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141.
Christian Bellec Daniel Bertin Roger Colau StPhane Deswarte Pierre Maitte Claude Viel 《Journal of heterocyclic chemistry》1979,16(8):1657-1659
Nitration of α-acylstilbenes with dinitrogen tetroxide leads to corresponding Z-α-acyl-β-nitrostilbenes. By chemical or electochemical reduction of these compounds, trisubstituted isoxazoles were prepared in good yields. 相似文献
142.
143.
Watzke A Brunsveld L Durek T Alexandrov K Rak A Goody RS Waldmann H 《Organic & biomolecular chemistry》2005,3(7):1157-1164
Rab/Ypt guanosine triphosphatases (GTPases) represent a family of key membrane traffic regulators in eukaryotic cells. For their function Rab/Ypt proteins require double modification with two covalently bound geranylgeranyl lipid moieties at the C-terminus. Generally, prenylated proteins are very difficult to obtain by recombinant or enzymatic methods. We generated prenylated RabGTPases using a combination of chemical synthesis and protein engineering. This semi-synthesis depends largely on the availability of functionalized prenylated peptides corresponding to the proteins' native structure or modifications. We developed solution phase and solid phase strategies for the generation of peptides corresponding to the prenylated C-terminus of Rab7 GTPase in preparative amounts enabling us to crystallize the mono-prenylated Ypt1:RabGDI complex. The structure of the complex provides a structural basis for the ability of RabGDI to inhibit the release of nucleotide by Rab proteins and a molecular basis for understanding a RabGDI mutant that causes mental retardation in humans. 相似文献
144.
Fei Yu Yuhuan Liu Xuejun Pan Xiangyang Lin Chengmei Liu Paul Chen Roger Ruan 《Applied biochemistry and biotechnology》2006,130(1-3):574-585
This research investigated a novel process to prepare polyester from corn stover through liquefaction and crosslinking processes.
First, corn stover was liquefied in organic solvents (90 wt% ethylene glycol and 10 wt% ethylene carbonate) with catalysts
at moderate temperature under atmospheric pressure. The effect of liquefaction temperature, biomass content, and type of catalyst,
such H2SO4, HCl, H3PO4, and ZnCl2, was evaluated. Higher liquefaction yield was achieved in 2 wt% sulfuric acid, 1/4 (w/w) stover to liquefying reagent ratio;
160°C temperature, in 2h. The liquefied corn stover was rich in polyols, which can be directly used as feedstock for making
polymers without further separation or purification. Second, polyester was made from the liquefied corn stover by crosslinking
with multifunctional carboxylic acids and/or cyclic acid anhydrides. The tensile strength of polyester is about 5 MPa and
the elongation is around 35%. The polyester is stable in cold water and organic solvents and readily biodegradable as indicated
by 82% weight loss when buried in damp soil for 10 mo. The results indicate that this novel polyester could be used for the
biodegradable garden mulch film production. 相似文献
145.
Ralf Wischnat Joachim RudolphRoman Hanke Roger KaeseAchim May Heidi TheisUndine Zuther 《Tetrahedron letters》2003,44(23):4393-4394
An efficient scalable synthesis of 2-halothiazolium-type peptide coupling reagents has been developed. The key step is the formation of the 2-bromothiazole scaffold through cyclization of α-thiocyanato ketones with hydrogen bromide. Using this method, the new coupling reagent 2-bromo-N-methylthiazolium bromide (BMTB) was synthesized. BMTB was tested in a difficult model coupling reaction of two sterically hindered N-methylated amino acids and showed higher activity than the well-established peptide coupling reagent HATU. 相似文献
146.
The Synthesis of some 4-Nitroisoxazoles from Pseudonitrosites of Chalcones The action of dinitrogen trioxide with α,β-unsaturated ketones of the chalcone type leads to pseudonitrosites (Table 1) which are cyclized to 4-nitro-3,5-diarylisoxazoles (Table 2). 相似文献
147.
Russell J. Waugh John H. Bowie Roger N. Hayes 《Journal of mass spectrometry : JMS》1991,26(4):250-256
Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH2CH(R1) CONHCH(R2)CO2? → NH2?C(R1)CONHCH(R2)CO2H → ?NHCH(R2)CO2H + NH2C(R1)?C?O, when R1 and R2 = H or alkyl. However, when one of the constituent amino acids is either aspartic acid or glutamic acid, the standard cleavage becomes minor in comparison with fragmentation through the α-side-chain of Asp or Glu. For example, [Asp-Leu - H]? and [Leu-Asp - H]? both fragment principally by loss of water, a fragmentation not normally noted for peptides. In addition, [Leu-Asp - H]? loses CO2 and also forms HO2CCH?CHCO2?˙. These fragmentations establish that Asp is the C-terminal amino acid. In contrast, isomeric Glu dipeptides, e.g. [Glu-Ala - H]? and [Ala-Glu - H]? undergo similar fragmentation, both competitively losing H2O and CO2. Both spectra also contain a product ion at m/z 128, identified as the pyroglutamate anion. Product ion and deuterium-labelling studies have been used in an attempt to elucidate the complex fragmentation mechanisms in these systems. 相似文献
148.
Fukuzumi S Okamoto K Tokuda Y Gros CP Guilard R 《Journal of the American Chemical Society》2004,126(51):17059-17066
Dehydrogenation of 10-methyl-9,10-dihydroacridine (AcrH(2)) by dioxygen (O(2)) proceeds efficiently, accompanied by the two-electron and four-electron reduction of O(2) to produce H(2)O(2) and H(2)O, which are effectively catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins in the presence of perchloric acid (HClO(4)) in acetonitrile (MeCN) and benzonitrile (PhCN), respectively. The cobalt porphyrin catalyzed two-electron reduction of O(2) also occurs efficiently by 9-alkyl-10-methyl-9,10-dihydroacridines (AcrHR; R = Me, Et, and CH(2)COOEt) to yield 9-alkyl-10-methylacridinium ion (AcrR+) and H(2)O(2). In the case of R = Bu(t) and CMe(2)COOMe, however, the catalytic two-electron and four-electron reduction of O(2) by AcrHR results in oxygenation of the alkyl group of AcrHR rather than dehydrogenation to yield 10-methylacridinium ion (AcrH+) and the oxygenated products of the alkyl groups, i.e., the corresponding hydroperoxides (ROOH) and the alcohol (ROH), respectively. The catalytic mechanisms of the dehydrogenation vs the oxygenation of AcrHR in the two-electron and four-electron reduction of O(2), catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins, respectively, are discussed in relation to the C(9)-H or C(9)-C bond cleavage of AcrHR radical cations produced in the electron-transfer oxidation of AcrHR. 相似文献
149.
Roger Alperin 《Monatshefte für Mathematik》1982,93(4):261-265
FollowingKazhdan, a separable locally compact groupG is said to have propertyT if the trivial representation is isolated in the dual space,, of equivalence classes of continuous irreducible unitary representations ofG. We generalize results ofMargulis—Tits by showing that groups which have propertyT can not be amalgams.Research supported by NSF. 相似文献
150.
Isabel W. C. E. Arends Roger A. Sheldon Martin Wallau Ulf Schuchardt 《Angewandte Chemie (International ed. in English)》1997,36(11):1144-1163
Zeolites are viewed by some as the “philosopher's stone” of modern chemistry.[1] They are more or less indispensable in oil refining and petrochemicals manufacture where they are widely applied as solid acid catalysts. More recently attention has been focused on their use in the manufacture of fine chemicals. The synthetic utility of zeolites and related molecular sieves (zeotypes) has been considerably extended by the incorporation of redox metals into their frameworks. The resulting redox molecular sieves catalyze a variety of selective oxidations under mild conditions in the liquid phase. Their structural diversity–including variation of the redox metal, incorporation of metal complexes, and the size and polarity of the micropores–provides the possibility of designing tailor-made solid catalysts (“mineral enzymes”) for liquid-phase oxidations with clean oxidants such as O2, H2O2, and RO2H. Hence, they have enormous potential in industrial organic synthesis as environmentally friendly alternatives to traditional oxidations employing inorganic oxidants in stoichiometric amounts. A primary aim of this review is to familiarize organic chemists with the synthetic potential of redox molecular sieves. An outline of their synthesis, structures, and chemical properties, highlighting their unique advantages, is followed by a discussion of general (mechanistic) features that influence the choice of a suitable catalyst for a particular type of oxidation. The main part of the review deals with the oxidation of various substrates of synthetic interest–such as alkanes, alkenes, (alkyl)arenes, alcohols, and amines–and emphasizes the advantages of redox molecular sieves (including selectivity and stability) over their homogeneous counterparts. New directions towards truly biomimetic solid catalysts, for example zeolite-encapsulated chiral metal complexes as heterogeneous catalysts for asymmetric oxidations, are high-lighted. 相似文献