Home About us Contact
|
Home About us Contact | ||
|
|
||
Cis Isomers (cis + isomer)
Selected AbstractsStructures and Thermodynamics of the Sulfuranes SF3CN and SF2(CN)2 as well as of the Persulfurane SF4(CN)2 , An ab initio MO Study by the G3(MP2) MethodEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2003Yana Steudel Abstract At the G3(MP2) level of theory the trans isomer 1a of the hypothetical molecule SF4(CN)2 is more stable than the cis isomer 1b by 8 kJ·mol,1. The isomerization of 1a to 1b requires an activation enthalpy of 319 kJ·mol,1 at 298 K. The decomposition of trans -SF4(CN)2 to SF2(CN)2 and F2 is endothermic (,Ho298 = 395 kJ·mol,1) but the elimination of FCN from trans -SF4(CN)2 is exothermic by ,7 kJ·mol,1. The elimination of (CN)2 from cis -SF4(CN)2 is exothermic by ,137 kJ·mol,1. The activation enthalpies for the latter two reactions were calculated as 251 and 311 kJ·mol,1, respectively. Thus, SF4(CN)2 should be a thermally stable compound. In the sulfuranes SF3CN and SF2(CN)2 the CN ligands prefer the equatorial positions; mutual exchange of an axial F atom by an equatorial CN group requires a reaction enthalpy of 51 kJ·mol,1 [SF3CN] or 58 kJ·mol,1 [SF2(CN)2]. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] An Efficient synthesis of orthogonally protected trans - and cis -4-aminopipecolic acidJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2006István Szatmári A straightforward synthesis of orthogonally N,/N, -protected trans - and cis -4-aminopipecolic acid is reported, starting from methyl cis -4-hydroxypiperidine-2-carboxylate. The two diastereomers were synthesized with the aid of C-4 inversion (the trans isomer) or double C-4 inversion (the cis isomer). [source] The Effect of Molecular Environment on the Photoisomerization of Urocanic Acid,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2004Richard A. Wallis ABSTRACT Urocanic acid, imidazole propenoic acid, is a metabolic product of histidine, which accumulates in skin and is excreted in sweat. It absorbs UV radiation at wavelengths shorter than 340 nm, and its principal photochemical reaction is a trans-cis isomerization about the propenyl double bond. This isomerization to the biologically active cis isomer is implicated in the photo-induced suppression of the immune system of skin. The kinetics of the trans,cis photoisomerization of urocanic acid has been determined in a number of solvents, spanning a range of polarities. The initial rates of isomerization and the photostationary trans-cis compositions, in all solvents except water, correlate linearly with solvent polarity. This indicates that the isomerization proceeds through a polar intermediate that is stabilized by coulombic interactions with the molecular environment. [source] Synthesis and anticancer activity of chalcogenide derivatives and platinum(II) and palladium(II) complexes derived from a polar ferrocene phosphanyl,carboxamideAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 5 2010í Schulz Abstract The polar phosphanyl-carboxamide, 1,-(diphenylphosphanyl)-1-[N -(2-hydroxyethyl)carbamoyl]ferrocene (1), reacts readily with hydrogen peroxide and elemental sulfur to give the corresponding phosphane-oxide and phosphane-sulfide, respectively, and with platinum(II) and palladium(II) precursors to afford various bis(phosphane) complexes [MCl2(1 -,P)2] (M = trans -Pd, trans -Pt and cis -Pt). The anticancer activity of the compounds was evaluated in vitro with the complexes showing moderate cytotoxicities towards human ovarian cancer cells. Moreover, the biological activity was found to be strongly influenced by the stereochemistry, with trans -[PtCl2(1 -,P)2] being an order of magnitude more active than the corresponding cis isomer. Copyright © 2010 John Wiley & Sons, Ltd. [source] A new polymorph of cis,transoid,cis -dicyclohexano-18-crown-6.ACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2003Erratum In the Comment of the paper by Kravtsov et al. [Acta Cryst. (2002), C58, o683o684], there in an error in a cited Cambridge Structural Database (Allen, 2002) refcode. The correct text is `Only recently, the atomic coordinates for the known monoclinic polymorph of the cis,transoid,cis isomer became available from a private communication (Nazarenko, 2002; CCDC refcode DCHXCS01).' The updated reference is given below. [source] A Photoactivated trans -Diammine Platinum Complex as Cytotoxic as CisplatinCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2006Fiona S. Mackay Abstract The synthesis and X-ray structure (as the tetrahydrate) of the platinum(IV) complex trans,trans,trans -[Pt(N3)2(OH)2(NH3)2] 3 are described and its photochemistry and photobiology are compared with those of the cis isomer cis,trans,cis -[Pt(N3)2(OH)2(NH3)2] 4. Complexes 4 and 3 are potential precursors of the anticancer drug cisplatin and its inactive trans isomer transplatin, respectively. The trans complex 3 is octahedral, contains almost linear azide ligands, and adopts a layer structure with extensive intermolecular hydrogen bonding. The intense azide-to-platinum(IV) charge-transfer band of complex 3 (285 nm; ,=19,500,M,1,cm,1) is more intense and bathochromically shifted relative to that of the cis isomer 4. In contrast to transplatin, complex 3 rapidly formed a platinum(II) bis(5,-guanosine monophosphate) (5,-GMP) adduct when irradiated with UVA light, and did not react in the dark. Complexes 3 and 4 were non-toxic to human skin cells (keratinocytes) in the dark, but were as cytotoxic as cisplatin on irradiation for a short time (50 min). Damage to the DNA of these cells was detected by using the "comet" assay. Both trans- and cis -diammine platinum(IV) diazide complexes therefore have potential as photochemotherapeutic agents. [source] Synthesis of 1,3-Diphospha-2,3-dihydro-1H -phenalenesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2010Arkadii Tarasevych Abstract The reaction of 1,8-dilithionaphthalene (1) with methylenebis[(dialkylamino)chlorophosphanes] [R2N(Cl)PCH2P(Cl)NR2; R = Me (2a), Et (2b)] leads to the formation of new heterocyclic compounds, 1,3-diphospha-2,3-dihydro-1H -phenalenes 3a,b as a mixture of cis and trans isomers. DFT calculations indicate that the cis isomers are thermodynamically more stable by about 1,3 kcal/mol than the trans isomers. Compounds 3a,b can be converted into dithio the derivatives 5a,b and the borane complexes 6a,b, which were characterized by NMR spectroscopy and investigated by X-ray diffraction analysis. The dialkylamino groups in 3 can be substituted by chlorine to give the chlorophosphane 7. [source] Geometrical isomerisation of eicosapentaenoic and docosahexaenoic acid at high temperaturesEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 7 2006Svein A. Mjøs Abstract Concentrates of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were heated at 140,240,°C for 2,8,h under nitrogen. The trans isomers were analysed by gas chromatography-mass spectrometry on a BPX-70 cyanopropyl column. All geometrical isomers of EPA and DHA with one trans double bond were observed. The rate constants (k) for the isomerisation of the all- cis isomers were calculated and found to be higher than previously reported for linoleic acid and ,-linolenic acid. Arrhenius plots showed a linear relationship between ln,k and the reciprocal absolute temperature above 180,°C. The distribution patterns of isomers with one trans double bond are approximately constant up to a degree of isomerisation of 25%. The degree of isomerisation can therefore be estimated from selected trans peaks. [source] Activation Parameters for the Epoxidation of Substituted cis/trans Pairs of 1,2-Dialkylalkenes by DimethyldioxiraneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2006Brian S. Crow Abstract The first activation parameter data for the reaction of dimethyldioxirane (1) with five cis/trans pairs of alkenes are reported. The epoxidation of cis -1,2-dialkylalkenes (2cis: R1 = Me, R2 = iPr; 3cis: R1 = Me, R2 = tBu; 4cis: R1 = R2 = Et; 5cis: R1 = Et, R2 = iPr; 6cis: R1 = Et, R2 = tBu) and trans -1,2-dialkylalkenes (2trans: R1 = Me, R2 = iPr; 3trans: R1 = Me, R2 = tBu; 4trans: R1 = R2 = Et; 5trans: R1 = Et, R2 = iPr; 6trans: R1 = Et, R2 = tBu) by 1 produced the corresponding epoxides, quantitatively and stereospecifically, as the sole observable products. Activation parameters of the epoxidation of the five pairs of alkenes, 2cis,6cis and 2trans,6trans, by 1 were determined using the Arrhenius method. Enhanced selectivity for cis - vs. trans -alkene epoxidation was observed at lower temperatures. In general, the ,G, terms were larger and showed more variability for the reaction of 1 with trans -alkenes as compared to those for the corresponding cis isomers. The ,H, terms mirrored trends observed in ,G, because ,S, terms for all ten of the compounds were roughly identical. The ,,G, values, a comparison of the trans to the cis isomer data, yielded positive values of 1.2 to 1.8 kcal/mol for the five sets of data and appeared to be dependent on relative steric interactions. The experimental activation parameter data, consistent with predictions from ab initio calculations based on a spiro transition-state model, showed that the lower reactivity of trans -alkenes is due to enthalpy factors. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Cycloaddition Behavior of 1,2-Thiaphospholes: Reactions with Diazocumulenes and with CyclopentadieneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2003Jochen Kerth Abstract 1,2-Thiaphospholes 3a,b react with (1-diazo-2-oxoalkyl)silanes 1a,c to form [1,2]thiaphospholo[2,,3,:3,4][1,3]diphospholo[1,5- b][1,2]thiaphosphole systems 4 with cis - anti - cis configuration of the tricyclic framework. They are accompanied by small amounts of compounds 5 that are presumably the cis - syn - cis isomers of 4, and 6-alkylidene-1-phospha-2-thiabicyclo[3.1.0]hex-3-enes 6. It is likely that these reactions proceed by [3+2] cycloaddition of diazocumulenes, which coexist with diazo compounds as minor equilibrium components, at the P=C bond of the heterophospholes, followed by N2 elimination and formation of short-lived 2-alkylidene-1,2(,5)thiaphospholes. The latter can either add to excess thiaphosphole to form the tricyclic products or undergo electrocyclization to form bicyclic alkylidenephosphiranes. Thiaphosphole 3a does not seem to react directly with cyclopentadiene in a [4+2] or [2+4] cycloaddition. Reaction with excess cyclopentadiene at 120 °C yields the polycyclic compounds 15 and 16, which are likely to arise from a Diels,Alder reaction of 3a, reacting as a heterodiene, with the cyclopentadiene dimer. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Functional implications of pigments bound to a cyanobacterial cytochrome b6f complexFEBS JOURNAL, Issue 2 2005Stephan-Olav Wenk A highly purified cytochrome b6f complex from the cyanobacterium Synechocystis sp. PCC 6803 selectively binds one chlorophyll a and one carotenoid in analogy to the recent published structure from two other b6f complexes. The unknown function of these pigments was elucidated by spectroscopy and site-directed mutagenesis. Low-temperature redox difference spectroscopy showed red shifts in the chlorophyll and carotenoid spectra upon reduction of cytochrome b6, which indicates coupling of these pigments with the heme groups and thereby with the electron transport. This is supported by the correlated kinetics of these redox reactions and also by the distinct orientation of the chlorophyll molecule with respect to the heme cofactors as shown by linear dichroism spectroscopy. The specific role of the carotenoid echinenone for the cytochrome b6f complex of Synechocystis 6803 was elucidated by a mutant lacking the last step of echinenone biosynthesis. The isolated mutant complex preferentially contained a carotenoid with 0, 1 or 2 hydroxyl groups (most likely 9- cis isomers of ,-carotene, a monohydroxy carotenoid and zeaxanthin, respectively) instead. This indicates a substantial role of the carotenoid , possibly for strucure and assembly , and a specificity of its binding site which is different from those in most other oxygenic photosynthetic organisms. In summary, both pigments are probably involved in the structure, but may also contribute to the dynamics of the cytochrome b6f complex. [source] The specificity of alcohol dehydrogenase with cis -retinoidsFEBS JOURNAL, Issue 9 2004Activity with 11- cis -retinol, localization in retina Studies in knockout mice support the involvement of alcohol dehydrogenases ADH1 and ADH4 in retinoid metabolism, although kinetics with retinoids are not known for the mouse enzymes. Moreover, a role of alcohol dehydrogenase (ADH) in the eye retinoid interconversions cannot be ascertained due to the lack of information on the kinetics with 11- cis -retinoids. We report here the kinetics of human ADH1B1, ADH1B2, ADH4, and mouse ADH1 and ADH4 with all- trans -, 7- cis -, 9- cis -, 11- cis - and 13- cis -isomers of retinol and retinal. These retinoids are substrates for all enzymes tested, except the 13- cis isomers which are not used by ADH1. In general, human and mouse ADH4 exhibit similar activity, higher than that of ADH1, while mouse ADH1 is more efficient than the homologous human enzymes. All tested ADHs use 11- cis -retinoids efficiently. ADH4 shows much higher kcat/Km values for 11- cis -retinol oxidation than for 11- cis -retinal reduction, a unique property among mammalian ADHs for any alcohol/aldehyde substrate pair. Docking simulations and the kinetic properties of the human ADH4 M141L mutant demonstrated that residue 141, in the middle region of the active site, is essential for such ADH4 specificity. The distinct kinetics of ADH4 with 11- cis -retinol, its wide specificity with retinol isomers and its immunolocalization in several retinal cell layers, including pigment epithelium, support a role of this enzyme in the various retinol oxidations that occur in the retina. Cytosolic ADH4 activity may complement the isomer-specific microsomal enzymes involved in photopigment regeneration and retinoic acid synthesis. [source] Complexation of 4-dimethylaminoazobenzene with various kinds of cyclodextrins: Effects of cyclodextrins on the thermal cis-to-trans isomerizationINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2002Yoshimi Sueishi On the basis of the change in electronic and induced circular dichroism spectra for complex formation, the complexation of 4-dimethylaminoazobenzene (DAAB) with four kinds of cyclodextrins (,- and ,-cyclodextrin (CD), heptakis(2,6-di- O -methyl)-,-cyclodextrin, and heptakis(2,3,6-tri- O -methyl)-,-cyclodextrin) was studied in methanol,water and dimethyl sulfoxide,water mixtures. It was found that the trans and cis isomers of DAAB form two different types of complex (inclusion and lid type) with CDs, depending on the kinds of CDs and solvents. Further, we have examined the effect of CDs on the thermal cis-to-trans isomerization of DAAB. The accelerated or decelerated effect on the thermal isomerization was observed upon adding CDs. The effects of CDs on the thermal isomerization are discussed in connection with the complexation of the cis-isomer of DAAB with CDs. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 481,487, 2002 [source] Effect of lipid bilayer alteration on transdermal delivery of a high-molecular-weight and lipophilic drug: Studies with paclitaxelJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 9 2004Ramesh Panchagnula Abstract Skin forms an excellent barrier against drug permeation, due to the rigid lamellar structure of the stratum corneum (SC) lipids. Poor permeability of drugs can be enhanced through alteration in partition and diffusion coefficients, or concentration gradient of drug with an appropriate choice of solvent system, along with penetration enhancers. The aim of the current investigation was to assess applicability of lipid bilayer alteration by fatty acids and terpenes toward the permeation enhancement of a high-molecular-weight, lipophilic drug, paclitaxel (PCL) through rat skin. From among the fatty acids studied using ethanol/isopropyl myristate (1:1) vehicle, no significant enhancement in flux of PCL was observed (p,>,0.05). In the case of cis mono and polyunsaturated fatty acids lag time was found to be similar to control (p,>,0.05). This suggests that the permeation of a high-molecular-weight, lipophilic drug may not be enhanced by the alteration of the lipid bilayer, or the main barrier to permeation could lie in lower hydrophilic layers of skin. A significant increase in lag time was observed with trans unsaturated fatty acids unlike the cis isomers, and this was explained on the basis of conformation and preferential partitioning of fatty acids into skin. From among the terpenes, flux of PCL with cineole was significantly different from other studied terpenes and controls, and after treatment with menthol and menthone permeability was found to be reduced. Menthol and menthone cause loosening of the SC lipid bilayer due to breaking of hydrogen bonding between ceramides, resulting in penetration of water into the lipids of the SC lipid bilayer that leads to creation of new aqueous channels and is responsible for increased hydrophilicity of SC. This increased hydrophilicity of the SC bilayer might have resulted in unfavorable conditions for ethanol/isopropyl myristate (1:1) along with PCL to penetrate into skin, therefore permeability was reduced. The findings of this study suggest that the permeation of a high-molecular-weight and lipophilic drug cannot be enhanced through bilayer alteration by penetration enhancers, and alteration in partitioning of drug into skin could be a feasible mode to enhance the permeation of drug. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:2177,2183, 2004 [source] Enthalpies of formation and isomerization of cis - and trans -decalin, and their oxa-analogs by G3(MP2)//B3LYP calculationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2009Esko Taskinen Abstract G3(MP2)//B3LYP calculations have been carried out on trans - and cis -decalin, and their mono-, di-, tri-, and tetraoxa-analogs. The main purpose of the work was to obtain enthalpies of formation for these compounds, and to study the relative stabilities of the cis,trans and positional isomers of the various (poly)oxadecalins. Comparison of the computational enthalpies of formation with the respective experimental ones, known only for the decalins and 1,3,5,7-tetraoxadecalins, shows that in both cases the computational values are more negative than the experimental ones, the deviations being ,5 to ,7,kJ,mol,1 for the decalins and ,12 to ,17,kJ,mol,1 for the 1,3,5,7-tetraoxadecalins. The respective computational enthalpies of cis,trans isomerization, however, are in excellent to satisfactory agreement with the experimental data. The cis,trans enthalpy differences vary from +11.0,kJ,mol,1 for decalin to ,15.4,kJ,mol,1 for 1,4,5,8-tetraoxadecalin. Low relative enthalpy values were also calculated for the cis isomers of 1,8-dioxadecalin (,3.7,kJ,mol,1), 1,3,6-trioxadecalin (,4.6,kJ,mol,1), 1,3,8-trioxadecalin (,9.7,kJ,mol,1), 1,4,5- trioxadecalin (,5.6,kJ,mol,1), 1,3,5,8-tetraoxadecalin (,7.3,kJ,mol,1), and 1,3,6,8-tetraoxadecalin (,14.5,kJ,mol,1). Copyright © 2009 John Wiley & Sons, Ltd. [source] Photoinduced Microphase Separation in Block Copolymers: Exploring Shape Incompatibility of Mesogenic Side GroupsMACROMOLECULAR RAPID COMMUNICATIONS, Issue 11 2010Yi Zhao Abstract Photoinduced microphase separation in block copolymers (BCP) was achieved for the first time, using a rationally designed diblock copolymer composed of two side-chain liquid crystalline polymers (SCLCP). The miscibility of the two blocks was promoted by the miscibility between the two types of mesognic side groups, while upon UV exposure inducing the trans,cis isomerization of azobenzene mesogens on one SCLCP, the shape incompatibility of bent cis isomers with an ordered liquid crystalline phase drove the separation of the two blocks resulting in a microphase separated morphology. This result shows the perspective of using light to process and organize BCP morphology and related nanostructures in a lithography-free manner. [source] Trifluoroethanol and binding to model membranes stabilize a predicted turn in a peptide corresponding to the first extracellular loop of the angiotensin II AT1A receptorBIOPOLYMERS, Issue 1 2002Roberto K. Salinas Abstract Homology modeling of the angiotensin II AT1A receptor based on rhodopsin,s crystal structure has assigned the 92,100 (YRWPFGNHL) sequence of the receptor to its first extracellular loop. Solution and membrane-bound conformational properties of a peptide containing this sequence (EL1) were examined by CD, fluorescence, and 1H-NMR. CD spectra in aqueous solution revealed an equilibrium between less organized and folded conformers. NMR spectra indicated the coexistence of trans and cis isomers of the Trp3,Pro4 bond. A positive band at 226 nm in the CD spectra suggested aromatic ring stacking, modulated by EL1's ionization degree. CD spectra showed that trifluoroethanol (TFE), or binding to detergent micelles and phospholipid bilayers, shifted the equilibrium toward conformers with higher secondary structure content. Different media gave rise to spectra suggestive of different ,-turns. Chemical shift changes in the NMR spectra corroborated the stabilization of different conformations. Thus, environments of lower polarity or binding to interfaces probably favored the formation of hydrogen bonds, stabilizing ,-turns, predicted for this sequence in the whole receptor. Increases in Trp3 fluorescence intensity and anisotropy, blue shifts of the maximum emission wavelength, and pK changes also evinced the interaction between EL1 and model membranes. Binding was seen to depend on both hydrophobic and electrostatic interactions, as well as lipid phase packing. Studies with water-soluble and membrane-bound fluorescence quenchers demonstrated that Trp3 is located close to the water,membrane interface. The results are discussed with regard to possible implications in receptor folding and function. © 2002 Wiley Periodicals, Inc. Biopolymers 65: 21,31, 2002 [source] Chemoenzymatic Route to Both Enantiomers of a 1-Isopropyl-3a-methyloctahydroinden-4-one Derivative: A Synthetic Intermediate for Sesqui- and DiterpenoidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7-8 2005Shigeo Fujieda Abstract On the way to a chemoenzymatic synthesis of a key intermediate for sesquiterpenoids and diterpenoids, 2-methyl-2-(4-methyl-3-oxopentyl)-1,3-cyclohexanedione was reduced with the whole cells of yeast biocatalysts. Torulaspora delbrueckii NBRC10921 reduced a cyclic ketone of three carbonyl groups in an enantiofacially selective manner (re -face attack), but there was poor enantiotopic group selectivity between two carbonyl groups on the cyclohexane ring to yield a mixture of diastereomeric products. Candida floricola IAM13115 reduced mainly the pro -(R) carbonyl group. In contrast, the reduction proceeded in an enantiofacially poorly selective manner to give another set of diastereomeric products. In both cases, another carbonyl group on the side chain worked as a ,trapping arm' of the resulting secondary alcohol. The diastereomeric products were effectively separated as the ,syn' or ,cis' isomer exclusively exist in the intramolecular hemiacetal structure, while ,anti' or ,trans' isomer being an equilibrated mixture of cyclic hemiacetal and open-chain hydroxyketone (ca. 0.7,:,1). Starting separately from the enantiomerically enriched products as above, both enantiomers of the target compound, a key intermediate for terpenoids, were efficiently prepared via stereoselective ring closure under pinacol coupling reaction conditions. Furthermore, a daucane sesquiterpene intermediate, a hydroazulene derivative, was provided after one-carbon homologation of the six-membered ring. [source] | |||||||||||||||||||||||||