Search results for "Nuclear magnetic resonance spectroscopy"
showing 10 items of 843 documents
New Acylated Triterpene Saponins fromPolygala arenaria
2003
Eight new acylated triterpene saponins 1–8 were isolated from the roots of Polygala arenaria as four inseparable (E)/(Z) mixtures of the 4-methoxycinnamoyl and 3,4-dimethoxycinnamoyl derivatives by repeated MPLC over silica gel. Their structures were established mainly by 600-MHz 2D-NMR techniques (1H,1H-COSY, TOCSY, NOESY, HSQC, HMBC) as 3-O-(β-D-glucopyranosyl)presenegenin 28-(O-β-D-galactopyranosyl-(14)-O-[β-D-glucopyranosyl-(13)]-O-β-D-xylopyranosyl-(14)-O-α-L-rhamnopyranosyl-(12)-{4-O-[(E)-4-methoxycinnamoyl]}-β-D-fucopyranosyl) ester and its (Z)-isomer (1/2), 3-O-(β-D-glucopyranosyl)presenegenin 28-(O-β-D-galactopyranosyl-(14)-O-[β-D-glucopyranosyl-(13)]-O-β-D-xylopyranosyl-(14)-O-α-L…
Dihydrooxazine Oxides as Key Intermediates in Organocatalytic Michael Additions of Aldehydes to Nitroalkenes
2012
Pause and play: dihydrooxazine oxides are stable intermediates that are protonated directly, without the intermediacy of the zwitterions, in organocatalytic Michael additions of aldehydes and nitroalkenes (see scheme, R=alkyl). Protonation of these species explains both the role of the acid co-catalyst in these reactions, and the observed stereochemistry when the reaction is conducted with α-alkylnitroalkenes.
- conformational preference of , -9-oxobicyclo[3.3.1]nonane-2,4-dicarboxylic acid
1985
Abstract The 1 H and 13 C nmr spectra of exo , exo -9-oxobicyclo[3.3.1]nonane-2,4-dicarboxylic acid, 3, in DMSO-d 6 or alkaline D 2 O, clearly show that it exists in a boat - chair -conformation with equatorial carboxyl groups, thus being the first case of boat - chair preference of a bicyclo[3.3.1] nonan-9-one due to the presence of exo , exo -2,4- substituents.
Synthesis of peptides with α,β‐dehydroamino acids, III. Debenzyloxycarbonylation and detrifluoroacetylation of dehydroalanine and dehydrophenylalanin…
1985
The removal of amino protecting groups from a series of twelve model Z-3) or TFA-dipeptides of dehydroalanine and (Z)-dehydrophenylalanine was investigated. During this deprotection, peptides with Δ Ala are prone to side reactions to a higher extent than those with Δ Phe. Therefore it is interesting to note that from the Δ Ala peptides, the Z group can be split off in preparative manner with HCO2NH4 in the presence of Pd C (Table 4). Synthese von Peptiden mit α,β-Dehydroaminosauren, III.– Debenzyloxycarbonylierung und Detrifluoracetylierung von Dehydroalanin- und Dehydrophenylalanin-Peptiden An einer Serie von zwolf Z-3) oder TFA-Modellpeptiden mit Dehydroalanin und (Z)-Dehydrophenylalanin …
<strong>Synthesis and Platinum (II) Complexes of Different Polyazacyclophane Receptors</strong>
2015
During the last years, research on coordination chemistry of platinum has aroused great interest due to their potential biological applications. Herein, we report the interaction of PtCI42- with different polyazacyclophanes containing a pyridine unit as aromatic spacer. Formation of complexes has been studied by 1H and 195Pt NMR spectroscopy. Analysis of the recorded spectra of D2O solutions containing L and PtCl42- in a 1:1 molar ratio at acidic pH shows the evolution with time of the 1H and 195Pt signals. Different crystal structures have been solved by X-ray diffraction analysis. At acidic pHs, the metal ion is coordinated by the central amino group of the macrocyclic cavity and three ch…
Conformation of dehydropentapeptides containing four achiral amino acid residues - controlling the role of L-valine.
2014
Structural studies of pentapeptides containing an achiral block, built from two dehydroamino acid residues (ΔZPhe and ΔAla) and two glycines, as well as one chiral L-Val residue were performed using NMR spectroscopy. The key role of the L-Val residue in the generation of the secondary structure of peptides is discussed. The obtained results suggest that the strongest influence on the conformation of peptides arises from a valine residue inserted at the C-terminal position. The most ordered conformation was found for peptide Boc-Gly-ΔAla-Gly-ΔZPhe-Val-OMe (3), which adopts a right-handed helical conformation.
Deuteron Magnetic Resonance Study of Order and Dynamics in Liquid Crystal Polymers
1984
Abstract Thermotropic nematic polyesters, specifically deuterated at different positions of the polymer chain are studied by multiple pulse dynamic NMR. Analysis of the NMR experiments is achieved, employing a comprehensive model, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain reorientation and trans-gauche-isomerization are in the range of 0.1–10 ns. Decreasing the temperature of the solid polymer first freezes the intermolecular motions. Thus, below the glass transition only intramolecula…
Synthesis and thermoresponsive properties of four arm, amphiphilic poly(tert-butyl-glycidylether)-block-polyglycidol stars
2011
Abstract A series of four arm stars with copolymer arms composed of poly(tert-butyl-glycidylether)-b-polyglycidol were prepared using a multi-step process based on anionic ring-opening polymerization. Control of the length of the arms and the number of functional (hydroxyl) reactive groups was achieved by anionic polymerization. Stars with molar masses up to 12200 g/mol were prepared. The amphiphilic character of the star structure was varied using different polyglycidol block lengths. The star structure and molar mass of the obtained stars were characterized by SEC–MALLS and NMR spectroscopy. The temperature behavior of an aqueous solution of the obtained polymers was also investigated. Th…
Structure elucidation of new oleanane-type glycosides from three species of Acanthophyllum
2010
From the roots of three species of Acanthophyllum (Caryophyllaceae), two new gypsogenic acid glycosides, 1 and 2, were isolated, 1 from A. sordidum and A. lilacinum, 2 from A. elatius and A. lilacinum, together with three known saponins, glandulosides B and C, and SAPO50. The structures of 1 and 2 were established mainly by 2D NMR techniques as 23-O-β-D-galactopyranosylgypsogenic acid-28-O-β-D-glucopyranosyl-(13)-[β-D-glucopyranosyl-(16)]-β-D-galactopyranoside (1) and gypsogenic acid-28-O-β-D-glucopyranosyl-(13)-[β-D-glucopyranosyl-(16)]-β-D-galactopyranoside (2). The cytotoxicity of several of these saponins was evaluated against two human colon cancer cell lines (HT-29 and HCT 116). Copyr…
Iodonium complexes of the tertiary amines quinuclidine and 1-ethylpiperidine
2021
Iodonium complexes incorporating tertiary amines have been synthesised to study and explore why such species comprised of alkyl amines are relatively rare. The complexes were characterised in solution (1H and 15N NMR spectroscopy) and the solid state (SCXRD), and analysed computationally. peerReviewed