Home About us Contact
|
Home About us Contact | ||
|
|
||
Moderate-to-good Yields (moderate-to-good + yield)
Selected AbstractsDiastereoselective Synthesis of Heteroaromatic Glycine DerivativesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2008Trygve Andreassen Abstract A TMSOTf promoted addition of an N - tert -butanesulfinyl ,-imino ester to five-membered aromatic heterocycles furnishes optically active heteroaromatic glycine derivatives with moderate-to-good yield in diastereomeric ratios up to 99,%. The absolute configuration of two of the addition products were solved by X-ray analysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Facile Construction of Spirocyclopropanated Bi-, Tri- and Tetracyclic Skeletons by Novel Cascades Involving Intra- and Intermolecular Heck Reactions of 2-Bromo-1,6-enynes and BicyclopropylideneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 3 2005Michael Schelper Abstract Acyclic 2-bromo-1,6-enynes 5 -R, 9 -R and 11 -R with bulky substituents at the acetylenic terminus were co-cyclized with the highly strained bicyclopropylidene (12) under palladium catalysis at 80 °C to give the cross-conjugated tetraenes 13 -R, 18 -R and 19 -R in moderate-to-good yields (34,71%). Only the co-cyclization of 5 -Ph gave rise to an additional product, which was identified as the 11-membered ring 20. At elevated temperatures (120,140 °C) the initially formed tetraenes underwent 6,-electrocyclization to give spiro[cyclopropane-1,4,-bicylo[4.3.0]-1(6),2-dienes] 21 -R, 22 -R and 23 -R. This novel class of spirocyclopropanated oligocycles is also accessible by a one-pot protocol. The highest yields for both the tetraenes and bicyclo[4.3.0]nonadiene and its heteroanalogues were obtained with bulky substituents at the alkyne terminus of the precursors. Heteroatom-containing precursors 9 -R and 11 -R gave lower yields than their all-carbon analogues 5 -R. The acyclic 2-bromo-1,8-dien-6-ynes 28a,b,c upon palladium-catalyzed co-cyclization with bicyclopropylidene (12) at 110 °C gave spirocyclopropanated tricycles 31a,b and 32, respectively, in moderate yields (14, 31 and 32%). These products were formed by two consecutive 6,-electrocyclizations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] 9,10-Diarylanthracenes as Molecular Switches: Syntheses, Properties, Isomerisations and Their Reactions with Singlet OxygenCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2008Daniel Zehm Abstract A series of 9,10-diarylanthracenes with various substituents at the ortho positions have been synthesised by palladium-catalysed cross-coupling reactions. Such compounds exhibit interesting physical properties and can be applied as molecular switches. Despite the high steric demand of the substituents, products were formed in moderate-to-good yields. In some cases, microwave conditions further improved yields. Bis-coupling afforded two isomers (syn and anti) that do not interconvert at room temperature. These products were easily separated and their relative stereochemistries were unequivocally assigned by NMR spectroscopy and X-ray analysis. The syn and anti isomers exhibit different physical properties (e.g., melting points and solubilities) and interconversion by rotation around the aryl,aryl axis commences at <100,°C for fluoro-substituted diarylanthracenes and at >300,°C for alkyl- or alkoxy-substituted diarylanthracenes. The reactions with singlet oxygen were studied separately and revealed different reactivities and reaction pathways. The yields and reactivities depend on the size and electronic nature of the substituents. The anti isomers form the same 9,10-endoperoxides as the syn species, occasionally accompanied by unexpected 1,4-endoperoxides as byproducts. Thermolysis of the endoperoxides exclusively yielded the syn isomers. The interesting rotation around the aryl,aryl axis allows the application of 9,10-diarylanthracenes as molecular switches, which are triggered by light and air under mild conditions. Finally, the oxygenation and thermolysis sequence provides a simple, synthetic access to a single stereoisomer (syn) from an unselective coupling step. [source] Synthesis, Microwave-Assisted Polymerization, and Polymer Properties of Fluorinated 2-Phenyl-2-oxazolines: A Systematic StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2008Matthias Lobert Dr. Abstract We present a detailed systematic study of the synthesis and ability of fluorinated 2-phenyl-2-oxazolines to undergo polymerization. The synthesis of these compounds is based on a two-step procedure that gives the desired 2-oxazolines in moderate-to-good yields. All the compounds were fully characterized by IR and NMR (1H, 13C, and 19F) spectroscopy, mass spectrometry, and elemental analysis. The 2-oxazolines were subsequently used as monomers for living cationic ring-opening polymerization (CROP) with microwave irradiation as the heat source (T=140,°C), nitromethane as the solvent, and methyl tosylate as the initiator. The linear first-order kinetic plots of the polymerizations accompanied by a linear increase of the molecular weight with conversion and low polydispersity index (PDI) values (generally below 1.30) indicate a living polymerization mechanism. The resulting polymerization rates reflect a strong sensitivity to the quantity of fluorine substituents in general and the presence or absence of ortho -fluoro substituents of the phenyl ring in particular. All the polymers were isolated and characterized by size-exclusion chromatography and MALDI-TOF mass spectrometry. Finally, a detailed investigation of selected polymer properties was performed by using differential scanning calorimetry, thermogravimetric analysis, and contact-angle measurements, thus resulting in structure,property relationships. Whereas the thermal properties of the polymers are mostly influenced by the presence of ortho -fluoro substituents, the surface properties are mainly determined by the presence of para - and/or meta -fluoro substituents. [source] | ||||||||||