共查询到20条相似文献,搜索用时 453 毫秒
1.
Karen Wright Augustin de Castries Fernando Formaggio Antonio Toffoletti Jean-Paul Mazaleyrat 《Tetrahedron letters》2005,46(33):5573-5576
Enantiomerically pure (3R,4S) and (3S,4R) protected 4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acids were synthesized by reduction of the enamines resulting from the condensation of 3-carboxymethyl-1-oxyl-2,2,6,6-tetramethyl-4-piperidone with (R) or (S)-α-methylbenzylamine. While NaBH3CN/CH3COOH reduction gave predominantly a mixture of the two possible cis-diastereomers, the use of NaBH4/(CH3)2CHCOOH resulted in a mixture of only one trans- and one cis-diastereomer. Removal of the chiral auxiliary from the separated diastereoisomers by hydrogenolysis and regeneration of the nitroxide radical gave the desired β-amino esters. The ESR spectrum of the (3R,4S)-enantiomer is also reported. 相似文献
2.
The fluorination using TbF4 as fluorinating agent was successfully performed on poly(p-phenylene). The method allows the fluorine content of the polymer to be controlled and the formation of structural defects, such as dangling bonds, to be significantly decreased by comparison with the direct fluorination using pure F2 gas. The aromatic character of the phenyl ring is partly maintained through the fluorination contrary to the direct fluorination (using pure F2 gas), for which a quasi-perfluorination and a partial decomposition of the polymer occur. Complementary analytical techniques have been used, such as 19F and 13C solid state NMR, FT-IR and EPR to compare the samples as a function of the reaction conditions. 相似文献
3.
Alexander S. Kiselyov 《Tetrahedron letters》2005,46(26):4487-4490
The reaction of N-fluoropyridinium triflate with isonitriles in acetonitrile and propionitrile in the presence of NaBH(OAc)3 led to the formation of the corresponding imidazo[1,2-a]pyridines in 44-73% yields. The proposed reaction mechanism involves the intermediate formation of a highly reactive carbene species and apparent reduction of the pyridinium intermediate with NaBH(OAc)3 to yield the targeted heterocycles. 相似文献
4.
B.M. Westwood 《The Journal of chemical thermodynamics》2003,35(12):1965-1974
A novel pycnometer has been designed for density measurements at elevated temperatures (up to T=473.15 K). The pycnometer consists of a stainless steel cell connected to a small-bore tube. The main feature of the design includes a bored-through expansion fitting, which allows the overflow due to thermal expansion from the cell (via the small-bore tube) to collect in a pressurized line. Densities were measured for pure 1-butanol, pure n-heptane, and for mixtures {xbCH3(CH2)3OH + (1−xb)CH3(CH2)5CH3}, from T=316.85 K to T=458.15 K, at a pressure of 4.93 MPa. Excess volumes were calculated and reported for these mixtures. 相似文献
5.
C. Wade Downey Alan S. FleisherJames T. Rague Chelsea L. SafranMegan E. Venable Robert D. Pike 《Tetrahedron letters》2011,52(37):4756-4759
Secondary amides undergo in situ silyl imidate formation mediated by TMSOTf and an amine base, followed by addition to acetal acceptors to provide N-acyl-N,O-acetals in good yields. An analogous, high-yielding reaction is observed with 2-mercaptothiazoline as the silyl imidate precursor. Competing reduction of the acetal to the corresponding methyl ether via transfer hydrogenation can be circumvented by the replacement of CY2NMe with 2,6-lutidine under otherwise identical reaction conditions. 相似文献
6.
Miaojun Wang 《Journal of solid state chemistry》2005,178(4):1182-1189
Lithium substituted Li1+xMn2−xO4 spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na2O·4MoO3 at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn3+ to Mn4+ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn3+ to Mn4+ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction. 相似文献
7.
Marie-Christine DuclosYuttapong Singjunla Christelle PetitAlain Favre-Réguillon Erwann JeanneauFlorence Popowycz Estelle MétayMarc Lemaire 《Tetrahedron letters》2012,53(45):5984-5986
P-Chirogenic monodentate binaphthyl phosphines were prepared in five steps from enantiomerically pure BINOL. This approach supposes the utilization of two methods previously developed in our group, the formation of secondary phosphine oxide, and the reduction of tertiary phosphine oxide using the association of tetramethyldisiloxane and Ti(OiPr)4. During the last reduction step, only the formation of the more stable diastereoisomer was observed. This product was employed as a ligand for the palladium catalyzed hydrosilylation of styrene to afford the corresponding alcohol with high yield and enantiomeric excess. 相似文献
8.
The synthesis of enantiomerically pure C2-symmetric dipyridylmethane ligands and related N,N′-dioxides is reported. A procedure for the synthesis of a few new enantiomerically pure C2-symmetric pyridine N-oxides and the preparation of four pyridine N-oxides with oxygen and nitrogen atoms as further coordinating elements in the heterocycle framework is described. All compounds were prepared from naturally occurring monoterpenes. These new compounds were assessed as organocatalysts in two different reactions, namely the allylation of aldehydes with allyltrichlorosilane that afforded homoallylic alcohols in good yields and up to 85% ee and the stilbene oxide opening by the addition of tetrachlorosilane that gave chlorohydrin in quantitative yield and up to 70% ee. 相似文献
9.
Treatment of 4-nitrophenyl chloroformate with alkylammonium hydrochloride salts and solid anhydrous Na2CO3 in either CH2Cl2 or CH3CN gave 4-nitrophenyl N-methylcarbamate and other N-alkylcarbamate analogues in excellent yields. Of particular interest is the observation that 4-nitrophenyl N-methylcarbamate, a safer alternative to the highly toxic methyl isocyanate, is obtained in quantitative yield (?95% pure as determined by 1H NMR) after simple filtration and solvent evaporation. 相似文献
10.
Jun Nakano Tsuyoshi Ichiyanagi Hiromichi OhtaYukishige Ito 《Tetrahedron letters》2003,44(14):2853-2856
Direct formation of Asn-linked carbohydrate by N-glycosylation has been difficult, because of the lack of nucleophilicity of carboxamide nitrogen. We report here the novel N-glycosylation using Asn hydroxamate as a glycosyl acceptor. Reaction with glycosyl fluoride or glycosyl trichloroacetimidate afforded N-glycoside and subsequent reduction with SmI2 gave Asn-linked glucose. Carbamate derived hydroxamates proved to have even enhanced reactivity to give N-glycosides in high yields. 相似文献
11.
The synthesis of C8-glycomimetics is described from C2-symmetrical polyhydroxylated cyclooctenes derived from carbocyclisation of enantiomerically pure 1,9-dienes by ring closing metathesis. Their obtention notably involved either hydroboration followed by oxidation to carbasugars or to cyclooctanones then reductive amination, or formation of a cis-cyclic sulfate followed by successive introduction of an azido group, reduction and subsequent reductive amination. The biological activity of the C8-carbasugars and related aminocyclitols, analogous to voglibose, has been evaluated towards several commercially available glycosidases. 相似文献
12.
We report in here the preparation of CdS nanoparticles (Q-CdS) in N,N-dimethylformamide (DMF) and poly(tetrafluoroethylene-co-vinylidenefluoride-co-propylene) (PTFE-co-PVDF-co-Prop) by reaction of cadmium acetate and thiourea at room temperature. The formation and size evolution of Q-CdS particles were followed by optical absorption spectroscopy as a function of the aging process of the solutions. The obtained results indicated that not only the Q-CdS particles were more stable in DMF than in aqueous solutions, probably due to the interaction with the CHNO group, but also they were formed in smaller sizes. The particle size obtained in DMF was estimated in 4-6 nm against 10-20 nm in aqueous solution. The results also showed that the velocity of Q-CdS formation is lower when the reaction was carried out in PTFE-co-PVDF-co-Prop dissolved in DMF. The Q-CdS particles formed in this system were also less stable than in pure DMF, probably due to the interaction of the CHNO fragment and CF2 of polymer chain or phase segregation. 相似文献
13.
Shaowu Zha 《Journal of solid state chemistry》2005,178(6):1844-1850
Complex metal oxides with composition of La0.75Sr0.25Cr1−xMnxO3(x=0.4,0.5,0.6) (LSCM) have been synthesized and examined as anode materials for solid oxide fuel cells (SOFCs). LSCM compositions show excellent tolerance to both reduction and oxidation but the crystal structure transforms from hexagonal in air to orthorhombic in H2. The volume change associated with this phase transformation is only about 1%, thus having little effect on other properties. The total electrical conductivity increases with the content of Mn, whereas the resistance to sulfur poisoning increases with the content of Cr. Fuel cells using LSCM as the anode show very good performance when pure hydrogen is used as the fuel. However, they do not appear to be stable in fuels containing 10% of H2S. 相似文献
14.
Ceric ammonium nitrate (CAN) is found to be a suitable, inexpensive, and effective non-toxic catalyst for a smooth (3+2) cycloaddition of organic nitriles with NaN3 to afford 5-substituted 1H-tetrazoles in excellent yields. Shorter reaction times, easy work-up, and substantial and pure product formation are the key advantages of the present method. 相似文献
15.
Liping Li 《Journal of solid state chemistry》2007,180(10):2896-2904
Highly crystalline one-dimensional (1D) α-MnO2 nanostructures were synthesized by a hydrothermal method. All samples were characterized by X-ray diffraction, transmission electron microscope, thermogravimetric and differential scanning calorimeter, and infrared spectroscopy. During the formation reactions, the tunnel structure of 1D α-MnO2 was simultaneously modified by NH4+ species and water molecules. The amount of NH4+ species that were trapped in the tunnels is almost independent on the reaction temperature, while the total water content increased with the reaction temperature. The average diameter of α-MnO2 nanorods increased from 9.2 to 16.5 nm when the reaction temperature increased from 140 to 220 °C. 1D α-MnO2 was destabilized by a subsequent high-temperature treatment in air, which is accompanied by a structural transformation to 1D Mn2O3 of a cubic structure. At low temperatures, all 1D α-MnO2 nanorods showed two magnetic transitions that were characterized by a decreased Néel temperature with rod diameter reduction. According to the effective magnetic moments experimentally measured, Mn ions presented in the nanorods were determined to be in a mixed valency of high spin state Mn4+/Mn3+. 相似文献
16.
Sadula Sunitha 《Tetrahedron letters》2008,49(16):2527-2532
The first report of a Brønsted acidic ionic liquid, 1-methylimidazolium tetrafluoroborate [(HMIm)BF4], catalyzed efficient and chemoselective N-Boc protection of various amines using (Boc)2O is presented. Optically pure amino alcohols and amino acid esters were converted efficiently to their corresponding optically pure N-Boc derivatives. The reported method is mild, solvent-free and has the advantages of both homogeneous and heterogeneous catalysis with high product yields, selectivity and ease of product separation. 相似文献
17.
Carlos D. MagnussonAnna V. Gudmundsdottir Gudmundur G. Haraldsson 《Tetrahedron》2011,67(10):1821-1836
A highly efficient two-step chemoenzymatic synthesis of enantiopure structured ether lipids of the 1-O-alkyl-2,3-diacyl-sn-glycerol type has been developed. Chimyl, batyl and selachyl alcohols possessing pure saturated fatty acid (SFA) attached to the sn-3 position and pure EPA and DHA attached to the sn-2 position were obtained under full regiocontrol. This was offered by mild conditions and a highly efficient lipase that operated at room temperature. High-resolution 1H NMR spectroscopy was used to monitor the progress of the reactions and to evaluate the full regiocontrol of the reactions involved by keeping track of all prospective adducts involved in these reactions. This was extended to preparation of a focused library of eight monoacyl intermediate adducts for all even-numbered SFA ranging from C2-C16 and the corresponding EPA and DHA structured diacyl glycerol ethers (DAGE) products for chimyl, batyl and selachyl alcohols, the total of 72 compounds. 相似文献
18.
Kunihiko Takabe Masataka Sugiura Yuya Asumi Nobuyuki Mase Hidemi Yoda Hiroyasu Shimizu 《Tetrahedron letters》2005,46(20):3457-3460
A simple and practical synthesis of (R)-(−)-muscone was achieved by optical resolution of dl-muscone using tartaric acid derivatives. The acetalization of dl-muscone with N,N′-dibenzyl-l-tartaramide in the presence of Sc(OTf)3 and methyl orthoformate furnished a diastereomeric mixture of acetals, which were readily separated by simple recrystallization. Diastereomerically pure acetal was hydrolyzed to give optically pure muscone and recovered N,N′-dibenzyl-l-tartaramide. 相似文献
19.
The phase diagram, crystallization and melting behavior of poly(ethylene oxide) (PEO)/poly(n-butyl methacrylate) (PnBMA) blends have been investigated using differential scanning calorimetry and optical microscopy. The results show that the blends are miscible up to 85 °C and show an lower critical solution temperature-type demixing at a higher temperature. The isothermal crystallization studies of the blends indicate a reduction in the overall rate of crystallization. Analysis of isothermal crystallization data by means of Avrami equation leads to average values of the Avrami index of 2.5 for pure PEO and 3.0 for the different blend compositions. The melting behavior of the blends reveals double endotherms, which is ascribed to both secondary crystallization and recrystallization. The melting point depression study yielded χ12=0, indicating a relatively low interaction strength. 相似文献
20.
The formation of substituted 2-pyrrolidinones and indoles by the reduction of the secondary nitro group in appropriate 3-aryl-2-methylene-4-nitroalkanoates afforded by Baylis-Hillman chemistry via different reducing agents is described. The 3-aryl-2-methylene-4-nitroalkanoate obtained from SN2 nucleophilic reaction between the acetate of Baylis-Hillman adducts and ethyl nitroacetate upon reduction with indium-HCl furnishes a mixture of cis and trans substituted phenyl-3-methylene-2-pyrrolidinones. In contrast, similar reductions of analogous substrates derived from nitroethane stereoselectively furnished only the trans substituted phenyl-3-methylene-2-pyrrolidinones. On the other hand the SnCl2·2H2O-promoted reductions of substrates derived from nitro ethylacetate give oxime derivatives while the ones obtained from nitroethane yield a mixture of cis and trans 4-aryl-3-methylene-2-pyrrolidinones. Alternatively, the SnCl2·2H2O-promoted reduction of substituted 2-nitrophenyl-2-methylene-alkanoate furnished from ethyl nitroacetate yield 3-(1-alkoxycarbonyl-vinyl)-1H-indole-2-carboxylate while indium-promoted reaction of this substrate leads to a complex mixture. Analogous reactions with SnCl2·2H2O of substituted 2-nitrophenyl-2-methylene-alkanoate obtained from nitroethane yield 4-alkyl-3-methylene-2-quinolones in moderate yields. 相似文献