Search results for "stereochemistry"
showing 10 items of 4831 documents
Bonandiol: A new, irregular, monocyclic diterpene from (L.) Halacsy (umbelliferae)
1984
A new, irregular, monocyclic diterpene, bonandiol (1), has been isolated from Bonannia graeca. The structure of bonandiol has been deduced from spectral data and chemical evidence.
Mechanistic studies of the ring opening reactions of [1,2,3]triazolo[1,5-a]pyridines
1998
Abstract A mechanism with radical intervention is proposed for the opening of the triazole ring in [1,2,3] triazolo[1,5-a]pyridines which results in the production of 2- or 2,6-disubstituted pyridines.
Stereoselective Synthesis of Enantiomerically Pure Piperidine Derivatives byN-Galactosylation of Pyridones
2004
Stereoselective desymmetrization of 4-pyridone has been achieved through selective N-galactosylation, activation of the N-(galactosyl)pyridone by O-silylation and immediate addition of Grignard compounds. Chiral piperidine derivatives, e.g. (S)-(+)-coniine and (5S,9S)-(+)-indolozidine 167B, were synthesised in enantiomerically pure form using these highly regio- and stereoselective reactions. After N-galactosylation of 2-pyridone and O-silylation of the N-galactosyl-2-pyridone, addition of a Grignard compound proceeded with high 1,4-regioselectivity and complete diastereoselectivity, to furnish 4-substituted 5,6-dehydro-2-piperidones. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Ger…
Synthesis of Isoxazoline Derivatives through Boulton-Katritzky Rearrangement of 1,2,4-Oxadiazoles
2013
The base-induced rearrangement of 1,2,4-oxadiazoles into isoxazoline derivatives is reported. This represents the first example of a three-atom side-chain rearrangement that involves a saturated CCO side chain at C-3 of the oxadiazole. Nonaromatic 3-amino-isoxazoline derivatives are obtained in good yields. Interestingly, this reaction occurs through the rearrangement of aromatic oxadiazoles to form less stable bonds than those that are broken.
Sesquiterpene lactones, flavonoids and coumarins from Centaurea collina
1989
Abstract The isolation and characterization of two coumarins, five flavonoids and four sesquiterpene lactones from the aerial parts of Centaurea collina are reported. The 13 C NMR data are included.
Theoretical Studies of the Electronic Spectra of Organic Molecules
1995
The complete active space (CAS) SCF method in conjunction with multiconfigurational second-order perturbation theory (CASPT2) has been used to study the electronic spectra of a large number of molecules. The wave functions and the transition properties are computed at the CASSCF level, while dynamic correlation contributions to the excitation energies are obtained through the perturbation treatment. The methods yield energies, which are accurate to at least 0.2 eV, except in a few cases, where the CASSCF reference function does not characterize the electronic state with sufficient accuracy. The applications comprise: the polyenes from ethene to octatetraene (cis- and trans-forms); a number …
A minor diterpene from Amaracus akhdarensis
1985
Abstract A new isopimarane diterpenoid, isoakhdartriol, was isolated in very small amount from the aerial part of Amaracus akhdarensis. Its structure, isopimar-15-en-3β,8β,19-triol, was established by spectroscopic means.
Das durch das enzym amylomaltase eingestellte gleichgewicht oligomerer amylosen
1966
In der Reihe Glucose G, Maltose G2, Maltotriose G3 usw. stellt Amylomaltase die Gleichgewichte: 2 G2 = G3 + G (K1), G3 + G2 = G4 + G (K2) usw. ein. Die Gleichgewichtsgemische werden chromatographisch analysiert und daraus die Gleichgewichtskonstanten K1, K2 usw. berechnet. Diese steigen vom Wert K1 = 0,9 ab bis zu einem Grenzwert von etwa 1,05 asymptotisch an. Enthalpie, Entropie und freie Enthalpie liegen, auser fur den ersten Reaktionsschritt (K1), wie zu erwarten nahe bei 0. Das aus Escherichia coli (Stamm ML aus dem Institut Pasteur) isolierte Enzym Amylomaltase hat ein Molekulargewicht von etwa 130 000 und eine darauf bezogen Wechselahle von etwa 11 000 Mol Maltose pro Min. In the seri…
Über die Synthese und Eigenschaften von brückensubstituierten p-Oligophenylenen. 13. Mitteilung
1963
Es wird uber die Darstellung von p-Oligophenylenen berichtet, bei denen Rotationsmoglichkeiten der verknupften Benzolringe um Biarylbindungen durch Athyleno- oder Athylidenobrucken blockiert sind. Die Synthesen wurden im wesentlichen von der Fragestellung bestimmt, warum die Loslichkeit von p-Oligophenylenen durch seitenstandige Methylgruppen so stark erhoht wird. An Hand der bruckensubstituierten Verbindungen wird eine Arbeitshypothese bestatigt, welche die Wirksamkeit der Seitenketten auf eine Storung der Molekulsymmetrie zuruckfuhrt. Es konnen zwei Asymmetrieeffekte unterschieden werden: der Substituenteneffekt erfast dabei den durch die substituierten Gruppen schlechthin bedingten Symme…
Anion Receptors Based on a Quinoline Backbone
2007
2-Amido-8-urea substituted quinoline derivatives are potent receptors for the binding of halide or benzoate anions in chloroform. The selectivity and affinity of the receptors for fluoride can be tuned by variation of the substituents at the receptor side chains. Computational considerations show that the cleft of the receptors provides space for effective binding of F–, but not bigger anions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)