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Thermal Isomerization (thermal + isomerization)
Selected AbstractsDiradical-Promoted Two-Carbon Ring-Expansion Reactions by Thermal Isomerization: Synthesis of Functionalized Macrocyclic KetonesHELVETICA CHIMICA ACTA, Issue 7 2004Georg Rüedi A new method for the smooth and highly efficient preparation of functionalized macrocyclic ketones has been developed. Pyrolysis of medium- and large-ring 3-vinylcycloalkanones by dynamic gas-phase thermo-isomerization (DGPTI) at 600,630° yielded, under insertion of a previously attached vinyl side chain by means of a 1,3-C shift, the corresponding ,,, -unsaturated cycloalkanones. The yield of the two-carbon ring-expanded ketones greatly depended on the relative ring strains of substrate and product (5,87%, cf. Table,5). The formation of minor amounts of one-carbon ring-expanded cycloalkenes (<10%) can be ascribed to a subsequent decarbonylation step. A reaction mechanism involving initial cleavage of the weakest single bond in the molecule has been established (cf. Scheme,6). Recombination within the generated diradical intermediate in terminal vinylogous position led to the observed products, while reclosure gave recovered starting material. Substituents on the vinyl moiety were transferred locospecifically into the ring-expanded products. An isopropenyl group did not significantly affect the isomerization process, whereas substrates bearing a prop-1-enyl group in , -position enabled competing intramolecular H-abstraction reactions, leading to acyclic dienones (cf. Schemes,9,11). DGPTI of the 13-membered analogue directly yielded 4-muscenone, which, upon hydrogenation, led to the valuable musk odorant (±)-muscone. Increasing the steric hindrance on the vinyl moiety gave rise to diminishing amounts of the desired ,,, -unsaturated cycloalkanones. This novel two-carbon ring-expansion protocol was also successfully applied to 3-ethynylcycloalkanones, giving rise to the corresponding ring-expanded cyclic allenes (cf. Scheme,13). [source] Visible-Light Photoresponsivity of a 4-(Dimethylamino)azobenzene Unit Incorporated into Single-Stranded DNA: Demonstration of a Large Spectral Change Accompanying Isomerization in DMSO and Detection of Rapid (Z)-to-(E) Isomerization in Aqueous SolutionEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 11 2007Takashi Kamei Abstract We demonstrate significant visible-light photoresponsivity in a synthesized oligonucleotide containing a built-in pseudo-nucleotide possessing a 4-(dimethylamino)azobenzene (4-DMAzo) side chain. In dry DMSO as solvent, two clearly distinguishable spectra corresponding to the (E) and (Z) forms of the 4-DMAzo moiety tethered to the oligonucleotide were recorded with a conventional spectrophotometer before and after irradiation with 420 nm wavelength light, which induced (E)-to-(Z) isomerization. In addition, (Z)-to-(E) isomerization was accelerated by irradiation with either visible (, = 550 nm) or UV (, = 350 nm) light, demonstrating reversible photoresponsivity of the pseudo-oligonucleotide. In aqueous solutions the (Z)-to-(E) thermal isomerization of the photoresponsive pseudo-oligonucleotide was very rapid and was only detectable by laser flash photolysis.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Synthesis and Photochromism of Novel Chromene Derivatives Bearing a Monoazacrown Ether MoietyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 13 2003Saleh A. Ahmed Abstract Crowned chromenes , four novel naphthopyrans (chromenes) bearing monoaza-12-crown-4, -15-crown-5, -18-crown-6, and a noncyclic analogue at the 5-position , were synthesized, and their photochromism in acetonitrile was examined in the presence of alkali and alkaline-earth metal ions. Incorporation of crown ether units to chromene moieties facilitated to a great extent both the thermal isomerization and photoisomerization of the chromene moiety, reflecting the metal-ion-binding ability of the crown ether moiety. Comparison of the crowned chromenes with the corresponding crowned spirobenzopyrans reveals that photochromism of the crowned chromene reflects an interaction between the metal ion and the crown ether moiety that is stronger than that of the crowned spirobenzopyran, and shows only positive photochromism with significant red-shifts in the UV/Vis absorption spectra, in contrast to the crowned spirobenzopyrans reported previously. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] (E/Z)-Isomerization of 3,-EpiluteinHELVETICA CHIMICA ACTA, Issue 8 2004Péter Molnár 3,-Epilutein (=(all- E,3R,3,S,6,R)-4,,5,-didehydro-5,,6,-dihydro- ,,, -carotene-3,3,-diol; 1), isolated from the flowers of Caltha palustris, was submitted to both thermal isomerization and I2 -catalyzed photoisomerization. The structures of the main products (9Z)- 1, (9,Z)- 1, (13Z)- 1, (13,Z)- 1, (15Z)- 1, and (9Z,9,Z)- 1 were determined based on UV/VIS, CD, 1H-NMR, and MS data. [source] Preparation and (E/Z)-Isomerization of the Diastereoisomers of ViolaxanthinHELVETICA CHIMICA ACTA, Issue 1 2004Péter Molnár Violaxanthin A (=(all- E,3S,5S,6R,3,S,5,S,6,R)-5,6,:,5,,6,-diepoxy-5,6,5,,6,-tetrahydro- ,,, -carotene-3,3,-diol =syn,syn- violaxanthin; 5) and violaxanthin B (=(all- E,3S,5S,6R,3,S,5,R,6,S)-5,6,:,5,,6,-diepoxy-5,6,5,,6,-tetrahydro- ,,, -carotene-3,3,-diol=syn,anti- violaxanthin; 6) were prepared by epoxidation of zeaxanthin diacetate (1) with monoperphthalic acid. Violaxanthins 5 and 6 were submitted to thermal isomerization and I2 -catalyzed photoisomerization. The structure of the main products, i.e., (9Z)- 5, (13Z)- 5, (9Z)- 6, (9,Z)- 6, (13Z)- 6, and (13,Z)- 6, was determined by their UV/VIS, CD, 1H-NMR, 13C-NMR, and mass spectra. [source] Kinetics of the thermal isomerization of 1,1,2,2-tetramethylcyclopropaneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2006David K. Lewis Reaction rates for the structural isomerization of 1,1,2,2-tetramethylcyclopropane to 2,4-dimethyl-2-pentene have been measured over a wide temperature range, 672,750 K in a static reactor and 1000,1120 K in a single-pulse shock tube. The combined data from the two temperature regions give Arrhenius parameters Ea=64.7 (±0.5) kcal/mol and log10(A, s,1) = 15.47 (±0.13). These values lie at the upper end of the ranges of Ea and log A values (62.2,64.7 kcal/mol and 14.82,15.55, respectively) obtained from three previous experimental studies, each of which covered a narrower temperature range. The previously noted trend toward lower Ea values for structural isomerization of methylcyclopropanes as methyl substitution increases extends only through the dimethylcyclopropanes (1,1- and 1,2-); Ea then appears to increase with further methyl substitution. In contrast, the pre-exponential factors for isomerization of cyclopropane and all of the methylcyclopropanes through tetramethylcyclopropane lie within ±0.3 of log10(A, s,1) = 15.2 and show no particular trend with increasing substitution. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 483,488, 2006 [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] Gibbs energy of activation for thermal isomerization of (1Z)-acetaldehyde hydrazone and (1Z)-acetaldehyde N,N-dimethylhydrazone by Gaussian-4 theory and CCSD(T)/CBS computationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2009Shih-I Lu Abstract In this article, we examined the Gibbs energy of activation for the Z/E thermal isomerization reaction of (1Z)-acetaldehyde hydrazone and (1Z)-acetaldehyde N,N-dimethylhydrazone, at 298.15 K in the solvent of cyclohexane. We carried out computations employing both the Gaussian-4 (G4) theory and the coupled cluster method using both single and double substitutions and triple excitations noniteratively, CCSD(T). The CCSD(T) energy is extrapolated to the complete basis set (CBS). We compared the calculated results to the available experimental observation. It appeared that both G4 and CCSD(T)/CBS computations overestimated the experimental value by as much as about 6 and 12 kcal/mol in the present two cases. We discussed possible sources of error and proposed the experimental kinetic data could be questionable. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] The effect of temperature on the stability of compounds used as UV-MALDI-MS matrix: 2,5-dihydroxybenzoic acid, 2,4,6-trihydroxyacetophenone, ,-cyano-4-hydroxycinnamic acid, 3,5-dimethoxy-4-hydroxycinnamic acid, nor-harmane and harmaneJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 2 2009Olga I. Tarzi Abstract The thermal stability of several commonly used crystalline matrix-assisted ultraviolet laser desorption/ionization mass spectrometry (UV-MALDI-MS) matrices, 2,5-dihydroxybenzoic acid (gentisic acid; GA), 2,4,6-trihydroxyacetophenone (THA), ,-cyano-4-hydroxycinnamic acid (CHC), 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid; SA), 9H-pirido[3,4-b]indole (nor-harmane; nor-Ho), 1-methyl-9H-pirido[3,4-b]indole (harmane; Ho), perchlorate of nor-harmanonium ([nor-Ho + H]+) and perchlorate of harmanonium ([Ho + H]+) was studied by heating them at their melting point and characterizing the remaining material by using different MS techniques [electron ionization mass spectrometry (EI-MS), ultraviolet laserdesorption/ionization-time-of-flight-mass spectrometry (UV-LDI-TOF-MS) and electrospray ionization-time-of-flight-mass spectrometry (ESI-TOF-MS)] as well as by thin layer chromatography analysis (TLC), electronic spectroscopy (UV-absorption, fluorescence emission and excitation spectrosco y) and 1H nuclear magnetic resonance spectroscopy (1H-NMR). In general, all compounds, except for CHC and SA, remained unchanged after fusion. CHC showed loss of CO2, yielding the trans-/cis -4-hydroxyphenylacrilonitrile mixture. This mixture was unambiguously characterized by MS and 1H-NMR spectroscopy, and its sublimation capability was demonstrated. These results explain the well-known cluster formation, fading (vanishing) and further recovering of CHC when used as a matrix in UV-MALDI-MS. Commercial SA (SA 98%; trans -SA/cis -SA 5 : 1) showed mainly cis- to- trans thermal isomerization and, with very poor yield, loss of CO2, yielding (3,,5,-dimethoxy-4,-hydroxyphenyl)-1-ethene as the decarboxilated product. These thermal conversions would not drastically affect its behavior as a UV-MALDI matrix as happens in the case of CHC. Complementary studies of the photochemical stability of these matrices in solid state were also conducted. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||