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Side Reactions (side + reaction)
Selected AbstractsOxime Carbonates: Novel Reagents for the Introduction of Fmoc and Alloc Protecting Groups, Free of Side ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2010Sherine N. Khattab Abstract Fmoc and Alloc protecting groups represent a consistent alternative to classical Boc protection in peptide chemistry. The former was established in the last decades as the ,-amino protecting group of choice, whereas the latter allows a fully orthogonal protection strategy with Fmoc and Boc. Usually, the introduction of the Fmoc and Alloc moieties takes place through their halogenoformates, azides, or activated carbonates. This rather simple reaction is accompanied by several side reactions, specially the formation of Fmoc/Alloc dipeptides and even tripeptides. The present work describes new promising Fmoc/Alloc-oxime reagents, which are easy to prepare, stable, and highly reactive crystalline materials that afford almost contaminant-free Fmoc/Alloc-amino acids in high yields by following a conventional procedure. Amongst the Fmoc-oxime derivatives, the N -hydroxypicolinimidoyl cyanide derivative (N -{[(9H-fluoren-9-yl)methoxy]carbonyloxy}picolinimidoyl cyanide) gave the best results for the preparation of Fmoc-Gly-OH, which is the most predisposed to give side reactions. The same Alloc-oxime analogue afforded the preparation of Alloc-Gly-OH in good yield, purity, and extremely low dipeptide formation, as analyzed by reverse-phase HPLC and NMR spectroscopy. [source] Synthesis and Characterization of the Nitrogen-Rich Hyperbranched Polymers , Poly([1,2,3]-Triazole-[1,3,5]-Triazine)sPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 6 2008Georgiy Abstract Novel hyperbranched poly([1,2,3]-triazole-[1,3,5]-triazine)s (HBP TT) were synthesized by a 1,3-dipolar cycloaddition reaction from AB2 monomer , 2-azido-4,6-bis-prop-2-yn-1-yloxy- [1,3,5]-triazine (ABPOT). The monomer contains one azide group A and two terminal alkyne units B. Thermal polymerization of ABPOT in bulk or in DMF solution leads to hyperbranched polymers containing both 1,4- and 1,5-disubstituted [1,2,3]-triazoles. The monomer was also polymerized catalytically in the presence of Cu(I) salts under mild reaction conditions in DMSO solution and in bulk affording hyperbranched poly-[1,2,3]-triazoles 1,4-disubstituted only. The reactions lead to the products soluble in aprotic polar solvents like DMSO or DMF. Side reactions can proceed in a few cases, particularly: (i) homocoupling of alkyne groups, leading to the formation of insoluble products as a result of cross-linking, (ii) isomerization of propynyloxytriazine fragments to propynyl- or propadienyltriazinone ones, and (iii) hydrolysis of triple bonds without the loss of solubility. Heats of formation of monomer and synthesized polymers were calculated from their combustion heats. All products were characterized by NMR-, IR-spectroscopy, and size exclusion chromatography (SEC) data. The obtained results open the prospect for the use of HBP TT as the high-enthalpy modifiers for energetic and non-energetic binders. [source] An Electroanalytical Investigation on the Redox Properties of Calcium Antagonist DihydropyridinesELECTROANALYSIS, Issue 10 2003Rosanna Toniolo Abstract The antioxidant capacity of some calcium antagonists and one calcium agonist 1,4-dihydropyridines (DHPs) was evaluated by a competitive kinetic procedure. With the exception of Amlodipine, all the calcium antagonist DHPs display an unambiguous antioxidant capacity, while for the calcium agonist DHP (Bay K 8644) no measurable reactivity towards peroxyl radicals could be detected. The finding was corroborated by an electroanalytical investigation of the redox properties of DHPs compounds to get an insight about both the thermodynamic constraints of their oxidation process and reaction pattern. The oxidation potentials decrease with both antioxidant capacity and increasing basic character, thus suggesting the relevance of the electron density on the DHP ring. For all the compounds investigated, the overall oxidation process takes place through a primary one-electron step accompanied by a fast proton release and the formation of a neutral radical undergoing a second much easier one-electron step. The protonated form of the parent pyridine derivative is thus generated as the final product. This pattern is relevant for the antioxidant effect, since the radical intermediate is much more prone to be oxidized than to be reduced, thus fully preventing the propagation of the oxidative chain reaction. In the case of calcium antagonist DHPs, the above release of protons complicates the overall oxidation process by introducing a parasitic side reaction where a coupling between protons and the starting species takes place. This DHP self-protonation subtracts part of the original species from the electrode process because the parent cationic species is no longer electroactive. Conversely, the calcium agonist DHP, which is more difficult to be oxidized, turned out to be such a weak base as to be unable to undergo the self-protonation reaction. The combined effect of oxidation potentials and proton binding capacity of DHPs is a key element for the redox transition, which could support their antioxidant effect and should be considered to some extent in accounting for the calcium antagonist vs calcium agonist effect. [source] Structural and Theoretical Insights into Metal,Scorpionate Ligand ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2007Matthias Schwalbe Abstract The syntheses of the complexes [M(TmMe)(CO)2(NO)] (M = Mo, W) by reaction of NOBF4 with [M(TmMe)(CO)3], are reported and their spectroscopic characterisation and crystal structures are described. The analogous Cr complex could not be prepared by this methodology. The complexes adopt the expected pseudo-octahedral geometry. Complexes [M(L)(CO)2(NO)] (M = Cr, Mo, W; L = Cp, Tp and TmMe) together with the hypothetical [Mo(CO)2(NO)]+ cation were subjected to DFT calculations. Geometry-optimised structures closely parallel the crystallographic determinations and indicate that the complex [Cr(TmMe)(CO)2(NO)] is not inherently unstable. The DFT calculations allow the assignment of the C,O and N,O stretches in the IR spectrum and give insight into both the M,NO bonding and the metal to tripodal ligand bonding. The electron-donor strengths are confirmed to lie in the order TmMe > Tp > Cp. A side reaction of the B,H moiety of the TmMe anion with NO+ results in the isolation of the dimethylformamide adduct of (trismethimazolyl)borane, providing further evidence that the reaction pathways of the TmR ligands are more varied and less passive than in the chemistry of the nitrogen-based scorpionates.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Complexes of Bidentate Phosphane Selenide Ligands with Mesitylenetellurenyl Iodide and with Tellurium Diiodide,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2006Cristian G. Hrib Abstract Bis(diphenylphosphanyl)methane diselenide (dppmSe2, 1) and bis(diphenylphosphanyl)ethane diselenide (dppeSe2, 2) reacted with 2 equiv. Br2 or I2 to form the insoluble solid products, dppm(SeX2)2 (X = Br, 3; X = I, 4) and dppe(SeX2)2 (X = Br, 5; X = I, 6). However, using the iodine-like electrophile mesitylenetellurenyl iodide (MesTeI, 7), fairly soluble complexes, dppmSe2[Te(I)Mes]2 (8) and dppeSe2[Te(I)Mes]2 (9), were obtained. Complexes 8 and 9 contain two T-shaped (10-Te-3) Se,Te(Mes),I moieties bridged by dppm or dppe; solid 9 exhibits intermolecular soft,soft interactions between approximately linear Se,Te,I units. In a side reaction accompanying the crystallisation of complex 8, or by the reaction of 1 with Te and I2, a chelate complex dppmSe2TeI2 (10) was formed. Fortuitously, a crystal of the related compound dppeSe2TeI2 (11) was also obtained. In 10, a square planar cis -Se2TeI2 group is part of a six-membered ring, and 11 is a coordination polymer with trans -Se2TeI2 moieties bridged by dppe. Averaged 31P- and 77Se NMR signals including 77Se,31P couplings, together with broad 125Te NMR singlets indicate phosphane selenide ligand exchange in solution, that is, the kinetically labile behaviour of complexes 8 and 9. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Alkylzinc Complexes with Achiral and Chiral Monoanionic N,N,O Heteroscorpionate LigandsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2003Ina Hegelmann Abstract The synthesis of the new chiral ligand (3,5-di- tertbutylpyrazol-1-yl)(3,,5,-dimethylpyrazol-1-yl)aceticacid (bpaHtBu2,Me2) (4) has been achieved. Two different synthetic routes to its precursor 3,5-di- tert -butyl-1-[(3,5-dimethyl-1H -pyrazol-1-yl)methyl]-1H -pyrazole (bpmtBu2,Me2) (3) are reported. Deprotonation at the methylene group, followed by reaction with carbon dioxide, yielded a racemic mixture of 4. The chemical behaviour of bis(3,5-di- tert -butylpyrazol-1-yl)acetic acid (bdtbpzaH) (2) and the new chiral N,N,O scorpionate ligand 4 involving their coordination to zinc ions was studied. [Zn(bpatBu2,Me2)Cl] (5) was formed from a mixture of ZnCl2, 4 and base. Reaction of bis(3,5-di- tert -butylpyrazol-1-yl)acetic acid (bdtbpzaH) (2) with Zn(CH3)2 or Zn(CH2CH3)2 gave the alkylzinc complexes [Zn(bdtbpza)(CH3)] (6) and [Zn(bdtbpza)(CH2CH3)] (7). [Zn(bpatBu2,Me2)(CH3)] (8) was obtained from a synthesis analogous to that of 6 with 4. The further reactions of 6 and 8 with acetic acid resulted in the acetato complexes [Zn(OAc)(bdtbpza)] (9) and [Zn(OAc)(bpatBu2,Me2)] (10). The chiral methyl complex 8 may serve as a precursor for structural model complexes of the active sites of zinc enzymes, such as thermolysin or carboxypeptidase A. [Zn(bpatBu2,Me2)2] (11) was formed from a side reaction. Crystal structures of 4, 5, 8 and 11 were obtained; 5 crystallised as the dimer [Zn(bpatBu2,Me2)Cl]2; 11 presents an unusual zinc binding geometry. (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003) [source] Side chain specificity of ADP-ribosylation by a sirtuinFEBS JOURNAL, Issue 23 2009Kamau Fahie Endogenous mono-ADP-ribosylation in eukaryotes is involved in regulating protein synthesis, signal transduction, cytoskeletal integrity, and cell proliferation, although few cellular ADP-ribosyltransferases have been identified. The sirtuins constitute a highly conserved family of protein deacetylases, and several family members have also been reported to perform protein ADP-ribosylation. We characterized the ADP-ribosylation reaction of the nuclear sirtuin homolog Trypanosoma brucei SIR2-related protein 1 (TbSIR2RP1) on both acetylated and unacetylated substrates. We demonstrated that an acetylated substrate is not required for ADP-ribosylation to occur, indicating that the reaction performed by TbSIR2RP1 is a genuine enzymatic reaction and not a side reaction of deacetylation. Biochemical and MS data showed that arginine is the major ADP-ribose acceptor for unacetylated substrates, whereas arginine does not appear to be the major ADP-ribose acceptor in reactions with acetylated histone H1.1. We performed combined ab initio quantum mechanical/molecular mechanical molecular dynamics simulations, which indicated that sirtuin ADP-ribosylation at arginine is energetically feasible, and involves a concerted mechanism with a highly dissociative transition state. In comparison with the corresponding nicotinamide cleavage in the deacetylation reaction, the simulations suggest that sirtuin ADP-ribosylation would be several orders slower but less sensitive to nicotinamide inhibition, which is consistent with experimental results. These results suggest that TbSIR2RP1 can perform ADP-ribosylation using two distinct mechanisms, depending on whether or not the substrate is acetylated. Structured digital abstract ,,MINT-7288298: TbSIR2 (uniprotkb:O96670) adp ribosylates (MI:0557) histone H1.1 (uniprotkb:Q02539) by enzymatic studies (MI:0415) ,,MINT-7288305, MINT-7288325, MINT-7288338, MINT-7288352, MINT-7288370, MINT-7288395, MINT-7288412: TbSIR2 (uniprotkb:O96670) adp ribosylates (MI:0557) histone H1.1 (uniprotkb:P02253) by enzymatic studies (MI:0415) ,,MINT-7288385: TbSIR2 (uniprotkb:O96670) deacetylates (MI:0197) histone H1.1 (uniprotkb:Q02539) by deacetylase assay (MI:0406) ,,MINT-7288424: hADPRH (uniprotkb:P54922) cleaves (MI:0194) histone H1.1 (uniprotkb:Q02539) by enzymatic studies (MI:0415) [source] Specific Processes and Scrambling in the Dehydrogenation of Ethane and the Degenerate Hydrogen Exchange in the Gas-Phase Ion Chemistry of the Ni(C,H3,O)+/C2H6 CoupleHELVETICA CHIMICA ACTA, Issue 5 2007Maria Schlangen Abstract A mechanistically unprecedented situation characterizes the gas-phase ion chemistry of Ni(C,H3,O)+ when reacted under thermal, single-collision conditions with ethane. A dehydrogenation channel leading to Ni(C3,H7,O)+ is to 90% preceded by a complete loss of positional identity of all nine H-atoms of the encounter complex (,scrambling'), whereas ca. 10% of the reaction exhibit a selective CH bond activation of the alkane. In addition, a degenerate H exchange between ethane and the (C,H3,O) unit occurs as a side reaction, the mechanistic details of which remain unknown for the time being. [source] Synthesis of Cyclic Peptides by Photochemical Decarboxylation of N -Phthaloyl Peptides in Aqueous SolutionHELVETICA CHIMICA ACTA, Issue 12 2002The synthesis of a variety of cyclic peptides from N -phthaloyl-protected di-, tri-, tetra-, and pentapeptides with different aminocarboxylic acid tethers by photodecarboxylation , initiated by intramolecular electron transfer , has been explored in aqueous media. The progress and the chemoselectivity of the follow-up processes after CO2 extrusion were traced by the respective pH/time-profiles, as well as by the overall change in pH after completion of the reaction. The competition between cyclization and simple oxidative decarboxylation depends on spacer length and geometry, H-bonding interaction between the electron accepting phthalimide CO groups and amide H-atoms, as well as the geometric reorganization coupled with the radical combination step and the formation of the lactam rings. With progressing reaction, hydrolysis of the phthalimide chromophore becomes an increasingly important side reaction due to the constant increase in pH. The use of phosphate-buffered aqueous media consequently improved the cyclization yields. The ground-state interactions between amide groups and the terminal COO, group with the imide CO groups were studied for the model system [N -(phthaloyl)glycyl]sarcosine (1) by NMR spectroscopy where the amide (E/Z)-equilibrium depends on the presence of carboxylate vs. free carboxylic acid, demonstrating the role of H-bonding and metal coordination. [source] Kinetics of liquid phase synthesis of ethyl tert -butyl ether from tert -butyl alcohol and ethanol catalyzed by ,-zeolite supported on monolithINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2002S. Assabumrungrat This paper compared the performance of ,-zeolite and Amberlyst-15 catalysts on a liquid phase synthesis of ethyl tert -butyl ether (ETBE) from ethanol (EtOH) and tert -butyl alcohol (TBA) ,-Zeolite was synthesized and deposited on monolith support. Its structure was confirmed by an XRD measurement and its composition was analyzed by an XRF measurement. It was found that even though the catalytic activity of ,-zeolite was lower than that of Amberlyst-15, the selectivity of ETBE was much higher than that of Amberlyst-15, resulting in almost the same level of ETBE yield. The dehydration of TBA to isobutene (IB) was the major side reaction. The kinetic study of the reaction catalyzed by ,-zeolite supported on monolith was carried out by using a semibatch reactor. The effect of external mass transfer was investigated by varying stirring speeds. The activity-based rate expressions were developed taking into account of water inhibition. Three temperature levels of 323, 333, and 343 K were performed in the study to obtain the parameters in the Arrhenius's equation and the Van't Hoff's equation. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 292,299, 2002 [source] Kinetic Resolution of 1-Biaryl- and 1-(Pyridylphenyl)alkan-1-ols Catalysed by the Lipase B from Candida antarcticaADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 5 2005Robert Kourist Abstract Lipase B from Candida antarctica (CAL-B) catalyses the highly enantioselective (E>200) transesterification of some 1-biaryl-2-yl-, -3-yl-, and -4-ylethanols and -propan-1-ols, as well as 1-(o -, m -, and p -pyridylphenyl)ethanols, 6, with vinyl acetate, Kazlauskas' rule being obeyed in all cases. meta and para -Substituted substrates were transformed within several hours (conversion degree ranging from 23,50%), reaction rates for propan-1-ol derivatives being slower than those for ethanol derivatives. Transesterifications of ortho -substituted alcohols took several days and were accompanied by a chemoenzymatic side reaction: the formation of another acetate derived from the hemiacetal between 6 and acetaldehyde coming from vinyl acetate. This side reaction was suppressed in the presence of isopropenyl acetate as acyl donor, conversion degrees for transesterification ranging from 20,40% after ten days (E>200). The usefulness of (R)- 6p as ligand in the asymmetric addition of diethylzinc to benzaldehyde was also demonstrated. [source] ,O -Acyl isopeptide method' for peptide synthesis: Solvent effects in the synthesis of A,1,42 isopeptide using ,O -acyl isodipeptide unit'JOURNAL OF PEPTIDE SCIENCE, Issue 12 2007Atsuhiko Taniguchi Abstract ,O -Acyl isopeptide method' is an efficient synthetic method for peptides. We designed ,O -acyl isodipeptide units', Boc-Ser/Thr(Fmoc-Xaa)-OH, as important building blocks to enable routine use of the O -acyl isopeptide method. In the synthesis of an A,1,42 isopeptide using O -acyl isodipeptide unit Boc,Ser(Fmoc,Gly),OH, a side reaction, resulting in the deletion of Ser26 in the O -acyl isopeptide structure, was noticed during coupling of the unit. We observed that the side reaction occurred during the activation step and was solvent-dependent. In DMF or NMP, an intramolecular side reaction, originating from the activated species of the unit, occurred during the activation step. In non-polar solvents such as CHCl3 or CH2Cl2, the side reaction was less likely to occur. Using CH2Cl2 as solvent in coupling the unit, the target A,1,42 isopeptide was synthesized with almost no major side reaction. Copyright © 2007 European Peptide Society and John Wiley & Sons, Ltd. [source] Synthesis and polymerization reactions of cyclic imino ethers.JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 1 2006Abstract Homopolyaddition reactions of AB-type monomers containing a 2-oxazoline and a phenol group in different positions of the phenyl ring, namely, 2-(4-hydroxyphenyl)-2-oxazoline, 2-(3-hydroxyphenyl)-2-oxazoline, 2-(2-hydroxyphenyl)-2-oxazoline, and 2-(4-hydroxyphenyl)-4,4-dimethyl-2-oxazoline, were studied. Except for 2-(4-hydroxyphenyl)-4,4-dimethyl-2-oxazoline, the reaction carried out in bulk or a solution of highly boiling solvents resulted in the formation of poly(ether amide)s with molecular weights in the range of 103 to 104 as measured by vapor pressure osmometry and gel permeation chromatography. A mechanism of the growth reaction, including a nucleophilic attack of a phenol group to a 2-oxazoline ring in the 5-position, was suggested. The polymerization was accompanied by a side reaction of the amido groups formed by the primary reaction of the 2-oxazoline ring. This led to branching of the main chain. The thermal properties of the prepared polymers were evaluated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 343,355, 2006 [source] Solid-phase incorporation of gaseous carbon dioxide into oxirane-containing copolymersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 15 2004Bungo Ochiai Abstract Carbon dioxide was incorporated into poly(glycidyl methacrylate- co -methyl methacrylate) by a solid-phase reaction, which transformed the pendent oxirane moieties into cyclic carbonate moieties, with quaternary ammonium halide catalysts. The incorporation of carbon dioxide into the copolymer led to soluble carbonate-containing polymers, whereas the incorporation of carbon dioxide into the glycidyl methacrylate homopolymer produced an insoluble product. The copolymer composition, reaction temperature, and catalyst amount affected the incorporation efficiency and the side reaction that caused crosslinking. Effective incorporation was achieved under the following reaction conditions: the glycidyl methacrylate content was less than approximately 50%, the temperature was greater than the glass-transition temperature, and the catalyst concentration was 1.5,6 mol %. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3812,3817, 2004 [source] Determination of the equilibrium constant for the reaction between bisphenol A and diphenyl carbonateJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 1 2002Stephen M. Gross Abstract Despite the industrial significance of poly(bisphenol A carbonate), there is a scarcity of open literature on the equilibrium of the melt-phase process. In fact, the equilibrium constant (Keq) for this reaction has never been measured directly. This article describes a process on the basis of NMR for the measurement of Keq for the reaction between bisphenol A and diphenyl carbonate in the presence and absence of a catalyst. The apparent enthalpy and entropy were calculated using a van't Hoff plot. Decomposition of bisphenol A is a common side reaction in the melt-phase reaction performed at high temperatures in the presence of catalyst. The effect of these side reactions on the Keq in the presence of catalyst is determined. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 171,178, 2002 [source] Glass-Transition Temperature (Tg) of Free-Radically Prepared Polyacrylonitrile by Inverse Gas Chromatography, 2,.MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 17 2002Molecular-Weight Dependence of Tg of Two Different Types of Aqueous Polymers Abstract The molecular-weight dependence of the glass-transition temperature (Tg) of a series of atactic polyacrylonitriles (PAN)s was studied by inverse gas chromatographic (IGC) analysis. PANs having different molecular weights were prepared by either; (i) the addition of isopropyl alcohol as a chain-transfer agent, or (ii) a scission reaction induced by the addition of alkali (NaOH) to a solution (N,N -dimethylformamide solution, at 25,°C) of the resulting polymer. The intrinsic viscosity [,] was in the range of 10.9,0.1 (dl,·,g,1), which corresponds to a viscosity-averaged molecular weight (Mv) of 1,590,000,3,000. As part of the results, a side reaction, which saw the conversion of the nitrile (CN) groups of PAN into amide (CONH2) and/or carboxylic acids (COOH) groups by alkali, was found to occur. The typical molecular-weight dependence of the Tg in free-radically prepared PAN was discussed in connection with a chain-transfer mechanism in an aqueous medium. Molecular-weight dependence of the Tg for PAN (WA). An error bar is given by a short vertical arrow. [source] Mechanism study on carbon reducing reaction in the preparation process of strontium carbonate (SrCO3)ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Jun Meng Abstract The mechanism for preparation of strontium carbonate (SrCO3) by the method of carbon reducing reaction is studied in this paper. Based on the mass ratio of SrSO4/C = 1.5 for this reaction, thermodynamics and kinetics calculation are conducted and also verified by TG-DSC experimental methods. For each side reaction, the changes of standard molar Gibbs function at proposed temperature are calculated through Gibbs-Helmholtz, Van't Hoff and Kirchhoff equations. TG-DSC data are used to explore the mechanism of the reaction, and to calculate the apparent activation energy and reaction orders. It has been demonstrated by TG and DSC thermal analysis for the whole reducing reaction that the apparent activation energy E is 350.6 and 307.3 kJ·mol,1, and the reaction orders n are 0.91 and 0.88, respectively. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Structure of ST0929, a putative glycosyl transferase from Sulfolobus tokodaiiACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 4 2010Charles B. C. Cielo The Sulfolobus tokodaii protein ST0929 shares close structural homology with S. acidocaldarius maltooligosyl trehalose synthase (SaMTSase), suggesting that the two enzymes share a common enzymatic mechanism. MTSase is one of a pair of enzymes that catalyze trehalose biosynthesis. The relative geometries of the ST0929 and SaMTSase active sites were found to be essentially identical. ST0929 also includes the unique tyrosine cluster that encloses the reducing-end glucose subunit in Sulfolobus sp. MTSases. The current structure provides insight into the structural basis of the increase in the hydrolase side reaction that is observed for mutants in which a phenylalanine residue is replaced by a tyrosine residue in the subsite +1 tyrosine cluster of Sulfolobus sp. [source] Hydrolytic Reactions of Thymidine 5,- O -Phenyl- N -Alkylphosphoramidates, Models of Nucleoside 5,-Monophosphate ProdrugsCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2007Mikko Ora Dr. Abstract To obtain detailed data on the kinetics of hydrolytic reactions of triester-like nucleoside 5,- O -aryl- N -alkylphosphoramidates, potential prodrugs of antiviral nucleoside monophosphates, the hydrolysis of diastereomeric (RP/SP) thymidine 5,-{O -phenyl- N -[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (3), a phosphoramidate derived from the methyl ester of L -alanine, has been followed by reversed-phase HPLC over the range from H0=0 to pH,8 at 90,°C. According to the time-dependent product distributions, the hydrolysis of 3 proceeds at pH<4 by two parallel routes, namely by nucleophilic displacement of the alaninyl ester moiety by a water molecule and by hydrolysis of the carboxylic ester linkage that allows intramolecular attack of the carboxy group on the phosphorus atom, thereby resulting in the departure of either thymidine or phenol without marked accumulation of any intermediates. Both routes represent about half of the overall disappearance of 3. The departure of phenol eventually leads to the formation of thymidine 5,-phosphate. At pH>5, the predominant reaction is hydrolysis of the carboxylic ester linkage followed by intramolecular displacement of a phenoxide ion by the carboxylate ion and hydrolysis of the resulting cyclic mixed anhydride into an acyclic diester-like thymidine 5,-phosphoramidate. The latter product accumulated quantitatively without any indication of further decomposition. Hydroxide-ion-catalyzed POPh bond cleavage of the starting material 3 occurred as a side reaction. Comparative measurements with thymidine 5,-{N -[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (4) revealed that, under acidic conditions, this diester-like compound is hydrolyzed by PN bond cleavage three orders of magnitude more rapidly than the triester-like 3. At pH>5, the stability order is reversed, with 3 being hydrolyzed six times as rapidly as 4. Mechanisms of the partial reactions are discussed. [source] Oxime Carbonates: Novel Reagents for the Introduction of Fmoc and Alloc Protecting Groups, Free of Side ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2010Sherine N. Khattab Abstract Fmoc and Alloc protecting groups represent a consistent alternative to classical Boc protection in peptide chemistry. The former was established in the last decades as the ,-amino protecting group of choice, whereas the latter allows a fully orthogonal protection strategy with Fmoc and Boc. Usually, the introduction of the Fmoc and Alloc moieties takes place through their halogenoformates, azides, or activated carbonates. This rather simple reaction is accompanied by several side reactions, specially the formation of Fmoc/Alloc dipeptides and even tripeptides. The present work describes new promising Fmoc/Alloc-oxime reagents, which are easy to prepare, stable, and highly reactive crystalline materials that afford almost contaminant-free Fmoc/Alloc-amino acids in high yields by following a conventional procedure. Amongst the Fmoc-oxime derivatives, the N -hydroxypicolinimidoyl cyanide derivative (N -{[(9H-fluoren-9-yl)methoxy]carbonyloxy}picolinimidoyl cyanide) gave the best results for the preparation of Fmoc-Gly-OH, which is the most predisposed to give side reactions. The same Alloc-oxime analogue afforded the preparation of Alloc-Gly-OH in good yield, purity, and extremely low dipeptide formation, as analyzed by reverse-phase HPLC and NMR spectroscopy. [source] Improved Auxiliary for the Synthesis of Medium-Sized Bis(lactams)EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2008Jasper Springer Abstract Our auxiliary-based method for the synthesis of bis(lactams) has been optimized. A novel auxiliary is described that is inserted in the backbone of a linear peptide facilitating the mutually reactive terminal groups to approach one another for a cyclization reaction. A subsequent ring contraction mechanism leads to the bis(lactams) with the remainings of the auxiliary still attached. Functionalized seven- and eight-membered bis(lactams) have been prepared that are difficult to access using traditional methods. Removal of the auxiliary from the bis(lactams) has been described with the possible side reactions that can occur.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Biocatalytic Racemization of (Hetero)Aryl-aliphatic ,-Hydroxycarboxylic Acids by Lactobacillus spp.EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2006Proceeds via an Oxidation, Reduction Sequence Abstract The biocatalytic racemization of a range of (hetero)aryl- and (di)aryl-aliphatic ,-hydroxycarboxylic acids has been achieved by using whole resting cells of Lactobacillus spp. The essentially mild (physiological) reaction conditions ensure the suppression of undesired side reactions, such as elimination, decomposition or condensation. Cofactor/inhibitor studies using a cell-free extract of Lactobacillus paracasei DSM 20207 reveal that the addition of redox cofactors (NAD+/NADH) leads to a distinct increase in the racemization rate, while strong inhibition is observed in the presence of Thio-NAD+, which suggests that the racemization proceeds by an oxidation,reduction sequence rather than involvement of a "racemase" enzyme. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The Design and Realization of Flexible, Long-Lived Light-Emitting Electrochemical CellsADVANCED FUNCTIONAL MATERIALS, Issue 16 2009Junfeng Fang Abstract Polymer light-emitting electrochemical cells (LECs) offer an attractive opportunity for low-cost production of functional devices in flexible and large-area configurations, but the critical drawback in comparison to competing light-emission technologies is a limited operational lifetime. Here, it is demonstrated that it is possible to improve the lifetime by straightforward and motivated means from a typical value of a few hours to more than one month of uninterrupted operation at significant brightness (>100,cd m,2) and relatively high power conversion efficiency (2 lm W,1 for orange-red emission). Specifically, by optimizing the composition of the active material and by employing an appropriate operational protocol, a desired doping structure is designed and detrimental chemical and electrochemical side reactions are identified and minimized. Moreover, the first functional flexible LEC with a similar promising device performance is demonstrated. [source] A New and Efficient Synthesis of the HMG-CoA Reductase Inhibitor PitavastatinHELVETICA CHIMICA ACTA, Issue 6 2007Murat Acemoglu Abstract A new synthetic method for the preparation of pitavastatin is described. The approach circumvents various synthetic problems associated with the buildup of the 3,5-dihydroxy-C7 acid side chain of HMG-CoA reductase inhibitors (statins). The use of the C6 -amide derivative 5 instead of ester derivatives in the coupling reaction with carboxaldehyde 8 (Scheme,3) prevents undesired side reactions, such as eliminations and retro -aldol reactions. The method provides synthetic statins, such as pitavastatin, in >99% ee and exceptionally high overall yield. The enantiomerically pure starting material, (3S)-3-{[(tert -butyl)dimethylsilyl]oxy}-5-oxo-5-{[(1S)-1-phenylethyl]amino}pentanoic acid (3c), is prepared by an improved procedure from 3-{[(tert -butyl)dimethylsilyl]oxy}glutaric anhydride (1) and (1S)-1-phenylethylamine (2c; Scheme,1). [source] A Homogeneous Catalyst for Reduction of Optically Active Esters to the Corresponding Chiral Alcohols without Loss of Optical PuritiesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1 2010Wataru Kuriyama Abstract A ruthenium complex was found to catalyze the hydrogen reduction of esters under mild and neutral conditions. A variety of optically active esters can be reduced to the corresponding alcohols in excellent yield without loss of their optical purity or causing undesirable side reactions. Hydrogen reduction needs such simple operations , reaction, concentration, and purification , that the violent quench step and extraction step, which accompany conventional sodium borohydride or lithium aluminum hydride reduction, can be omitted. [source] Microwave-assisted depolymerization of poly(ethylene terephthalate) [PET] at atmospheric pressureADVANCES IN POLYMER TECHNOLOGY, Issue 4 2006Mir Mohammad Alavi Nikje Abstract Microwave-assisted hydroglycolysis of poly(ethylene terephthalate) using an excess of methanol, ethanol, 1-butanol, 1-pentanol, and 1-hexanol in the presence of different simple basic catalysts, namely, potassium hydroxide, sodium hydroxide, sodium acetate, and zinc acetate, is reported. Reactions were performed at short times without any side reactions, namely, oxidation of ethylene glycol. The products terephthalic acid and ethylene glycol were obtained in their pure form with sufficiently high yields with potassium hydroxide. The purified product was characterized by IR and nuclear magnetic resonance spectroscopy. The process of hydroglycolysis reported here is economically viable since yields of recycled products are high, and it has potential for further improvement to produce useful products. This process is of economic interest because much of the raw materials can be recovered and used for virgin PET resin synthesis. © 2007 Wiley Periodicals, Inc. Adv Polym Techn 25:242,246, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20080 [source] Reactive grafting of glycidyl methacrylate onto polypropyleneJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2010Emma-Louise Burton Abstract This work explored the melt-phase grafting of glycidyl methacrylate (GMA) onto polypropylene on a closely intermeshing corotating twin-screw extruder (16-mm screws, 40 : 1 length/diameter ratio). The modification of the base polypropylene to produce GMA-grafted polypropylene was achieved via peroxide-induced hydrogen abstraction from the polypropylene followed by the grafting of the GMA monomer or by the grafting of styrene followed by copolymerization with the GMA. In this study, both the position and order of the reactant addition were investigated as a route to improving graft yields and reducing side reactions (degradation). For the peroxide,GMA system, adding GMA to the melt before the peroxide resulted in significant improvements in the graft levels because of the improved dispersion of GMA in the melt. The addition of a comonomer (styrene) was explored as a second route to improving the graft yield. Although the addition of the comonomer led to a considerable rise in the level of grafted GMA, altering the order of the reactant addition was not found to contribute to an increase in the grafted GMA levels. However, variable levels of grafted styrene were achieved, and this may play an important role in the development of grafted polymers to suit specific needs. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Synthesis and characterization of dendronized aromatic polyamides with bromomethyl groups in the peripheryJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2008Yanru Miao Abstract This paper focused on the synthesis of novel functionalized dendronized aromatic polyamides, which contained a polyamide backbone and polyamide dendrons decorated with bromomethyl groups, by the macromonomer route. The new activating procedure of the synthesis of dendrons and macromonomers using thionyl chloride as an activating agent eliminated the protection/deprotection procedure and reduced side reactions. The structures of the dendrons, macromonomers, and polyamides were confirmed by FTIR, 1H NMR, 13C NMR, and elemental analysis. The properties of the resulting dendronized polyamides, which were compared with that of the linear polyamide (LP), were studied using GPC, XRD, TGA, DSC, inherent viscosity measurement, and solubility experiment. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Restraining the associations of anthracene fluorophore by chemically linking to poly(methyl methacrylate)JOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2008Fu-Kun Su Abstract Associations (dimer or aggregate) of anthracene (An) fluorophores tend to interrupt the monomer emission and reduce the quantum yield (,PL); therefore, poly(methyl methacrylate) (PMMA) chain was used in this study to chemically link to anthracene and to block the mutual associations among the anthracene fluorophores. With this aim, the target polymers were prepared by anionic polymerizations with 9,10-dibromoanthracene/s -butyllithium as initiating system to proceed polymerizations of methyl methacrylate (MMA) directly or in the presence of 1,1-diphenylethylene (DPE). Use of DPE before addition of MMA produces stable initiating anionic species and avoids potential side reactions during polymerization; however, it also introduces four ,-phenylene rings around the central anthracene ring, which interfere with the corresponding emission pattern and reduce the ,PL (32%) value due to potential interactions between phenylene rings and anthracene. On the contrast, polymerization without participation of DPE results in polymer with central anthracene ring directly connected to two PMMA chains, which gives clean vibronic emission pattern with limited association emissions and enhanced ,PL (52%) value. Physical blending of anthracene by PMMA is less efficient to restrain the associations and results in a film of lower ,PL (20%). © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Characterization of the Products Formed by the Reaction of Trichlorocyanuric Acid with 2-PropanolJOURNAL OF FORENSIC SCIENCES, Issue 6 2009P. Mark L. Sandercock Ph.D. Abstract:, We report a recent investigation into the death of a cat that was initially thought to involve intentionally burning the animal via the use of an ignitable liquid. The exposure of the animal to flame was ruled out. Instead, forensic investigation revealed the intentional mixing together of a common outdoor swimming pool chlorinator, trichlorocyanuric acid (TCCA), and 2-propanol (aka, isopropyl alcohol or rubbing alcohol). The reaction of these two chemicals resulted in the formation of cyanuric acid residue, hydrochloric acid, and the evolution of a significant volume of chlorine gas. Further ,-chlorination side reactions also occurred between 2-propanol and TCCA to produce a variety of chlorinated 2-propanone species that were detected on the submitted evidence. The identification of the products of both the main reaction and the side reactions allowed the authors to determine what chemicals were originally mixed together by the culprit. [source] | ||||||||||||||||||||||||||||