			Home About us Contact

Z Isomer (z + isomer)

Distribution by Scientific Domains

Chemistry	90%

Selected Abstracts

ChemInform Abstract: Selective Synthesis of E and Z Isomers of Oximes.

CHEMINFORM, Issue 17 2001
Hashem Sharghi
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

The First Enantioselective Halo Aldol Reaction of Ethyl Propiolate and Aldehydes

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 15 2004
Dianjun Chen
Abstract The first enantioselective halo aldol reaction of ethyl propiolate with aldehydes has been established by using Jacobsen's chiral cyclohexylsalen ligand. The reaction was conducted at ,20 °C in dichloromethane with Et2AlI as the source of halogen and Lewis acid promoter. Excellent geometric selectivity (only the Z isomer) has been achieved for all 14 examples examined. The reaction works well with aromatic aldehydes; it also works with aliphatic aldehydes and ,,,-unsaturated aldehydes, albeit with diminished yields and ee. This method provides the first enantioselective synthesis of ,-iodo Morita,Baylis,Hillman (MBH) esters. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Stereochemical Control by an Ester Group or Olefin Ligand in Platinum-Catalyzed Carboalkoxylation of 6-(1-Alkoxyethoxy)- hex-2-ynoates

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7-8 2009
Itaru Nakamura
Abstract The cyclization of 6-(1-alkoxyethoxy)hex-2-ynoantes in the presence of the platinum-olefin catalyst system gave the corresponding multisubstituted 2-[dihydrofuran-2(3H)-ylidene]acetates in good to high yields. The Z/E selectivity is controlled by the electronic property of the ester group; the 2,2,2-trichloroethyl ester yielded the Z isomer, while the phenyl ester gave the E isomer. Moreover, we found that the Z/E selectivities in the reaction of phenyl esters 1h, 1n, and 1o were controlled by the olefin ligand. For example, the platinum-catalyzed reaction of 1h using 1,5-hexadiene as the olefin ligand gave E - 2h as the major product, while that using 1,5-cyclooctadiene produced mainly the Z isomer. [source]

Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

MAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2009
Andrei V. Afonin
Abstract In the 1H and 13C NMR spectra of 1-(2-selenophenyl)-1-alkanone oximes, the 1H, the 13C-3 and 13C-5 signals of the selenophene ring are shifted by 0.1,0.4, 2.5,3.0 and 5.5,6.0 ppm, respectively, to higher frequencies, whereas those of the 13C-1, 13C-2 and 13C-4 carbons are shifted by 4,5, ,11 and ,1.7 ppm to lower frequencies on going from the E to Z isomer. The 15N chemical shift of the oximic nitrogen is larger by 13,16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the 77Se resonance positions is revealed in the studied oxime isomers, the 77Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the 1H, 13C, 15N and 77Se shielding constants in the energy-favorable conformation with the syn orientation of theCNOH group relative to the selenophene ring. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Alpha-retinals as Rhodopsin Chromophores,Preference for the 9- Z Configuration and Partial Agonist Activity,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008
Yajie Wang
The visual pigment rhodopsin, the photosensory element of the rod photoreceptor cell in the vertebrate retina, shows in combination with an endogenous ligand, 11- Z retinal, an astonishing photochemical performance. It exhibits an unprecedented quantum yield (0.67) in a highly defined and ultrafast photoisomerization process. This triggers the conformational changes leading to the active state Meta(rhodopsin) II. Retinal is covalently bound to Lys-296 of the protein opsin in a protonated Schiff base. The resulting positive charge delocalization over the terminal part of the polyene chain of retinal creates a conjugation defect that upon photoexcitation moves to the opposite end of the polyene. Shortening the polyene as in 4,5-dehydro,5,6-dihydro (alpha), 5,6-dihydro or 7,8-dihydro-analogs might facilitate photoisomerization of a 9- Z and a 11- Z bond. Here we describe pigment analogs generated with bovine opsin and 11- Z or 9- Z 4,5-dehydro,5,6-dihydro-retinal that were further characterized by UV,Vis and FTIR spectroscopy. The preference of opsin for native 11- Z retinal over the 9- Z isomer is reversed in 4,5-dehydro,5,6-dihydro-retinal. 9- Z 4,5-dehydro,5,6-dihydro-retinal readily generated a photosensitive pigment. This modification has no effect on the quantum yield, but affects the Batho,blueshifted intermediate (BSI) equilibrium and leads to a strong decrease in the G-protein activation rate because of a downshift of the pKa of the Meta I,Meta II equilibrium. [source]

The Z isomer of 2,4-diaminofuro[2,3- d]pyrimidine antifolate promotes unusual crystal packing in a human dihydrofolate reductase ternary complex

ACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 8 2009
Vivian Cody
The crystal structure of the ternary complex of human dihydrofolate reductase (hDHFR) with NADPH and the Z isomer of 2,4-diamino-5-[2-(2,-methoxyphenyl)propenyl]-furo[2,3- d]pyrimidine (Z1) shows that the Z isomer binds in the normal antifolate orientation in which the furo oxygen occupies the 8-amino position observed in the binding of 2,4-diaminopteridine antifolates such as methotrexate and with the methoxyphenyl moiety cis to and coplanar with the furo[2,3- d]pyrimidine ring. The hDHFR ternary complex crystallized in the orthorhombic space group P212121 and its structure was refined to 1.7,Å resolution. Although other hDHFR complexes crystallize in this space group, these data provide only the second example of an unusual packing arrangement in which the conserved active-site Arg70 forms a salt bridge to the side chain of Glu44 from a symmetry-related molecule. As a result, the conformations of Phe31 and Gln35 shift with respect to those observed in the structure of mouse DHFR bound to Z1, which crystallizes in the monoclinic space group P21 and shows that Gln35 interacts with Arg70. [source]

A Formal Total Synthesis of Eleutherobin Using the Ring-Closing Metathesis (RCM) Reaction of a Densely Functionalized Diene as the Key Step: Investigation of the Unusual Kinetically Controlled RCM Stereochemistry

CHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2006
Damiano Castoldi Dr.
Abstract Asymmetric oxyallylation reactions and ring-closing metathesis have been used to synthesize compound 3, a key advanced intermediate used in the total synthesis of eleutherobin reported by Danishefsky and co-workers. The aldehyde 6, which is readily prepared from commercially available R -(,)-carvone in six steps in 30,% overall yield on multigram quantities, was converted into the diene 5 utilizing two stereoselective titanium-mediated Hafner,Duthaler oxyallylation reactions. The reactions gave the desired products (8 and 12) in high yields (73 and 83,%, respectively) as single diastereoisomers, with the allylic alcohol already protected as the p -methoxyphenyl (PMP) ether, which previous work has demonstrated actually aids ring-closing metathesis compared to other protective groups and the corresponding free alcohol. Cyclization under forcing conditions, using Grubbs' second-generation catalyst 13, gave the ten-membered carbocycle (E)- 14 in 64,% yield. This result is in sharp contrast to similar, but less functionalized, dienes, which have all undergone cyclization to give the Z stereoisomers exclusively. A detailed investigation of this unusual cyclization stereochemistry by computational methods has shown that the E isomer of the ten-membered carbocycle is indeed less thermodynamically stable than the corresponding Z isomer. In fact, the selectivity is believed to be due to the dense functionality around the ruthenacyclobutane intermediate that favors the trans -ruthenacycle, which ultimately leads to the less stable E isomer of the ten-membered carbocycle under kinetic control. During the final synthetic manipulations the double bond of enedione (E)- 16 isomerized to the more thermodynamically stable enedione (Z)- 4, giving access to the advanced key-intermediate 3, which was spectroscopically and analytically identical to the data reported by Danishefsky and co-workers, and thereby completing the formal synthesis of eleutherobin. [source]

Synthesis of 3-Fluorofuran-2(5H)-ones Based on Z/E Photoisomerisation and Cyclisation of 2-Fluoro-4-hydroxybut-2-enoates

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 35 2007
Karel Pomeisl
Abstract Mixtures of some (E)- and (Z)-2-fluoroalk-2-enoates prepared from the corresponding 2-hydroxycarbonyl compounds and ethyl 2-(diethoxyphosphoryl)-2-fluoroacetate have been transformed in high conversions into the target 3-fluorofuran-2(5H)-ones by an efficient Z/E photoisomerisation of noncyclisable Z isomers followed by acid-catalysed cyclisation. In contrast, the acid-catalysed deprotection of ethyl (E)- and (Z)-4-[tert -butyl(dimethyl)silyloxy]-2-fluoro-4-phenylbut-2-enoates resulted in the displacement of vinylicfluorine, affording ethyl (E)-2-oxo-4-phenylbut-3-enoate. 3-Fluoro-4-phenylfuran-2(5H)-one was transformed into 2-[tert -butyl(dimethyl)silyloxy]-3-fluoro-5-methylfuran as a novel fluorinated building block.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

Pentiptycene-Derived Light-Driven Molecular Brakes: Substituent Effects of the Brake Component

CHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2010
Wei-Ting Sun
Abstract Five pentiptycene-derived stilbene systems (1,R; R=H, OM, NO, Pr, and Bu) have been prepared and investigated as light-driven molecular brakes that have different-sized brake components (1,H<1,OM<1,NO<1,Pr<1,Bu). At room temperature (298,K), rotation of the pentiptycene rotor is fast (krot=108,109,s,1) with little interaction with the brake component in the trans form ((E)- 1,R), which corresponds to the brake-off state. When the brake is turned on by photoisomerization to the cis form ((Z)- 1,R), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)- 1,NO, (Z)- 1,Pr, and (Z)- 1,Bu, the rotation is nearly blocked (krot=2,6,s,1) at 298,K. It is also demonstrated that the rotation is slower in [D6]DMSO than in CD2Cl2. A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)- 1,H, (Z)- 1,OM, and (Z)- 1,NO, and it levels off on going from (Z)- 1,NO to (Z)- 1,Pr and (Z)- 1,Bu. DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E,Z photoswitching is up to 46,%, and both the E and Z isomers are stable under the irradiation conditions. [source]

A [3]Rotaxane with Three Stable States That Responds to Multiple-Inputs and Displays Dual Fluorescence Addresses

CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2005
Da-Hui Qu
Abstract A [3]rotaxane molecular shuttle containing two ,-cyclodextrin (,-CD) macrocycles, an azobenzene unit, a stilbene unit, and two different fluorescent naphthalimide units has been investigated. The azobenzene unit and the stilbene unit can be E/Z -photoisomerized separately by light excited at different wavelengths. Irradiation at 380 nm resulted in the photoisomerization of the azobenzene unit, leading to the formation of one stable state of the [3]rotaxane (Z1- NNAS-2CD); irradiation at 313 nm resulted in the photoisomerization of the stilbene unit, leading to the formation of another stable state of the [3]rotaxane (Z2- NNAS-2CD). The reversible conversion of the Z1 and Z2 isomers back to the E isomer by irradiation at 450 nm and 280 nm, respectively, is accompanied by recovery of the absorption and fluorescence spectra of the [3]rotaxane. The E isomer and the two Z isomers have been characterized by 1H NMR spectroscopy and by two-dimensional NMR spectroscopy. The light stimuli can induce shuttling motions of the two ,-CD macrocycles on the molecular thread; concomitantly, the absorption and fluorescence spectra of the [3]rotaxane change in a regular way. When the ,-CD macrocycle stays close to the fluorescent moiety, the fluorescence of the moiety become stronger due to the rigidity of the ,-CD ring. As the photoisomerization processes are fully reversible, the photo-induced shuttling motions of the ,-CD rings can be repeated, accompanied by dual reversible fluorescence signal outputs. The potential application of such light-induced mechanical motions at the molecular level could provide some insight into the workings of a molecular machine with entirely optical signals, and could provide a cheap, convenient interface for communication between micro- and macroworlds. [source]