aspergillus species (AA-I) afforded the chiral building blocks ( S )- 5 and ( S )- 6 , respectively; which were used for the syntheses of the title natural products and derivatives of ( S )-pipecolic acid. The syntheses were short (2–4 steps) and proceeded with satisfactory overall yield.
TL;DR: In this paper, a compound (4R,5R)- or (4S,5S)-2,2-dimethyl-α,α, α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanol (TADDOL) was shown to C-alkylation of aldimine Schiff's bases of alanine esters under phase-transfer catalysis conditions (solid NaOH, toluene, ambient temperature, 10% TADDOL).
Abstract: Compound (4R,5R)- or (4S,5S)-2,2-dimethyl-α,α,α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanol (TADDOL) was shown to catalyze C-alkylation of aldimine Schiff's bases of alanine esters under phase-transfer catalysis conditions (solid NaOH, toluene, ambient temperature, 10% TADDOL) with the ee of the final α-methylphenylalanine or α-allylalanine reaching 82%
TL;DR: In this paper, a chiral P,N-ferrocene ligand, 1-diphenylphosphino-1-oxazolinylferrocenes, was prepared from ferrocene via 1,1′-bis(tributylstannyl)FERCORENE and 1-methoxycarbonylferricene as intermediates, and with this new kind of ligand up to 91% ee with 99% chemical yield was afforded for the palladium-catalyzed allylic alkylation of 1,3-
Abstract: Novel chiral P,N-ferrocene ligands, 1-diphenylphosphino-1′-oxazolinylferrocenes, were prepared from ferrocene via 1,1′-bis(tributylstannyl)ferrocene and 1-diphenylphosphino-1′-methoxycarbonylferrocene as intermediates, and with this new kind of ligand up to 91% ee with 99% chemical yield was afforded for the palladium-catalyzed allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethylmalonate anion. The complexation behavior of this kind of ligand with dichlorobis(acetonitrile)palladium and bis(μ-chloro)(1,3-diphenyl-η3-allyl)dipalladium was investigated.
TL;DR: In this paper, the new C 2 -symmetrical ferrocenyl diamines 5 − 7 and 13 proved to be good ligands for the ruthenium catalyzed enantioselective transfer hydrogenation of unsymmetric ketones.
Abstract: The new C 2 -symmetrical ferrocenyl diamines 5 – 7 and 13 proved to be good ligands for the ruthenium catalyzed enantioselective transfer hydrogenation of unsymmetrical ketones. The stereocontrolled and highly flexible synthetic route to the new diamines made it possible to vary the ligand structure in an efficient manner. A short trial and error process led to a very active catalytic system with diamine 6a as the ligand, which is capable of reducing ketones even at −30°C using 2-propanol as a hydrogen source. Enantioselectivities up to 90% were reached in the reduction of 1′-acetonaphthone.
TL;DR: In this article, a facile two-step synthesis of chiral ligands for bonding to transition metals was reported, which were easily prepared from trans-1,2-diaminocyclohexane by reaction with sulfonyl chlorides to give amino-sulfonamide compounds.
Abstract: We report a facile two step synthesis of chiral ligands for bonding to transition metals. The ligands are easily prepared from trans -1,2-diaminocyclohexane by reaction with sulfonyl chlorides to give amino-sulfonamide compounds. These intermediates are then condensed with salicylaldehyde derivatives to provide sulfonamide/Schiff base compounds which represent a new class of chiral ligands.
TL;DR: Several kinds of micelle forming amphiphiles were tested in the asymmetric hydrogenation of the prochiral dialkyl 1-benzamido-2-phenyl-ethenephosphonates in aqueous media as discussed by the authors.
Abstract: Several kinds of micelle forming amphiphiles were tested in the asymmetric hydrogenation of the prochiral dialkyl 1-benzamido-2-phenyl-ethenephosphonates in aqueous media. The chiral catalytic system [Rh(cod) 2 ]BF 4 /BPPM or amphiphilized PPM proved to be suitable for the hydrogenation reaction affording enantiomeric excesses up to 99%. A chiral induction is possible to a certain extent (up to 11% ee ) by selected chiral amphiphiles in the presence of achiral rhodium catalysts.
TL;DR: In this paper, unsaturated α-amino acids are synthesized through condensation of allyl and propargyl bromides or of 9-bromoundecene with a Ni(II) complex of the Schiff base derived from glycine and BPB.
Abstract: ω-Unsaturated α-amino acids are synthesized through condensation of allyl and propargyl bromides or of 9-bromoundecene with a Ni(II) complex of the Schiff base derived from glycine and BPB. Hydroboration with Ipc 2 BH followed by oxidation with acetaldehyde affords enantiomerically pure ω-borono-α-aminocarboxylic acids.
TL;DR: In this article, the additive effects of imides and 1,3-indandione were found in an iridium-BICP catalyzed asymmetric hydrogenation of imines.
Abstract: Remarkable additive effects of imides and 1,3-indandione were found in an iridium–BICP catalyzed asymmetric hydrogenation of imines. The highest enantioselectivity of 95% ee has been obtained for the hydrogenation of 2,3,3-trimethylindolenine catalyzed by an Ir–BICP catalyst.