Search results for "acid catalysi"
showing 10 items of 58 documents
A Study of the Effects of the Lewis Acid Catalysts on Oxa‐Diels‐Alder Reactions through Molecular Electron Density Theory
2020
Zur protonenkatalysierten elektrophilen substitution von 2,3-dimethylindol mit arylaldehyden: C6-regioselektivität am indol-gerüst
1986
The proton catalyzed reaction of 2,3-dimethylindole (1) with several arylaldehydes 2 yields the 6,6′bisindolylarylmethanes 5 regioselectively. No mechanistic information about the predominant attack of the aldehydes on the indole nucleus is obtained, because the reaction proceeds too fast to control. The electron-rich aldehydes, 2,4,6-trimethoxybenzaldehyde and indole-3-carbaldehyde, yield exclusively the 6,6′,6″-trisin-dolylmethane 9. The constitutions of all isolated methanes were elucidated by 400 MHz 1H-nmr spectroscopy.
A DFT study of the molecular mechanisms of the nucleophilic addition of ester-derived lithium enolates and silyl ketene acetals to nitrones: Effects …
2006
9 pages, 6 schemes, 3 figures, 2 tables.
Carbohydrates as chiral templates: asymmetric ugi-synthesis of alpha-amino acids using galactosylamines as the chiral matrices
1988
Abstract In the presence of Lewis acid catalysts O-acetyl- (1) and O-pivaloyl- (2) protected β-D-galactopyranosylamines react with aldehydes, isocyanides and carboxylic acids in Ugi-four-component-condensations to give the corresponding N-galactosyl-amino acid amide derivatives 3,5 in almost quantitative yields. Zinc chloride is the most effective Lewis acid catalyst. At 0°C or even at room temperature the (2R,β-D)-diastereomers of the amino acid derivatives 3,5 are formed with high diastereoselectivity. If the sterically more demanding O-pivaloyl galactosylamine 2 is used at -78°C to -25°C the stereoselectivity often exeeds 20:1 favouring the (2R,β-D) diastereomers 5. After one recrystalli…
On the Dichotomic Behavior of the Z-2,4-Dinitrophenylhydrazone of 5-Amino-3-benzoyl-1,2,4-oxadiazole with Acids in Toluene and in Dioxane/Water: Rea…
2004
The mononuclear rearrangement (MRH) of the Z-2,4-dinitrophenylhydrazone (4a) and of the Z-phenylhydrazone (4b) of 5-amino-3-benzoyl-1,2,4-oxadiazole into the relevant triazoles 5a and 5b in toluene has been quantitatively investigated in the presence of trichloroacetic acid (TCA) and of piperidine at 313.1 K. While the behavior in the presence of piperidine recalls the one previously evidenced for some Z-hydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole, the study of the reactivity in the presence of TCA has most interestingly evidenced a general-acid-catalyzed rearrangement for “both” 4a and 4b. Thus, 4a offers the first example of a solvent-dependent dichotomic behavior in MRH processes o…
On the use of multi-parameter free energy relationships: the rearrangement of (Z)-arylhydrazones of 5-amino-3-benzoyl-1,2,4-oxadiazole into (2-aryl-5…
2010
Abstract By using a multi-parameter approach (a combination of Hammett/Ingold-Yukawa-Tsuno/Fujita-Nishioka free energy relationships) the mononuclear rearrangements of heterocycles (MRH) rates for five new ortho-substituted and ten new di-, tri-, or tetra-substituted (Z)-arylhydrazones of 5-amino-3-benzoyl-1,2,4-oxadiazole into the relevant (2-aryl-5-phenyl-2H-1,2,3-triazol-4-yl)ureas (in dioxane/water and in a large range of pS+ values) have been related to the electronic and proximity effects exerted by the present substituents, also considering previous results on some mono meta- and para-substituted (Z)-arylhydrazones. In every case, excellent correlation coefficients have been calculat…
A DFT Study of the Domino Inter [4 + 2]/Intra [3 + 2] Cycloaddition Reactions of Nitroalkenes with Enol Ethers
2000
The molecular mechanism of the domino inter [4 + 2]/intra [3 + 2] cycloaddition reactions of nitroalkenes with enol ethers to give nitroso acetal adducts has been characterized using density functional theory methods with the B3LYP functional and the 6-31G* basis set. The presence of Lewis acid catalyst and solvent effects has been taken into account to model the experimental environment. These domino processes comprise two consecutive cycloaddition reactions: the first one is an intermolecular [4 + 2] cycloaddition of the enol ether to the nitroalkene to give a nitronate intermediate, which then affords the final nitroso acetal adduct through an intramolecular [3 + 2] cycloaddition reacti…
Über die Synthese von o-Acylamino-ß-dimethylamino-propiophenonen 1. Mitt.: Verhalten von o-Nitro- und o-Carbäthoxyamino-acetophenon bei der Mannich-R…
1970
Das bei der Mannich-Reaktion von o-Carbathoxyamino-acetophenon mit Paraformaldehyd und Dimethylamin-hydrochlorid erhaltene Kondensationsprodukt wurde als ein Gemisch von o-Carbathoxyamino-β-dimethylamino-propiophenon und o-Carbathoxyamino-α-dimethylaminomethyl-acrylophenon identifiziert. Durch Variation der Versuchsbedingungen konnte das Acrylophenonderivat als Hauptprodukt erhalten werden. Es war durch Saurekatalyse in 1-N-Carbathoxy-3-dimethylaminomethyl-4-oxo-1,2,3,4-tetrahydrochinolin uberfuhrbar. Synthesis of o-Acylamino-β-dimethylamino-propiophenones The aminomethylation product of o-carbethoxyamino-acetophenone, paraformaldehyde and dimethylamine-hydrochloride was identified as a mix…
Competition between decarboxylation and isomerization in the C3H5O 2+ energy surface. Justification of the experimental results by molecular orbital …
1994
In constrast with recent molecular orbital calculations on the decarboxylation of O-protonated 2-oxetanone, this experimental work indicates that no decarboxylation of this cation occurs in sulphuric acid solution up to 150°C, but instead a clean isomerization to protonated acrylic acid takes place. Parallel theoretical work shows that the gas-phase model is too crude to account successfully for the experimental facts obtained in acidic media. However, the latter are well reproduced when the effect of the solvent is taken into account. The present findings do not necessarily invalidate the reaction mechanism currently accepted to explain the rate enhancement and change of stereochemistry ac…
Proton Acid-induced Rearrangements of α-Alkynylestradiol Methyl Ethers
1993
When subjected to proton catalysis, the α-alkynylestradiol methyl ethers 1a and 1b furnish the Rupe or Meyer-Schuster rearrangement products 2, 4, and 5a. In the presence of sulfuric acid, α-ethynylestradiol 3-methyl ether (1a) is converted into the D-homosteroid 3a. The use of deuterated acids gives rise to the formation of the corresponding deuterated steroids 2b, 2c, 3b, and 5b. Some mechanistic implications of these results are also discussed.