Search results for "acids"
showing 10 items of 3520 documents
Understanding the formation of [3+2] and [2+4] cycloadducts in the Lewis acid catalysed reaction between methyl glyoxylate oxime and cyclopentadiene:…
2013
The formation of the formal [3+2] and [2+4] cycloadducts in the Lewis acid (LA) catalysed reaction of cyclopentadiene (Cp, 1) with methyl glyoxylate oxime (MGO, 2a) has been theoretically studied using DFT methods. Coordination of BF3 LA to the oxygen atom of MGO 2a not only increases the electrophilicity of the oxime, but also makes the corresponding tautomeric BF3:nitrone complex 8b the reactive species. The reaction is characterised by the nucleophilic attack of Cp 1 on the carbon atom of the corresponding BF3:nitrone complex 8b. The subsequent ring closure at the end of the reaction allows the formation of the [3+2] or [2+4] cycloadducts. ELF bonding analysis of selected points on the i…
Method for evaluating the bioconversion of radioactive polyunsaturated fatty acids by use of reversed-phase liquid chromatography
1988
Abstract Reversed-phase high-performance liquid chromatography on a thermostatted octadecylsilyl column was used to separate mixtures of labelled polyunsaturated fatty acids (as their methyl esters) formed by successive desaturations and elongations of labelled linoleic (18:2 n - 6) or linolenic (18:3 n -3) acid by rat liver microsomes. Acetonitrile-water mixtures were used for elution of the esters. Unsaturated and saturated esters were detected by their refractive indices. The order of elution of fatty acid methyl esters in complex mixtures varies as a function of the chain length and unsaturation, analysis temperature, water concentration and solvent flow-rate. The peak areas vary as a f…
Absorption and intestinal catabolism of fatty acids in the rat: effect of chain length and unsaturation
1991
Simultaneous portal blood absorption and intestinal mucosal catabolism of labelled fatty acids were investigated. Anaesthetized adult Wistar rats were infused intraduodenally either with 90 mumol of capric (C10:0), oleic (C18:1), linoleic (C18:2) or arachidonic (C20:4) 1-14C acids or with 30 mumol of each labelled fatty acid in addition to 30 mumol of oleic acid and 30 mumol of monopalmitin. For mixed infusates, experiments were carried out with two additional long-chain fatty acids: palmitic (C16:0) and erucic (C22:1) 1-14C acids. Radioactivity was quantified in the lipids and in the catabolic products in portal blood recovered at 5 min intervals for 1 h after infusion. At the end of the e…
Determination of the protein and free amino acid content in a sample using o-phthalaldehyde and N-acetyl-L-cysteine
1990
A spectrophotometric method is proposed for determining the protein content in a sample after total acid hydrolysis. In the procedure, free amino acids are caused to react with o-phthalaldehyde and N-acetyl-L-cysteine at pH 9.5, using isoleucine as the reference compound. Correction factors are used to take into account the differences between the molar absorptivities of the amino acid isoindoles and the recoveries of the amino acids after the hydrolysis treatment. The limit of detection was in the range 40-50 micrograms of protein, and the recoveries were usually 101 +/- 3% with a coefficient of variation lower than 4%. The free amino acid content in a partially hydrolysed protein was also…
o-Boronato- and o-Trifluoroborato-Phosphonium Salts Supported by L-α-Amino Acid Side Chain.
2015
The synthesis of o-boronato- and o-trifluoroborato-phosphonium salts supported by the L-amino acid side chain is described. The synthesis of these new class of amino acid derivatives was achieved by stereoselective quaternization of o-(pinacolato)boronatophenylphosphine with β- or γ-iodo amino acid derivatives which are prepared from L-serine or L-aspartic acid, respectively. The quaternization of the phosphine was performed using either iodo amino ester or carboxylic acid derivatives. In addition, free carboxylic acid and amine derivatives were obtained by saponification or HCl acidolysis of o-boronato-phosphonium amino esters, respectively. The usefulness of these compounds in peptide cou…
Synthesis of cyclic dipeptide templates, their incorporation into peptides and studies on their conformational and biological properties.
2009
This study investigated the diastereoselective synthesis of three dipeptide templates 1, 2 and 3, which may be regarded as conformationally restricted analogs of H-Gly-Xaa-OH, in which Xaa constitutes an aromatic amino acid. Bond formation between α-C of Gly and the aromatic moiety was achieved by proton-catalyzed intramolecular electrophilic aromatic substitution. The absolute configuration of the dipeptide templates was determined by single-crystal X-ray crystallography or by nuclear Overhauser enhancement measurements. A protective group strategy was elaborated to allow their incorporation into peptide sequences by liquid phase as well as by solid-phase peptide synthesis. The templates w…
Glycoside synthesis via electrophile-induced activation of N-allyl carbamates
1993
Abstract O-Benzyl-, O-acyl-, N-acyl- and isopropylidene-protected glycosyl N-allylcarbamates, obtained from anomerically unprotected monosaccharides and allyl isocyanate, are activated by an electrophile-induced cyclisation and react with hydroxyl compounds to form the corresponding glycosides.
Lithium enediolates and dienediolates of carboxylic acids in synthesis: Alkylation with secondary halides
1998
Abstract High yields in the alkylation of dianions of α,β-unsaturated carboxylic acids with secondary halides can be obtained despite elimination reactions occurring. α-Regioselectivity for the alkylation of but-2-enoic acids ( 1–4 ) is seldom obtained. Although double bond stereoselectivity is higher than 99% for γ-alkylated products, stereoselectivity is rather poor for most of the α-alkylated products.
Direct observation of a borane-silane complex involved in frustrated Lewis-pair-mediated hydrosilylations.
2014
Perfluorarylborane Lewis acids catalyse the addition of silicon-hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si-H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interactio…
The conformation cis of N-acetyl-N-methyl-α,β-dehydroalanine N′-methylamide and saturated analogues
2007
A series of three homologous amino acids derivatives: N-acetyl-N-methyl-α,β–dehydroalanine N′-methylamide (1), N-acetyl-N-methyl-L-alanine N′-methylamide (2), and N-acetyl-N-methyl-DL-alanine N′-methylamide have been synthesised. The racemic species undergoes spontaneous separation into L and D-enantiomers. From these two chiral forms, the structure of L-enantiomer (3) was analysed. The molecules of 1 – 3 adopt the cis arrangement of the N-terminal amide bond. The molecular conformations are similar for 1 (φ, ψ = 94.6(1)°, −1.7(1)°) and 3 (φ, ψ = 111.5(1)°, −23.8(1)°), and also 2 (φ, ψ = −114.8(2)°, 29.5(2)°), if inversion through the chiral C2 carbon is considered. They are stabilised by i…