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Substitution Pattern (substitution + pattern)
Selected AbstractsElectrochemical Polymerisation of N -Arylated and N -Alkylated EDOT-Substituted Pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) Derivatives: Influence of Substitution Pattern on Optical and Electronic PropertiesMACROMOLECULAR RAPID COMMUNICATIONS, Issue 21 2009Kai Zhang Abstract New pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) derivatives carrying 3,4-ethylenedioxy-thiophenylphenyl (EDOT-phenyl) substituent groups in the 3- and 6-position, or in the 2- and 5-position of the DPP chromophore were synthesised and electrochemically polymerised. The properties of the polymers were investigated using cyclic voltammetry and UV/Vis absorption spectroscopy. It was found that the optical and electronic properties differ greatly between the two polymers. Materials with EDOT-phenyl groups in the 3- and 6-positions represent conjugated polymers with a low oxidation potential and reversible electrochromic properties, whereas the polymer with EDOT-phenyl groups in the 2- and 5-positions is non-conjugated and possesses a high oxidation potential and irreversible redox behaviour. [source] Mild Protocols for Generating Molecular Complexity: A Comparative Study of Hetero-Domino Reactions Based on the Oxidant and the Substitution Pattern.CHEMINFORM, Issue 24 2005Jose I. Candela Lena Abstract For Abstract see ChemInform Abstract in Full Text. [source] A New Metabolite with a New Substitution Pattern from the Seeds of the Chinese Yew, Taxus maireiCHEMISTRY & BIODIVERSITY, Issue 1 2007Chang-Hong Huo Abstract A new taxoid metabolite with a new substitution pattern was isolated from the methanol extract of the seeds of Taxus mairei, and its structure was established as 5,,15-dihydroxy-7,,9, -diacetoxy-11(15,1)abeo- taxa-4(20),11-dien-13-one (1) on the basis of spectral analysis including 1H- and 13C-NMR, HMQC, HMBC, NOESY, and confirmed by HR-FAB mass spectrometry. [source] Optical, Redox, and NLO Properties of Tricyanovinyl Oligothiophenes: Comparisons between Symmetric and Asymmetric Substitution PatternsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2006Juan Casado Dr. Abstract A series of tricyanovinyl (TCV)-substituted oligothiophenes was synthesized and investigated with a number of physical methods including UV/Vis, IR, and Raman spectroscopy, nonlinear optical (NLO) measurements, X-ray diffraction, and cyclic voltammetry. Mono- or disubstituted oligomers were prepared by the reaction of tetracyanoethylene with mono- or dilithiated oligomers. The comparative effects of the symmetric and asymmetric substitutions in the electronic and molecular properties have been addressed. These oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison with unsubstituted molecules. The analysis of the electronic properties of the molecules was assisted by density functional theory calculations, which are in excellent agreement with the experimental data. TCV substitution influences the energies of the frontier orbitals, especially with respect to the stabilization of LUMO orbitals. X-ray structural characterization of a monosubstituted oligomer exhibits ,-stacking with favorable intermolecular interactions. NLO results agree with the role of the intramolecular charge-transfer feature in the asymmetric samples. These results furthermore exalt the role of conformational flexibility in the disubstituted compounds and reveal an unexpected nonlinear optical activity for symmetric molecules. Regarding the electronic structure, the interpretation of the vibrational data reflects the balanced interplay between aromatic and quinoid forms, finely tuned by the chain length and substitution pattern. The electronic and structural properties are consistent with the semiconducting properties exhibited by these materials in thin film transistors (TFTs). [source] Regioselective Dendritic Functionalization of CelluloseMACROMOLECULAR RAPID COMMUNICATIONS, Issue 24 2004Mohammad L. Hassan Abstract Summary: A series of regioselectively dendronized cellulose derivatives has been prepared by the treatment of cellulose in a N,N -dimethylacetamide/LiCl solvent system with dendrons possessing isocyanate focal substituents. These new materials were characterized using FT-IR and 13C NMR spectroscopies, and thermogravimetric analysis; the products were highly soluble in a wide range of organic solvents. Substitution pattern of the dendronized cellulose. [source] Accumulation and distribution of polychlorinated dibenzo- p -dioxin, dibenzofuran, and polychlorinated biphenyl congeners in atlantic salmon (Salmo salar)ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 7 2004Pirjo Isosaari Abstract Adult Atlantic salmon (Salmo salar) were fed on four diets containing polychlorinated dibenzo- p -dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs) for 30 weeks. Lipid-normalized concentrations showed that all congeners were equally partitioned between whole-fish and fillet samples. Skinned fillet accumulated approximately 30% of the total PCDD/F and PCB content in fish. Accumulation efficiencies in whole fish were 43% for 2,3,7,8-chlorinated dibenzo- p -dioxins and dibenzofurans, 83% for dioxin-like PCBs, and 78% for other PCB congeners. Among PCDD/Fs, tetra- and pentachlorinated congeners were preferentially accumulated in salmon, whereas hepta- and octachlorinated dibenzo- p -dioxins were excreted in the feces. Substitution patterns that were associated with a preferential accumulation of PCBs in salmon included non- ortho substitution and tetrachlorination. Accumulation efficiencies and lipid-normalized biomagnification factors (BMFs) were not influenced by the PCDD/F and PCB concentrations of the diets. Biomagnification (BMF > 1) of tetra- and pentachlorinated dibenzo- p -dioxins and dibenzofurans and of all the PCBs was observed. Differences in the behavior of PCDD/F and PCB congeners resulted in a selective enrichment of the most toxic congeners in salmon. [source] Pauson,Khand Reaction of Allenic Hydrocarbons: Synthesis of 4-AlkylidenecyclopentenonesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2010Frédéric Antras Abstract The carbonyldicobalt-mediated alkyne/allene/CO cocyclization gives 4-alkylidenecyclopentenones as the major [2+2+1] cycloadducts. The regio- and stereoselectivities depend mainly on the substitution pattern of both the alkyne and the allenic moieties, which can be rationalized using the Magnus mechanism. However, contrary to this model, and in agreement with more recent mechanistic studies, our results provide evidence that both initial pseudo-equatorial and pseudo-axial coordination modes of the allenic hydrocarbons onto one of the cobalt atoms of the primary alkyne,dicobalt complex are involved. DFT calculations supporting both these coordination modes are given. [source] Regioselective SN2 Opening of Vinylic Epoxides with Trialkylzincates and TrialkylaluminatesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2004Olivier Equey Abstract The use of trialkylorganozincates and tetraalkylaluminates allows regioselective SN2 nucleophilic opening of vinylic epoxides. The reaction occurs with an anti -substitution pattern and can be applied to a wide range of substrates. We also show that the solvent and the structure of the epoxide have an influence on the substitution products' ratio. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] High-Performance Solution-Processable Poly(p -phenylene vinylene)s for Air-Stable Organic Field-Effect TransistorsADVANCED FUNCTIONAL MATERIALS, Issue 5 2005Abstract The influence of the substitution pattern (unsymmetrical or symmetrical), the nature of the side chain (linear or branched), and the processing of several solution processable alkoxy-substituted poly(p -phenylene vinylene)s (PPVs) on the charge-carrier mobility in organic field-effect transistors (OFETs) is investigated. We have found the highest mobilities in a class of symmetrically substituted PPVs with linear alkyl chains (e.g., R1, R2,=,n -C11H23, R3,=,n -C18H37). We have shown that the mobility of these PPVs can be improved significantly up to values of 10,2,cm2,V,1,s,1 by annealing at 110,°C. In addition, these devices display an excellent stability in air and dark conditions. No change in the electrical performance is observed, even after storage for thirty days in humid air. [source] Bismuth-catalyzed cyclization of amino-1,6-enynesHETEROATOM CHEMISTRY, Issue 6 2008Kimihiro Komeyama Bi(OTf)3 effectively catalyzed the cyclization of amino-1,6-enynes, leading to bicyclic amine and cyclopentadienyl amino compounds. The selectivity of the products depends on the substitution pattern of the olefinic moiety. 19F NMR trace experiment of the reaction and other results indicate a novel alkynophilicity of the bismuth species. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:644,648, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20490 [source] Tunable Organophosphorus Dopants for Bright White Organic Light-Emitting Diodes with Simple StructuresADVANCED MATERIALS, Issue 12 2009Omrane Fadhel Efficient white-light-emitting LEDs have been constructed by varying the substitution pattern of phosphole derivatives and chemical modification of their P atoms, producing thermally stable derivatives that are used as suitable dopants in a blue-light-emitting host. The devices exhibit high brightness and, due to their simple structures, current-independent CIE coordinates. [source] Electrospinning of cellulose-based nanofibersJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2007Audrey Frenot Abstract Cellulose derivatives of carboxymethyl cellulose sodium salt (CMC), hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), and enzymatically treated cellulose have been electrospun, and the microstructure of the resulting nanofibers has been analyzed by scanning electron microscopy (SEM). Before electrospinning, the solutions were characterized by viscometry and surface tension measurements, and the results were correlated with spinnability. Four different CMC derivatives, varying in molecular weight (Mw), degree of substitution (DS), and substitution pattern, have been electrospun in mixtures with poly(ethylene oxide) (PEO), and nanofibers of various characteristics have formed. The CMC-based nanostructures, i.e., the nonwoven sheet and individual nanofibers, proved to be independent of Mw and DS but largely dependent on the substitution pattern. The nonwoven sheets varied in homogeneity, and beads appeared on the individual fibers. Depending on the chemical nature of the CMC, the extraction of PEO resulted in pure CMC nanostructures of varying appearance, indicating that the distribution of PEO and CMC in the nanofibers also varied. Two different HPMC derivatives, varying in DS, were electrospun into nanofibers. Homogeneous nonwoven sheets based on nanofibers of similar appearance are formed, independent of the substitution content of the HPMC sample. Preliminary fibers were obtained from enzymatically treated cellulose in a solvent system based on lithium chloride dissolved in dimethyl acetamide (LiCl: DMAc). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1473,1482, 2007 [source] Computational modeling of tetrahydroimidazo-[4,5,1-jk][1,4]-benzodiazepinone derivatives: An atomistic drug design approach using Kier-Hall electrotopological state (E-state) indicesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2008Nitin S. Sapre Abstract Quantitative structure-activity relationships (QSAR), based on E-state indices have been developed for a series of tetrahydroimidazo-[4,5,1-jk]-benzodiazepinone derivatives against HIV-1 reverse transcriptase (HIV-1 RT). Statistical modeling using multiple linear regression technique in predicting the anti-HIV activity yielded a good correlation for the training set (R2 = 0.913, R = 0.897, Q2 = 0.849, MSE = 0.190, F -ratio = 59.97, PRESS = 18.05, SSE = 0.926, and p value = 0.00). Leave-one-out cross-validation also reaffirmed the predictions (R2 = 0.850, R = 0.824, Q2 = 0.849, MSE = 0.328, and PRESS = 18.05). The predictive ability of the training set was also cross-validated by a test set (R2 = 0.812, R = 0.799, Q2 = 0.765, MSE = 0.347, F -ratio = 64.69, PRESS = 7.37, SSE = 0.975, and p value = 0.00), which ascertained a satisfactory quality of fit. The results reflect the substitution pattern and suggest that the presence of a bulky and electropositive group in the five-member ring and electron withdrawing groups in the seven-member ring will have a positive impact on the antiviral activity of the derivatives. Bulky groups in the six-member ring do not show an activity-enhancing impact. Outlier analysis too reconfirms our findings. The E-state descriptors indicate their importance in quantifying the electronic characteristics of a molecule and thus can be used in chemical interpretation of electronic and steric factors affecting the biological activity of compounds. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Synthesis of thiophene/phenylene co-oligomers.JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2003We report the synthesis of various thiophene/phenylene co-oligomers with a total number of thiophene and benzene (phenylene) rings of 6 to 8. These compounds include a phenyl-capped sexithiophene, a thienyl-capped quaterphenylene, as well as block and alternating co-oligomers. The synthesis is based on either the Suzuki coupling reaction or the direct dimerization coupling. The latter method produces symmetric molecules with an even total ring number. These reaction schemes enabled us to obtain the target compounds in high quality. Although the resulting materials are difficult to dissolve in organic solvents and therefore difficult to identify by usual 1H nmr spectroscopy, they have successfully been identified through Fourier-transform ir spectroscopy. The specific group frequencies of ring-stretching and out-of-plane deformation modes are characteristic of the substitution pattern of the individual thiophene and benzene rings. [source] Synthesis and characterization of stable betainic pyrimidinaminidesJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2002Andreas Schmidt Depending on electronically or kinetically stabilizing effects determined by the substitution pattern or the reaction conditions, 6-amino substituted (5-chloropyrimidine-2,4-diyl)bis(hetarenium) salts or 5-chloro-2,6-bis-(pyridinio)-pyrimidin-4-aminides are formed on nucleophilic substitution of 4-(dimethylamino)pyridine, 4-(pyrrolidin-1-yl)pyridine, or 1-methylimidazole on 4-amino substituted 2,5,6-trichloropyrimidines. [source] Radiosyntheses and reactivities of novel [18F]2-fluoroethyl arylsulfonatesJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 10 2005John L. Musachio Abstract [18F]2-Fluoroethyl tosylate ([18F]FEOX, X=Ts) is widely used for labeling radiotracers for positron emission tomography (PET). Little work has been reported on syntheses of other [18F]2-fluoroethyl arylsulfonates ([18F]FEOX) that bear a less electron-rich aryl group, even though these might offer enhanced reactivities. Thus, a series of novel [18F]FEOX (X=benzenesulfonyl, brosyl, nosyl, 3,4-dibromobenzenesulfonyl) were synthesized and reactivities compared to [18F]FEOTs. Precursors for radiolabeling (bis -ethylene glycol arylsulfonates) and reference FEOX were synthesized (alcohol+arylsulfonyl chloride+KOSiMe3 in THF). Regardless of substitution pattern, [18F]FEOX (110°C, 5 min, acetonitrile) were obtained in similar decay-corrected isolated radiochemical yields (RCY; 47,53%). All [18F]FEOX gave excellent RCYs (64,87%) of the dopamine uptake radioligand, [18F]FECNT (130°C, 10 min, acetonitrile). The 3,4-dibromobenzensulfonate gave the highest RCY of [18F]FECNT (87%) and this HPLC-purified labeling agent was used directly for efficient [18F]FECNT production. When the secondary aniline of an amyloid probe (HM-IMPY) or p -nitrophenol was reacted with [18F]FEOX, RCYs were appreciably higher for brosylate and nosylate than for tosylate, while 3,4-dibromobenzenesulfonate again gave the highest RCY. Owing to the high reactivity of the new [18F]FEOX and their ease of syntheses via stable precursors, such agents (particularly 3,4-dibromobenzenesulfonate) should be considered as alternatives to [18F]FEOTs. Copyright © 2005 John Wiley & Sons, Ltd. [source] Structural determination of a new 2(3 , 20)abeotaxane with an unusual 13,-substitution pattern and a new 6/8/6-ring taxane from Taxus cuspidataMAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2007Changhong Huo Abstract A new 2(3 , 20)abeotaxane with an unusual 13,-substitution pattern and a new 6/8/6-ring taxane were isolated from the methanol extract of the needles of Taxus cuspidata. The structures were established as 2,,7,-diacetoxy-5,,10,,13,-trihydroxy-2(3 , 20)abeotaxa-4(20), 11-dien-9-one (1) and 2,,5,,7,,9,,13,-pentahydroxy-10,-acetoxytaxa-4(20),11-diene (2) on the basis of 1D and 2D NMR spectral data and high-resolution FAB-MS analyses. Copyright © 2007 John Wiley & Sons, Ltd. [source] NMR study on 9- S and 9- R oxirane derivatives of ascomycinMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2002K. Baumann Abstract Structure elucidation of 9- S and 9- R oxirane derivatives of ascomycin, a 23-membered immunomodulating macrolactam, was performed using NMR spectroscopy. The total 1H and 13C signal assignments required the gradient-selected versions of COSY (gs-COSY), heteronuclear multiple quantum-correlation spectroscopy (gs-HSQC), heteronuclear multiple-bond correlation spectroscopy (gs-HMBC), and nuclear Overhauser methods. The data sets then were used to examine the dependence of ketone,hemiketal and cis,trans amide equilibria on the substitution pattern and the absolute configuration of the chiral oxirane. Copyright © 2002 John Wiley & Sons, Ltd. [source] Novel Pirinixic Acids as PPAR, Preferential Dual PPAR,/, AgonistsMOLECULAR INFORMATICS, Issue 5 2009Heiko Zettl Abstract Pirinixic acid is a moderate agonist of both the alpha and the gamma subtype of the peroxisome proliferator activated receptor (PPAR). Previously, we have shown that ,-alkyl substitution leads to balanced low micromolar-active dual agonists of PPAR, and PPAR,. Taking ,-hexyl pirinixic acid as a new scaffold, we further optimized PPAR activity by enlargement of the lipophilic backbone by substituting the 2,3-dimethylphenyl with biphenylic moieties. Such a substitution pattern had only minor impact on PPAR, activity but further increased PPAR, activity leading to nanomolar activities. Supporting docking studies proposed that the (R)-enantiomer should fit the PPAR, ligand-binding pocket better and thus be more active than the (S)-enantiomer. Single enantiomers of selected active analogues were then prepared by enantio-selective synthesis and enantio-selective preparative HPLC, respectively. Biological data for the distinct enantiomers fully corroborated the docking experiments and substantiate a stereochemical impact on PPAR activation. [source] Synthesis and insecticidal activity of new 3-benzylfuran-2-yl N,N,N,,N,-tetraethyldiamidophosphate derivativesPEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 8 2008Vanderlúcia F Paula Abstract BACKGROUND: A series of 3-benzylfuran-2-yl N,N,N,,N,-tetraethyldiamidophosphate derivatives were synthesized as potential new agents to control insects. Their structures were confirmed on the basis of IR, NMR and MS analyses. RESULTS: Ten 3-benzylfuran-2-ylN,N,N,,N,-tetraethyl derivatives were prepared from the compound furan-2-yl N, (N,N,N,,N,-tetraethyldiamidophosphate). The contact toxicity of all derivatives, at a dose of 10 µg mg,1 insect, was evaluated against four insect species, Ascia monuste orseis Latr. (Lepidoptera: Pyralidae), Diaphania hyalinata (L.) (Lepidoptera: Pyralidae), Sitophilus zeamais Mots. (Coleoptera: Bruchidae) and Solenopsis saevissima (Smith) (Hymenoptera: Formicidae). The mortality range observed for some derivatives, such as 3-(3-methylbenzyl)furan-2-yl N,N,N,,N,-tetraethyldiamidophosphate (82.5% mortality against D. hyalinata; 100% mortality against S. saevissima), was comparable with that of the commercial insecticide chlorpyrifos-methyl. The biological activity of the derivatives depended on the substitution pattern of the benzylic ring. Furan-2-yl N,N,N,,N,-tetraethyldiamidophosphate, furan-2-yl N,N -diethylamidochlorophosphate and difuran-2-yl N,N -diethylamidophosphate were also evaluated, displaying, in some cases, activity comparable with that of chlorpyrifos-methyl (90%, 100% and 97.5% respectively against A. monuste orseis). Considerable activity was observed for some furan-2(5H)-ones evaluated. CONCLUSION: Ten 3-benzylfuran-2-yl N,N,N,,N,-tetraethyldiamidophosphate derivatives were synthesized and fully characterized from a chemical point of view. The results obtained from the biological assays indicate that this class of compounds can be utilized for the design of new substances endowed with insecticidal activity. Copyright © 2008 Society of Chemical Industry [source] Acridizinium Salts as a Novel Class of DNA-binding and Site-selective DNA-photodamaging Chromophores,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2001Heiko Ihmels ABSTRACT It was demonstrated that the interaction of the aminoacridizinium salts 2a,2d with DNA depends on the substitution pattern of the chromophore. Spectrophotometric and fluorometric titrations of the acridizinium salts 2a,2d with natural and synthetic polynucleotides reveal that the degree of interaction of the acridizinium salts 2a,2d with the nucleic acid differs significantly. The binding mode of the dyes with DNA was evaluated by circular dichroism and linear dichroism spectroscopy and compared with the parent system 2c. Whereas the 9-aminoacridizinium (2a) mainly intercalates into DNA, the salts 2b,c show a higher degree of association to the DNA backbone. The intercalated aminoacridizinium 2a caused few strand breaks upon UVA exposure, whereas the salts 2b,2d exhibit relatively efficient DNA-damaging properties. All acridizinium salts showed a sequence-selective strand cleavage for guanine-rich DNA regions. [source] Differentiation of fluoronitroaniline isomers by negative-ion electrospray mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 3 2006ej Gierczyk Tetra- and trifluoronitroanilines were studied by electrospray ionization mass spectrometry. These compounds gave signals only in the negative-ion mode. It was found that the so-called ,in-source' fragmentation, induced by cone voltage increase, enables differentiation of isomers. For para -nitroanilines, in contrast to ortho derivatives, the loss of NO2 was the most favored process and other fragment ions were characterized by low abundances. For trifluoro conjugates the substitution pattern of aromatic ring by fluorine atoms also affected their fragmentation patterns. For example, in 2,3,6-trifluoro-4-nitroaniline, in contrast to 2,3,5-trifluoro-4-nitroaniline, efficient NO loss, followed by HF loss, took place. Copyright © 2005 John Wiley & Sons, Ltd. [source] Tunable haptotropic metal migration in fused arenes: towards organometallic switches,THE CHEMICAL RECORD, Issue 2 2004Karl Heinz Dötz Abstract Tricarbonyl chromium complexes of naphthalene derivatives are synthesized by chromium-templated [3,+,2,+,1]-benzannulation and subjected to thermally induced haptotropic rearrangement experiments. Thermodynamic and kinetic parameters for the metal shift demonstrate the influence of the arene substitution pattern. In turn, the chromium template may be tuned as well by phosphorus coligands which allow to accelerate or slow down the isomerization process; this effect quantitatively reflects the steric and electronic properties of the coligand sphere. Proper adjustment of the template allows for a photo-induced reverse migration of the chromium moiety which results in a switchable organometallic device. Experiments with enantiopure arene chromium complexes indicate a stereospecific metal migration. The rearrangement proceeds by an intramolecular mechanism in both directions. Haptotropic isomerization reactions are not limited to bicyclic arenes and can be extended from naphthalenes to phenanthrene or tetra- and pentacyclic heteroarene systems. © 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 4: 61,71; 2004: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20007 [source] Ethyl[tris(3- tert -butyl-5-methylpyrazol-1-yl)hydridoborato]zinc(II)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2010Mukesh Kumar The X-ray crystal structure of the title compound, [Zn(C2H5)(C24H40BN6)], or TptBu,MeZnEt [TptBu,Me is tris(3- tert -butyl-5-methylpyrazolyl)hydridoborate], reveals a distorted tetrahedral geometry around the Zn atom. The Zn center is coordinated by three N atoms of the borate ligand and by one C atom of the ethyl group. The present structure and other tetrahedral Tp zinc alkyl complexes are compared with similar Ttz ligands (Ttz is 1,2,4-triazolylborate), but no major differences in the structures are noted, and it can be assumed that variation of the substitution pattern of Tp or Ttz ligands has little or no influence on the geometry of alkylzinc complexes. Refinement of the structure is complicated by a combination of metric pseudosymmetry and twinning. The metrics of the structure could also be represented in a double-volume C -centered orthorhombic unit cell, and the structure is twinned by one of the orthorhombic symmetry operators not present in the actual structure. The twinning lies on the borderline between pseudomerohedral and nonmerohedral. The data were refined as being nonmerohedrally twinned, pseudomerohedrally twinned and untwinned. None of the approaches yielded results that were unambiguously better than any of the others: the best fit between structural model and data was observed using the nonmerohedral approach which also yielded the best structure quality indicators, but the data set is less than 80% complete due to rejected data. The pseudomerohedral and the untwinned structures are complete, but relatively large residual electron densities that are not close to the metal center are found with values up to three times higher than in the nonmerohedral approach. [source] Application of NMR, molecular simulation, and hydrodynamics to conformational analysis of trisaccharides,BIOPOLYMERS, Issue 4 2003Ann M. Dixon Abstract The preferred conformations and conformational flexibilities of the trisaccharides ,-D-Glcp -(1,2)-,-D-Glcp -(1,3)-,-D-Glcp -OMe (I) and ,-D-Glcp -(1,3)[,-D-Glcp -(1,4)]-,-D-Glcp -OMe (II) in aqueous solution were determined using nuclear magnetic resonance (NMR) spectroscopy, molecular dynamics (MD) and Langevin dynamics (LD) simulations, and hydrodynamics calculations. Both trisaccharides have a vicinal substitution pattern in which long range (nonsequential) interactions may play an important role. LD simulation at 600 K indicated that the all- syn conformation predominated, though other conformations were apparent. NOE data and MD and LD simulations at 298 K all indicated that trisaccharide I is a single all- syn conformer in solution. Given that previous studies showed evidence of anti -conformers in ,-D-Glcp -(1,2)-,-D-Glcp -(1,3)-,-D-Glcp -OMe, this result provides an example of how changing the anomeric configuration of one residue from , to , can make an oligosaccharide more rigid. Discrepancies in inter-ring distances obtained by experiment and by simulation of the all- syn conformer suggest the presence of an anti -, conformation at the ,-(1,4)-linkage for II. A combined analysis of measured and calculated translational diffusion constants and 13C T1 relaxation times yield order parameters of 0.9 for each trisaccharide. This implies that any interconversion among conformations is significantly slower than tumbling. Anisotropies of approximately 1.6 and 1.3 calculated for I and II, respectively, are consistent with the observed relatively flat T1 profiles because the tumbling is not in the motional narrowing regime. Published 2003 Wiley Periodicals, Inc. Biopolymers 69: 448,460, 2003 [source] Disaccharide Mimetics of the Aminoglycoside Antibiotic NeamineCHEMBIOCHEM, Issue 9 2004Andre Venot Dr. Abstract A highly convergent approach has been employed for the facile synthesis of a library of 24 disaccharides that are ,(1,3), ,(1,3), ,(1,4), or ,(1,4) linked and contain 2,4 amino groups. Fourier-transformation ion cyclotron resonance mass spectrometry (FT-ICR MS) has been used to determine dissociation constant (Kd) values for the binding of the disaccharides to a prototypical fragment of 16S ribosomal RNA. Several derivatives bound with affinities similar to that of neamine. Structure,activity relationships have revealed the substitution pattern that is important for high-affinity binding. The compounds described here are unique lead compounds for the design of novel aminoglycoside antibiotics. [source] Regio- and Stereoselective Biomimetic Synthesis of Oligostilbenoid Dimers from Resveratrol Analogues: Influence of the Solvent, Oxidant, and SubstitutionCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2008Saraswati Abstract Oligostilbenoids are polyphenols that are widely distributed in nature with multifaceted biological activities. To achieve biomimetic synthesis of unnatural derivatives, we subjected three resveratrol analogues to oligomerization by means of one-electron oxidants. Upon varying the metal oxidant (AgOAc, CuBr2, FeCl3,6,H2O, FeCl3,6,H2O/NaI, PbO2, VOF3), the solvent (over the whole range of polarities), and the oxygenated substitution pattern of the starting material, stilbenoid oligomers with totally different carbon skeletons were obtained. Here we propose to explain the determinism of the type of skeleton produced with the aid of hard and soft acid/base concepts in conjunction with the solvating properties of the solvents and the preferred alignment by the effect of , stacking. [source] High Diversity on Simple Substrates: 1,4-Dihalo-2-butenes and Other Difunctionalized Allylic Halides for Copper-Catalyzed SN2, ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2008Caroline A. Falciola Dr. Abstract Enantioselective allylic alkylation with an organomagnesium reagent catalyzed by copper thiophene carboxylate (CuTC) was carried out on difunctionalized substrates, such as commercially available 1,4-dichloro-2-butene and 1,4-dibromo-2-butene, and on similar compounds of higher substitution pattern of the olefin for the formation of all-carbon chiral quaternary centers. The high regioselectivity obtained throughout the reactions favored good regiocontrol for the addition of phenyl Grignard reagents. Other difunctionalized substrates (allylic ethers and allylic alcohols) also underwent asymmetric SN2, substitution. [source] Functional [6]Pericyclynes: Aromatization to Substituted carbo -Benzenes,CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2007Catherine Saccavini Dr. Abstract Reductive treatment of stereoisomeric mixtures of variously substituted hexaoxy[6]pericyclynes with SnCl2/HCl led to the corresponding substituted carbo -benzenes. Tetramethoxyhexaphenyl[6]pericylynediol and dimethoxyhexaphenyl[6]pericyclynetetrol thus proved to be alternative precursors of hexaphenyl- carbo -benzene, previously described. Another hexaaryl- carbo -benzenic chromophore with 4-pyridyl and 4-anisyl substituents was targeted for its second-order nonlinear optical properties and was obtained by aromatization of a dimethoxy[6]pericyclynetetrol. Two alkynyl substituents in para positions were also found to be compatible with the C18carbo -benzene ring, provided that the four remaining vertices are substituted by phenyl groups. In the protected series, bis(trimethylsilylethynyl)hexaphenyl- carbo -benzene (C18Ph4(CCTMS)2) could be isolated and fully characterized, even by X-ray crystallography. In the bis-terminal series, the diethynylhexaphenyl- carbo -benzene C18Ph4(CCH)2 could not be isolated in the pure form. It could, however, be generated by two different methods and identified by the corresponding 1H NMR spectra. Unsubstituted carbo -benzene C18H6 remains unknown, but tetraphenyl- carbo -benzenes C18Ph4H2 with two unsubstituted vertices proved to be viable molecules. Whereas the "para" isomer could be characterized by MS and 1H and 13C NMR spectroscopy only in a mixture with polymeric materials, the "ortho" isomer (with adjacent CH vertices) could be isolated, and its structure was determined by using X-ray crystallography. The structure calculated at the B3PW91/6-31G** level of theory turned out to be in excellent agreement with the experimental structure. The 1H and 13C NMR chemical shifts of hexa- and tetraphenyl- carbo -benzenes were also calculated at the B3LYP/6-31+G** level of theory and were found to correlate with experimental spectra. The remote NMR deshielding of peripheral protons (through up to five bonds) revealed a very strong diatropic circulation around the C18 ring, regardless of the substitution pattern. In full agreement with theoretical investigations, it has been demonstrated experimentally that the carbo -benzene ring is "independently" aromatic, in accord with structural,energetic and ,magnetic criteria. [source] Catalytic Ethylene Polymerisation in Carbon Dioxide as a Reaction Medium with Soluble Nickel(II) CatalystsCHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2006Amaia Bastero Dr. Abstract A series of neutral NiII,salicylaldiminato complexes substituted with perfluorooctyl- and trifluoromethyl groups, [Ni{,2 - N,O -6-C(H)NAr-2,4-R,2C6H2O}(Me)(pyridine)] (6,a: Ar=2,6-{4-(F17C8)C6H4}2C6H3, R,=I; 6,b: Ar=2,6-{4-(F3C)C6H4}2C6H3, R,=I; 6,c: Ar=2,6-{3,5-(F3C)2C6H3}2C6H3, R,=3,5-(F3C)2C6H3; 6,d: Ar=2,6-{4-(F17C8)C6H4}2C6H3, R,=3,5-(F3C)2C6H3; 6,e: Ar=2,6-{3,5-(F3C)2C6H3}2C6H3, R,=I) were studied as catalyst precursors for ethylene polymerisation in supercritical CO2. Catalyst precursors 6,a and 6,c, which are soluble in scCO2, afford the highest polymer yields, corresponding to 2×103 turnovers. Semicrystalline polyethylene (Mn typically 104 g,mol,1) is obtained with variable degrees of branching (11 to 24 branches per 1000 carbon atoms, predominantly Me branches) and crystallinities (54 to 21,%), depending on the substitution pattern of the catalyst. [source] | |||||||||||||||||||||||||||||||||||||