Search results for "stereochemistry"
showing 10 items of 4831 documents
Transition metal–saccharide chemistry: synthesis, characterization and solution stability studies of cis-dioxomolybdenum saccharide complexes
1998
Six cis-dioxomolybdenum(VI) complexes of simple monosaccharides (D-glucose, D-fructose, D-galactose, D-mannose, D-ribose and D-xylose) have been synthesized and characterized by a variety of analytical and spectral methods. Both the solution and solid-state studies have supported the presence of dimeric structures, formed through the cis-MoO2 moieties and the bridging saccharide units. Solution stability of these complexes as a function of time has also been addressed.
Triterpenoid saponins from Piptadeniastrum africanum (Hook. f.) Brenan
2013
One new triterpenoid saponin, named piptadeniaoside (1), along with two known saponins (2–3) have been isolated from the stem bark of Piptadeniastrum africanum. After previous isolation of flavone derivatives from this plant, new phytochemical investigations were performed for its saponin content. Their structures were established by direct interpretation of their spectral data, mainly HRESIMS, 1D NMR (1H, 13C NMR, DEPT) and 2D NMR (COSY, NOESY, HSQC and HMBC), and by comparison with the literature data.
The role of the transfer group in the intramolecular [5+2] cycloadditions of substituted β-hydroxy-γ-pyrones: a DFT analysis
2005
The intramolecular [5+2] cycloaddition reactions of a series of 3-OR (R=SiMe3, H, CHO, Me) substituted β-hydroxy-γ-pyrones bearing tethered alkenes were studied using DFT methods at the B3LYP/6–31G* level. The role of the transfer R group was analyzed considering two alternative channels: (i) in channel a the process is initialized by the transfer of the R group with formation of an oxidopyrylium ylide intermediate, which is followed of an intramolecular [5+2] cycloaddition; (ii) in channel b, the process is initialized by the intramolecular [5+2] cycloaddition on the γ-pyrone followed by the transfer of the R group. Copyright © 2005 John Wiley & Sons, Ltd.
The role of the trifluoromethyl group in reactivity and selectivity in polar cycloaddition reactions. A DFT study
2012
Abstract The role of the trifluoromethyl group in reactivity, regio-, and stereoselectivity in cycloaddition reactions has been analyzed by studying the 1,3-dipolar cycloaddition (13DC) reactions of nitrone 4 with 3,3,3-trifluoro-1-nitropropene 7b and 3,3,3-trifluoro-2-nitropropene 14 using density functional theory (DFT) methods at the B3LYP/6-311G(d) level. The recent M06-2X functional has also been evaluated but, although the thermodynamic performance is improved, the kinetic data are not necessarily better than those obtained by the popular B3LYP functional for cycloaddition reactions. Four reactive channels associated with the meta and ortho regio- and endo and exo stereoselective appr…
Oxo and Hydroxo Tantalocene Complexes: Synthesis and Reactivity. X-ray Molecular Structures of [(η5-Cp*)TaCl(OH)(η5-C5H4SiMe3)]+Cl-, (η5-Cp*)TaCl(O)…
2005
Exposure of a THF solution of tantalocene dichloride complexes (η 5 -Cp*)TaCl 2 (η 5 -C 5 H 4 -SiMe 3 ) (1) and (η 5 -Cp*)TaCl 2 (η 5 -C 5 H 5 ) (2) to the ambient atmosphere for 12 h leads to the formation of new tantalum(V) cationic hydroxo complexes [(η 5 -Cp*)TaCl(OH)(η 5 -C 5 H 4 SiMe 3 )] + -Cl- (3) and [(η 5 -Cp*)TaCl(OH)(η 5 -C 5 H 5 )]+Cl - (4). This oxidation reaction is more rapid under oxygen atmosphere since the hydroxo complexes are obtained in better yields within only 15 min. The hydroxo complexes are easily deprotonated by base to generate oxo complexes 5 and 6, which, in turn, react with trimethylsilyl triflate, giving the corresponding cationic silylated complexes 7 and 8…
Bis[(trimethylsilyl)methyl]manganese: Structural Variations of Its Solvent-Free and TMEDA-, Pyridine-, and Dioxane-Complexed Forms
2009
First synthesized in 1976 and recently taking on a new significance as a key precursor to heterobimetallic alkali-metal-manganese(II) complexes, bis[(trimethylsilyl)methyl] manganese has been structurally characterized by X-ray crystallography. It forms a polymeric chain structure of formula [{Mn(CH2SiMe3)(2)}(infinity)], 1, in which distorted tetrahedral, spiro Mn atoms are linked together via mu(2)-bonding alkyl ligands. The structure is notable for displaying two distinct categories of Mn-C bond lengths with a mean size differential of 0.225 angstrom and for being the first fully crystallographically characterized polymeric manganese(II) dialkyl compound. Magnetic measurements of 1 indic…
Stereoselective synthesis of C-glycosyl compounds via Michael addition of trimethylsilyl enol ethers and enamines to hex-1-enopyran-3-uloses
1987
Abstract The titanium(IV)-catalyzed addition of trimethylsilyl enol ethers to 2,4,6-tri- O -acylhex - 1 - enopyran - 3 - uloses ( 3 and 4 ) gave stereoselectively 3,6 - di - O - acyl - 4 - deoxy - β - d - glycero -hex-3-enopyranosyl-2-ulose derivatives. The formation of the C -glycosyl bond was accompanied by a 2→3 acyl shift, followed by the elimination of the 4-acyloxy substituent. Similarly, the reaction of 1-pyrrolidinocyclohexene with 3 and 4 also led stereoselectively to 2-(2,4,6-tri- O -acyl-β- d - ribo -hexopyranosyl)cyclohexanones. In this Michael addition, among the eight possible diastereomers, only one was formed. The high stereocontrol of both C -glycosyl compound syntheses is …
Characterization of some triphenyltin(IV) complexes of 5-(arylazo)salicylaldehyde ligands using IR, 1H, 13C, 119Sn NMR and 119Sn Mössbauer spectrosco…
1994
Abstract Coordination complexes of 5-(arylazo)salicylaldehyde with triphenyltin chloride have been prepared and subjected to IR, multinuclear NMR (1H, 13C, 119Sn) and 119Sn Mossbauer spectroscopy. 5-(Arylazo)salicylaldehyde forms two types of complexes viz. triorganotin adducts and phenoxides. In the adducts, the protonated ligand coordinates through the aldehydic oxygen atom while in phenoxides, the coordination occurs via the phenolic oxygen atom and the oxygen atom of the aldehydic group. The spectroscopic data suggest that both types of complexes adopt a cis-trigonal bipyramidal geometry around tin.
Enantiomer Separation of Tris(2,2′-bipyridine)ruthenium(II): Interaction of a D3-Symmetric Cation with a C2-Symmetric Anion
2015
A compound widely used in the separation of the enantiomers of Δ,Λ-[Ru(bipy)3]2+ (bipy = 2,2′-bipyridine) and originally described as “a curious lattice compound” with the formula Δ-[Ru(bipy)3]3[Sb2(R,R-tart)2]2I2·18H2O (tart = tetradeprotonated, carboxyl and hydroxyl, tartaric acid anion) has been crystallographically characterized as this species with a slightly higher degree of hydration (19.5H2O). The crystal lattice has a layered structure in which sheets containing Δ-[Ru(bipy)3]2+ cations and iodide anions alternate with those containing [Sb2(R,R-tart)2]2– anions and water. The role of the iodide ions, which lie in pseudohexagonal cavities formed by the array of three inequivalent but…
SULFUR ANALOGUES OF POLYCHLORINATED DIBENZO-p-DIOXINS, DIBENZOFURANS AND DIPHENYL ETHERS AS INDUCERS OF CYP1A1 IN MOUSE HEPATOMA CELL CULTURE AND STR…
1994
Three sulfur-containing compounds, 2,3,7,8-tetrachlorothianthrene (TCTA), 2,3,7,8-tetrachlorodibenzothiophene (TCDT), and 3,3[prime],4,4[prime]-tetrachlorodiphenyl sulfide (TCDPS), were analyzed for their CYP1A1-inducing potencies--measured as aryl hydrocarbon hydroxylase (AHH) and 7-ethoxyresorufin O-deethylase (EROD) activities--in mouse hepatoma cell culture Hepa-1. Marked differences in the induction potencies were observed among the three compounds studied and between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its sulfur analogue. The estimated EC50 values for TCDD, TCTA, and TCDT were about 8 pM, 700 pM, and 7.5 nM, respectively. TCDPS did not elicit any AHH/EROD induction. Compar…