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Results 551 - 600 of 2323
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Zhou Lei L Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, - - 2009
Lewis acids catalyze the cyclization of methyl phenyldiazoacetates with an ortho-imino group, prepared from o-aminophenylacetic acid, to give 2,3-substituted indoles in quantitative yields.
Lee Doris - - 2009
Interactions between pyruvic acids and diphenylborinic acid form the basis of an efficient, direct, boron-catalyzed aldol reaction that takes place in water at room temperature with low catalyst loadings. Both boronic and borinic acids function as catalysts, with the latter demonstrating particularly high activity. A wide range of aldehydes, including ...
Yoshikawa Takashi - - 2009
A highly torquoselective olefination of alkynoates to provide functionalized tetrasubstituted olefins, (E)-2-en-4-ynoic acids, is described. Addition of Br??nsted acids dramatically switched the mode of the Ag(I)-catalyzed cyclization of the resulting enyne carboxylic acids to give either tetronic acids or 2-pyrones.
Cornella Josep - - 2009
A palladium catalyzed C-H activation of indoles and a silver catalyzed decarboxylative C-C activation of ortho substituted benzoic acids are synergistically combined to synthesize indoles arylated exclusively in the C-3 position. This novel decarboxylative C-H arylation methodology is compatible with electron-donating and -withdrawing substituents in both coupling partners.
Sibi Mukund P - - 2009
Lewis acid complexation is generally required for chiral-auxiliary-controlled stereoselectivity, and chiral Lewis acid catalysis is frequently optimal for introducing asymmetry. In this work, we show that nitrile ylide cycloadditions to electron-poor acceptors attached to chiral auxiliaries proceed in high yield and stereoselectivity in the absence of Lewis acids. In contrast, ...
Pahadi Nirmal K - - 2009
This manuscript details the development of a palladium-catalyzed hydroamination of vinyl ethers. It is proposed that palladium catalyzes the hydroamination via Bronsted base catalysis, where palladium is protonated by the relatively acidic sulfonamide to generate a palladium hydride as well as the active anionic sulfonamide nucleophile. Thus, this process is ...
Fernandes Rodney A - - 2009
The total syntheses of all stereoisomers of phenatic acid B and determination of their absolute configuration are described. The synthetic strategy is based on an efficient combination of the Sharpless asymmetric dihydroxylation, the Johnson-Claisen rearrangement, and hydroboration-oxidation. It involves 11-12 steps and overall yield of 5-8%.
Wilsily Ashraf - - 2009
A systematic study outlining the enantioselective 1,4-addition of dialkylzinc reagents to 5-(1-arylalkylidene) and indenylidene Meldrum's acids is presented. Variation of the aryl and alkyl groups present on the alkylidene was thoroughly explored. The 1,4-addition displayed compatibility with a wide range of heteroaromatics and functional groups, and the arene pattern of ...
Mirebeau Jean-Hugues - - 2009
The rearrangement of 2,5-bis(silylated)-N-Boc pyrroles in their 2,4-isomers is shown to proceed under mild acidic conditions. A reasonable mechanism, based on literature as well as experiments, is proposed to rationalize this transformation.
Larson Shawn E SE Department of Chemistry, University of South Florida, 4202 East Fowler Avenue CHE205A, Tampa, Florida 33620, - - 2009
Conditions for the phosphoric acid-catalyzed highly enantioselective ring-opening of meso-aziridines with a series of functionalized aromatic thiol nucleophiles are described. The procedure utilizes commercially available aromatic thiols, a series of meso-aziridines, and a catalytic amount of VAPOL phosphoric acid to explore the substrate scope of this highly enantioselective reaction.
Saulnier Mark G - - 2009
Generation of imino-quinone methide type intermediates from 2-aminothiazole-5-carbinols using alkylsulfonic acids in nitromethane followed by trapping with a wide range of nucleophiles effects C-C, C-O, C-N, C-S, and C-P bond formation.
Mahoney Stuart J - - 2009
Herein, we describe enantioselective addition of alkenyltin reagents possessing a reactive and sensitive allylic functionality not readily available to other classes of alkenyl metals. This method is enabled by the use of highly electrophilic alkylidene Meldrum's acids as acceptors and a cationic Rh(I)-diene complex as catalyst.
Yadav Lal Dhar S - - 2010
This letter describes two unprecedented one-pot high yielding synthetic approaches to imidazo[1,2-a]pyridine scaffolds from carbohydrates. The first approach involves microwave-assisted acid-catalyzed domino reactions of unprotected D-glucose/D-xylose with ammonium acetate and benzoin to afford polyhydroxy iminosugar-bearing tetrahydroimidazo[1,2-a]pyridines. In the second approach, polyhydroxy iminosugar-bearing tetrahydrobenzimidazo[1,2-a]pyridines were synthesized by using unprotected D-glucose/D-xylose and ...
Nodes William J - - 2009
A highly stereoselective synthesis of conformationally constrained cyclic gamma-amino acids has been devised. The key step involves an intramolecular cyclization of a nitronate onto a conjugated ester, promoted by a bifunctional thiourea catalyst. This methodology has been successfully applied to generate a variety of gamma-amino acids, including some containing three ...
Wingstrand Erica - - 2009
A minor enantiomer recycling one-pot procedure employing two reinforcing chiral catalysts has been developed. Continuous regeneration of the achiral starting material is effected via selective enzyme-catalyzed hydrolysis of the minor product enantiomer from Lewis acid-Lewis base catalyzed addition of acyl cyanides to prochiral aldehydes in a two-phase solvent system. The ...
Draghici Cristian - - 2009
A three-step sequence to prepare polycyclic 2,5-dihydropyrroles from alpha-silyloxy ketones is presented. A Lewis acid-mediated ring fragmentation of cyclic gamma-silyloxy-beta-hydroxy-alpha-diazo esters provided tethered aldehyde ynoate intermediates which, when treated with amino acid silyl esters, underwent intramolecular azomethine ylide 1,3-dipolar cycloadditions. The 2,5-dihydropyrrole products were formed in good to excellent yield ...
Yu Jie - - 2009
A bisphosphoric acid-catalyzed 1,3-dipolar cycloaddition of buta-2,3-dienoates with azomethine ylides yields 3-methylenepyrrolidine derivatives with excellent enantioselectivity (up to 97% ee).
Anthony Savarimuthu Philip - - 2009
Unusual intermolecular interactions of organic luminescent acid, 2-cyano-3(4-(diphenylamino)phenyl)acrylic acid (CDPA), with amines lead to the formation of supramolecular luminescence systems with switchable and tunable solid-state luminescence.
Al-Harrasi Ahmed - - 2009
1,3-Dioxolanyl-substituted 1,2-oxazines, such as syn-1 and anti-1, rearrange under Lewis acidic conditions to provide bicyclic products 2-5. Subsequent reductive transformations afforded enantiopure 3-aminopyran derivatives such as 7 and 9 or their protected diastereomers 16 and 18, which can be regarded as carbohydrate mimetics. An alternative sequence of transformations including selective ...
Gotor-Fernández Vicente - - 2009
Biocatalysis is a well-known and well-established technology that allows clean and straightforward stereoselective transformations to be conducted under mild reaction conditions. Traditionally, hydrolases, oxidoreductases and lyases have been used for these transformations to give the final products in excellent yields and with a high level of enantiopurity. Particular attention has ...
Chandra Tilak - - 2009
An efficient and selective method was developed for the deprotection of triethylsilyl (TES) ethers using formic acid in methanol (5-10%) or in methylene chloride 2-5%) with excellent yields. TES ethers are selectively deprotected to the corresponding alcohols in high yields using formic acid in methanol under mild reaction conditions. Other ...
Dong Haitao - - 2009
The effects of ring substitution on the pK(a) value of benzenesulfonic acid (BSA) were investigated using a combined quantum mechanical and classical approach. Ring substitution with strong electron-withdrawing elements such as F, Cl, and Br is found to enhance the acidity of the BSA. More importantly, ring substitution with -NO(2) ...
Figlus Marek - - 2010
Asymmetric reduction of ketimines with trichlorosilane can be catalysed by the Lewis-basic N-methylvaline-derived formamide anchored to a soluble dendron () with good enantioselectivity (</=94% ee) and low catalyst loading (typically 5 mol%) at room temperature in toluene. This protocol represents an improvement and simplification of the isolation procedure and recovery ...
Ahlford Katrin - - 2009
Amino acid based thioamides, hydroxamic acids, and hydrazides have been evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. Catalysts containing thioamide ligands derived from L-valine were found to selectively generate the product with an R configuration (95 % ee), whereas the corresponding L-valine-based hydroxamic acids ...
Fillion Eric - - 2009
The enantioselective alkynylation of benzylidene Meldrum's acids has been successfully achieved through rhodium-catalyzed addition of TMS-acetylene in the presence of bisphosphine ligand 3,5-Xylyl-MeOBIPHEP. The resulting Meldrum's acids were obtained in good yields and up to 99% enantiomeric excess. The alkynylation method is carried out under mild reactions conditions and is ...
Lagiakos Helen Rachel - - 2009
A mild, general method for the [3,3]-sigmatropic rearrangement of N,O-diacylhydroxylamines, employing a combination of mild base and Lewis acid, is described. Employing stoichiometric amounts of reagents with respect to substrate provides alpha-acyloxyamides, whereas an excess of reagents favors formation of cyclic orthoamides.
Zuend Stephan J SJ Harvard University, Department of Chemistry and Chemical Biology, Cambridge, Massachusetts 02138, - - 2009
Alpha-amino acids are the building blocks of proteins and are widely used as components of medicinally active molecules and chiral catalysts. Efficient chemo-enzymatic methods for the synthesis of enantioenriched alpha-amino acids have been developed, but it is still a challenge to obtain non-natural amino acids. Alkene hydrogenation is broadly useful ...
Gross Ulrike - - 2009
A short, convergent, and asymmetric synthesis of the epicoccin core was achieved using a phosphite-promoted one-step condensation of a complex proline-type amino acid. Key features of the assembly of this amino acid were a double-bond isomerization/vinylation/ring-closing metathesis strategy as well as an efficient, highly diastereoselective [2 + 2] cycloaddition of ...
Xu Zhenghu - - 2009
The first example of metal Lewis acid-primary amine bifunctional cooperative catalyst derived from primary amino acids was developed, and it was found to catalyze aldol reactions of cyclic ketones highly efficiently with very good to excellent stereoselectivities.
Machin Ben P - - 2009
Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids were investigated. The use of [RhCl(COD)](2), (+/-)-BINAP, and NaHCO(3) in MeOH at 60 degrees C was found to be the optimized conditions for the ring-opening reactions to give the 1,2-ring-opened products. Various arylboronic acids were examined, and low to moderate yields ...
Hennig Andreas - - 2009
We report the use of exciton-coupled circular dichroism (ECCD) spectroscopy in multianalyte sensing systems in complex matrices. To prepare ECCD sensors, anionic anthrylhydazides are reacted with formylphenylboronic acids to give hydrazinoanthrylboronic acids that in turn are reacted with multivalent catechols in aqueous solution. The ECCD signal between the anthracene chromophores ...
Takizawa Shinobu - - 2009
This review describes our recent efforts in the development of acid-base organocatalysts, (S)-3-(N-isopropyl-N-3-pyridinylaminomethyl) BINOL (6a) and (S)-3-[2-(diphenylphosphino)phenyl] BINOL (13a), with dual activation mechanism for the aza-Morita-Baylis-Hillman (aza-MBH) reaction. In these catalysts, chiral Br??nsted acid units are connected with a Lewis base unit via a spacer. The acid-base moieties act cooperatively ...
Ciesielski Jennifer - - 2009
Beta-iodoallenolates II, generated from alkynones I with tetra-n-butylammonium iodide and a Lewis acid, underwent selective single or double annulation, depending on the Lewis acid promoter. Treatment with TiCl(4) gave cyclohexenyl alcohols III, whereas BF(3) x OEt(2) gave oxadecalins IV. The scope and limitations of the two annulation reactions are described.
Zhu Dunming - - 2009
Transaminases catalyze amino transfer reactions from amino donors such as amino acids or amines to keto acids or ketones to give chiral amino acid or amines in optically pure form. alpha-Amino acid dehydrogenases catalyze the asymmetric reductive amination of alpha-keto acids using ammonia as amino donor to furnish L-amino acids. ...
Bagmare Seema - - 2009
The chirality of the amide linker in dimer blocks was found to have a profound effect on the orientation of base stacking interactions as studied by CD and NMR spectroscopy.
Ren Kai - - 2009
A novel Lewis acid InBr3-catalyzed direct cross-coupling reaction of arylboronic acids with diorgano diselenides and ditellurides without any additive has been developed. The reactions generated the corresponding unsymmetrical diorgano monoselenides and monotellurides in good to excellent yields. The method has advantages of broad substrate scope, simple operation, mild reaction conditions ...
Mishler Richard E RE - - 2009
We report a new and simple one-pot synthetic method to produce mesoporous silica and nanoporous solid acid catalyst capable of catalyzing pinacole-pinacolone rearrangement and esterification reactions, by preparing a solvent washable phosphonated triblock copolymer template and self-assembling it in the presence of alkoxysilane.
Smith Anna J - - 2009
A general strategy for the conjugate addition of 2-heteroaryl nucleophiles to cyclic enones, unsaturated lactones, and unsaturated lactams in high enantioselectivities and yields is reported. The use of 2-heteroaryl titanates and zinc reagents offers a practical alternative to 2-heteroarylboronic acids, which are prone to undergo protodeboronation.
Wagner Franziska M - - 2009
The (18)F-labeled aromatic amino acid 6-fluoro-3,4-dihydroxy-L-phenylalanine (6-(18)F-fluoro-L-DOPA) is widely used as a radiopharmaceutical in neurologic and oncologic PET. In this study, a novel approach to the preparation of carrier-added (CA) 6-(18)F-fluoro-L-DOPA in 3 radiosynthesis steps was developed and evaluated; in this approach, direct nucleophilic (18)F fluorination of a protected amino ...
Harmer Mark A - - 2009
We have developed a simple and effective route for the high yield extraction of sugars from cellulosic based biomass. This process uses a combination of a cellulose decrystallization step with a mixture of phosphoric and sulfuric acid, followed by a hydrolysis step producing sugars (xylose and glucose) with yields of ...
Zhao Fei - - 2009
Mediated by AlCl(3), substituted spiro[4.5]deca-tetraenes underwent novel and selective skeletal rearrangement to generate indene derivatives in high to excellent yields.
Peng Zhi-Yong - - 2009
Using cheap and readily available AlCl(3) as Lewis acid, functionalized aldehydes react with organozinc reagents to give (E)-alkenes stereoselectively in high yields.
Gourdain St??phanie - - 2009
UV irradiation of 5-azido-2'-deoxyuridine in water provides up to seven products. All likely result from a pivotal azirene, formed by the intramolecular rearrangement of the initially formed nitrene, that undergoes nucleophilic addition at its C5 position. This study strongly suggests that only nucleophilic amino acid residues in close proximity are ...
García Jesús M - - 2009
An unprecedented highly selective direct conjugate addition of prochiral nitroalkanes (R not equal H) to acrylate equivalents is described. The method employs a unique Lewis acid/Brønsted base/MS ternary catalytic system and affords products with dr up to 97/3. With beta-substituted (R' not equal H) acceptors unprecedented levels of anti/syn selectivity ...
B??uerlein Patrick S - - 2009
Ionic pi-acidic alkene ligands based on chalcone and benzylidene acetone frameworks have been "doped" into ionic liquids to provide functional reaction media for Pd-catalysed cross-couplings of a cyclohexenyl carbonate with aryl siloxanes that allow simple product isolation, free from Pd (<50 ppm) and ligand contamination.
Johnson Tarn C - - 2010
This tutorial review describes recent progress in the development of homogeneous catalytic methodology for the direct generation of hydrogen gas from formic acid and alcohols.
Berini Christophe - - 2009
The stereoselective synthesis of penmacric acid, an optically active C-4 substituted pyroglutamic acid, has been efficiently achieved through an unusual 11-step sequence starting from simple N-triisopropylsilylpyrrole. The key-steps are the initial addition of the pyrrole nucleus onto a chiral nitrone and the obtention of the pyroglutamic acid moiety by reductive ...
Wang Xiaochen - - 2009
Lewis bases, including reactant aldehydes, inhibit the rate of conversion for cycloaddition and those, like aldehydes or nitriles, cause a decrease in enantiocontrol due to the Lewis acidity of the activated complex.
Donohoe Timothy J - - 2009
Replacing trifluoroacetic acid with a catalytic amount of Lewis acid in the osmium mediated oxidative cyclization results in higher yielding reactions that can proceed nearly an order of magnitude faster. The osmium loading can also be reduced to as little as 0.2 mol %. Furthermore, these mildly acidic conditions are ...
Chakraborti Asit K - - 2009
A convenient and clean procedure of esterification is reported by direct condensation of equimolar amounts of carboxylic acids with alcohols catalyzed by an easy to prepare catalyst system of perchloric acid immobilized on silica gel (HClO(4)-SiO(2)). The direct condensation of aryl, heteroaryl, styryl, aryl alkyl, alkyl, cycloalkyl, and long-chain aliphatic ...
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