Showing papers by "Artur M. S. Silva published in 1998"
TL;DR: In this article, the structures of Artemisia campestris subsp maritima six flavanones, two chromones and the coumarin scopoletin were reported for the first time in this species.
108 citations
TL;DR: In this paper, structural elucidation of 24 xanthones with different substitution patterns was performed by spectroscopic methods, namely 2D NMR techniques, such as correlation spectroscopy (COSY), nuclear Overhauser effect (NOE), heteronuclear correlation (HETCOR) and a 1d NMR technique, selective insensitive nuclei enhanced by polarization transfer (INEPT).
Abstract: The structural elucidation of 24 xanthones with different substitution patterns was performed by spectroscopic methods, namely 2D NMR techniques, such as correlation spectroscopy (COSY), nuclear Overhauser effect (NOE), heteronuclear correlation (HETCOR) and a 1D NMR technique, selective insensitive nuclei enhanced by polarization transfer (INEPT). © 1997 John Wiley & Sons, Ltd.
39 citations
TL;DR: In this article, the stereochemistry of (E,E)-cinnamylideneacetophenones 1 and (E)- and (Z)-2-styrylchromones 3 and 4 was established by NOE experiments.
39 citations
TL;DR: A significant increase in the antifungal effect of the substituted flavones could be observed if mixtures in combination with the unsubstituted flavone or flavanone were used.
38 citations
TL;DR: The structure of the new xanthone was confirmed by X-ray crystallography as discussed by the authors, and the new 7-hydroxy-1,2,3,8-tetramethoxyxanthone, was isolated from the wood of Cratoxylum maingayi.
37 citations
TL;DR: In this paper, photo-oxidation reactions of several meso-tetraphenylporphyrins have been studied in order to ascertain the chemical stability of this class of compounds towards singlet oxygen.
Abstract: The photo-oxidation reactions of several meso-tetraphenylporphyrins have been studied in order to ascertain the chemical stability of this class of compounds towards singlet oxygen. The 2,6-disubstituted ones showed an excellent stability, whilst this is not the case for other porphyrins with different substitution pattern on the phenyl rings. This parallels what has been previously found using mono-oxygenated donors. The steric effects in protecting the macrocycle seems to be predominant over the electronic ones; even with electron-donating groups in the 2,6-positions, e.g. methoxy groups, the porphyrin macrocycle was not degraded. The reaction on the β-tetrabrominated meso-tetraphenylporphyrin also proceeded easily and yielded a bilinone derivative. The structures of the final products were all elucidated by mass, 1H and 13CNMR spectroscopy.
34 citations
TL;DR: The first reported 1,3-dipolar cycloaddition of 2-styrylchromones with diazomethane afforded 4-aryl-3-(2-chromonyl)-2-pyrazolines as discussed by the authors.
29 citations
TL;DR: In this article, one of the fractions from the wood of Artocarpus elasticus gave two prenylated flavones artelastininin and artelasticin and cyclocommunin.
23 citations
TL;DR: One new and two known 2,5-diaryl-3,4-dimethyltetrahydrofuran lignans were isolated from the bark of Talauma hodgsonii.
19 citations
TL;DR: In this article, the synthesis of chalcone and pyrazole type derivatives of methyl dehydroabietate is described, which are exhaustively characterized using NMR and MS techniques.
Abstract: The synthesis of some new chalcone and pyrazole type derivatives of methyl dehydroabietate is described. All compounds were exhaustively characterized using NMR and MS techniques. In order to confirm the information obtained by those techniques, the X-ray single crystal structures of methyl 12-acetyldehydroabietate and methyl 12-(4-methoxycinnamoyl)-dehydroabietate were determined. Molecular mechanics calculations were performed in order to evaluate the steric influence of the bulk of the C-12 substituent on the conformational preference of the isopropyl group in these derivatives. The energy barriers of 52.3 and 53.6 kJ/mol found for the two compounds indicated that the C-12 substituent hinders the free rotation of the isopropyl group in both cases. Die Synthese einiger neuer Derivate des Dehydroabietinsauremethylesters vom Typ der Chalkone und Pyrazole wird beschrieben. Alle Verbindungen wurden mit Hilfe von NMR- und MS-Techniken charakterisiert. Um die Ergebnisse dieser Techniken zu sichern, wurden Rontgen-Einkristallstrukturanalysen des 12-Acetyldehydroabietinsauremethylesters und des entsprechenden (12-(4-Methoxycinnamoyl)-Derivates durchgefuhrt. Mittels molekulmechanischer Berechnungen wurde der Einflua der Raumerfullung des Substituenten in Position 12 auf die Konformation der Isopropylgruppe in diesen Derivaten studiert. Energiebarrieren von 52.3 und 53.6 kJ/mol fur diese beiden Verbindungen weisen darauf hin, daa die freie Rotation der Isopropylgruppe in beiden Fallen sterisch gehindert ist.
12 citations
TL;DR: In this paper, the structures of the methylaurones previously isolated from Cyperus capitatus were revised and the compound described before as 6,3′,4′-trihydroxy-4-methoxy-5-methylaurone is shown to be 6, 3′, 4′ -trihydroxyl-4methoxyl-7 methylaurone.
TL;DR: In this paper, the chlorosulfonyl flavones were converted into their corresponding sulfonamides and sulfonates by reaction with alkylamines, anilines, isopropyl alcohol and p−chlorophenol.
Abstract: The chlorosulfonation of flavone and 4′-methoxyflavone with chlorosulfonic acid occurs regioselectively in ring B. The chlorosulfonyl flavones were converted into their corresponding sulfonamides and sulfonates by reaction with alkylamines, anilines, isopropyl alcohol and p−chlorophenol. Two-dimensional (COSY and HETCOR) and one-dimensional selective INEPT experiments were carried out in order to unequivocally assign all the signals in the NMR spectra of compounds.
TL;DR: In this paper, the molecular structures of 2,6-bis-(2′-methoxyphenyl)pyridine 1, the tetrafluoroborate salt of 2.6-bis-(2.6)-methoxide-mithoxy-polymorphic pyridine 2 and 3,5-bimodal pyrazole 4 have been determined by X-ray analysis.
Abstract: Two new heterocyclic ligands, 2,6-bis-(2′-hydroxyphenyl)pyridine 2 and 3,5-bis-(2′-hydroxyphenyl)pyrazole 4 have been prepared. The molecular structures of 2,6-bis-(2′-methoxyphenyl)pyridine 1, the tetrafluoroborate salt of 2,6-bis-(2′-hydroxyphenyl)pyridine 3 (a monohydrate) and that of compound 4 have been determined by X-Ray analysis. In 1, the phenyl rings are oriented so that the oxygen atoms of the methoxy groups are placed away from the nitrogen of the pyridine to overcome electronic repulsion. In salt 3, intramolecular hydrogen bonds are found between the pyridinium proton and the hydroxyl atoms which are involved in strong OH…O and OH…F interactions. In addition, the water molecule participates in a bridging hydrogen bond network with the BF4−. The molecules in 4, are also held together by hydrogen bonds where the hydroxyl groups act as both a donor and an acceptor and are responsible for the formation of chains.
TL;DR: The first reported 1,3-dipolar cycloaddition of 2-styrylchromones with diazomethane afforded 4-aryl-3-(2-chromonyl)-2-pyrazolines as discussed by the authors.
Abstract: The first reported 1,3-dipolar cycloaddition of 2-styrylchromones with diazomethane afforded 4-aryl-3-(2-chromonyl)-2-pyrazolines. However, 3-aryl-4-(2-chromonyl)-1-pyrazolines have been also found as minor products of this reaction. These two series of pyrazolines have been fully characterized.