Search results for "stereochemistry"

Bisfunctionalized Janus Molecules

2004

[reaction: see text] Bisfunctionalized dendritic multiester molecules were synthesized by combined protection-deprotection and divergent-convergent-divergent sequences in high yields leading to dendritic molecules that combine two functionally different surfaces, polar aliphatic arborol and nonpolar gallate ether moieties, resulting in a two-faced Janus molecule.

A combined experimental and theoretical study of the alkylation of 3,5-dithioxo-[1,2,4]triazepines

2008

The chemo- and regioselective alkylation reactions of 3,5-dithioxo[1,2,4]triazepine 1 in a basic medium with α,ω-dibromoalkanes 2a–c, Br(CH2)nBr (n = 1–3), are studied experimentally and theoretically. These alkylations, which occur at the thioxo sulfur atom in position 5, afford mainly 5-bromomethylthio-2,7-dimethyl- 2,3-dihydro- 4H[1,2,4]triazepin-3-one 3 for n = 1, 6,8-dimethyl-5-thioxo-2,3,4,5-tetrahydro-6H[1,3]thiazolo[4,5-d][1,2,4]triazepine 4 for n = 2 and 7,9-dimethyl-6-thioxo-2,3,4,5,6,7-hexahydro[1,3]thiazino [4,5-d][1,2,4]triazepine 5 for n = 3. Theoretical calculations have been carried out at the B3LYP/6-31G* and B3LYP(benzene)/6-311+G*//B3LYP/6-31G* levels, in order to rationa…

Two New Flavonoids fromBonannia graeca: a DFT-NMR Combined Approach in Solving Structures

2007

Two new cyclized C-geranylated flavonoids, the dihydroflavonol bonanniol C (4a) and the flavanone bonannione B (6a), were isolated as minor compounds from the aerial parts of Bonannia graeca (Umbelliferae). Their structures were elucidated by a combined approach of extensive spectroscopic means and quantum mechanical methods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

1,2,4-Oxadiazole rearrangements involving an NNC side-chain sequence.

2009

The thermal rearrangement of N-1,2,4-oxadiazol-3-yl-hydrazones into 1,2,4-triazole derivatives is reported. This represents the first example of a three-atom side-chain rearrangement involving an NNC sequence linked at the C(3) of the oxadiazole. The reactions carried out under solvent-free conditions produced good to high yields of the final products.

Hyperconjugative Control by Remote Substituents of Diastereoselectivity in the Oxygenation of Hydrocarbons.

2000

The oxidation of 2-substituted adamantanes (2) with TFDO (1) is reported. The data show a stereodifferentiation of the chemical environments induced by remote electron-withdrawing substituents which produces remarkable Z/E diastereoselectivity in the oxidation of the tertiary C(5)-H and C(7)-H bonds. The results show a bell-shaped correlation between the Z/E stereoselectivity and the substituent constant sigma(I), which is interpreted in terms of hyperconjugative stabilization of the diastereomeric transition states.

The reaction of amino-imidazoles, -pyrazoles and -triazoles with α,β-unsaturated nitriles

2005

The reactions of α,β-unsaturated nitriles (1, 9, 12) as bielectrophiles with aminoazoles (2, 4, 6) as binu-cleophiles were investigated. Acrylonitrile (1) reacts almost exclusively in a chemoselective Michael-type addition yielding the substituted azoles 3, 5 and 7, respectively. Cinnamonitriles 9a,b behave in a similar way, but the free CN group adds a second molecule 4 yielding 10a,b and its cyclocondensation product 11a,b as minor component. The attempted formation of azolopyrimidines is best achieved by the reaction of the benzylidenemalononitriles 12a - f with 2 or 4. The process is chemo- and regioselective. The structure determinations were based on NMR measurements including DEFT, C…

Eudesmane and elemane derivatives from Onopordon acaulon

1993

Abstract From the methanol extract of Onopordon acaulon 11 sesquiterpenes were isolated. Two elemane and three eudesmane derivatives are described for the first time.

Stereoselective Syntheses of Piperidine-Containing Heterocycles Using Carbohydrate Auxiliaries

2003

E-Ring extended estrone derivatives: introduction of 2-phenylcyclopentenone to the estrone D-ring via an intermolecular Pauson–Khand reaction

2006

Abstract An expedient synthetic route to E-ring extended estrone derivatives is reported. Estrone-derived cyclopentenones were accessed by an intermolecular Pauson–Khand (PK) cycloaddition. It was found that electron donating and withdrawing substituents in the arylalkyne increased and decreased the yields of PK products, respectively. The stereochemistry of the products was elucidated by X-ray and NMR studies.

Synthese von methyl- und methylenosubstituierten p-Oligophenylenen durch cokondensierende ULLMANN-Reaktion. 12. Mitt.

1963

Es wird ein Syntheseverfahren fur p-Oligophenylene beschrieben, das eine Variante der ULLMANNschen Biarylverknupfungsreaktion darstellt. Dabei wird eine bifunktionelle Jodverbindung mit einem groseren Uberschus an einer monofunktionellen Komponente umgesetzt. Dieses als cokondensierende ULLMANN-Reaktion bezeichnete Syntheseprinzip, das Ausbeuten von 40–50% d. Th. ergibt, wird eingehend erlautert und die Leistungsfahigkeit an zahlreichen Beispielen belegt. Neben einer Reihe von methylsubstituierten p-Oligophenylenen, welche Derivate vom p-Terphenyl bis zum p-Septiphenyl einschliest, werden auch Oligophenylene mit endstandigem oder mittelstandigem Fluorenrest als Strukturelement hergestellt. …