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671.
The domino C,O-cyclodialkylation reaction of dilithiated 1,3-dicarbonyl compounds with 1,4-dibromo-2-butene resulted in regio- and diastereoselective formation of 2-alkylidene-5-vinyltetrahydrofurans. The cyclization of 1,3-dicarbonyl dianions with 1-bromo-2-chloroethane regio- and diastereoselectively afforded 2-alkylidenetetrahydrofurans under thermodynamic reaction control. 相似文献
672.
Rainaldi M Lancelot N Elbayed K Raya J Piotto M Briand JP Kaptein B Broxterman QB Berkessel A Formaggio F Toniolo C Bianco A 《Organic & biomolecular chemistry》2003,1(11):1835-1837
A series of [L-(alphaMe)Leu]n (n = 1-5) homo-peptides have been covalently linked to Tentagel and POEPOP resins and submitted to a conformational study using HRMAS NMR spectroscopy. Whereas the mono- and dipeptide are mainly fully-extended, stable 3(10)-helical structures are formed beginning from the trimer. 相似文献
673.
Karim Adil Amor Ben Ali Minh Trung Dang Marc Leblanc 《Journal of fluorine chemistry》2007,128(4):404-412
The environment of H3O+, H2O, HF and F− species (non-bonded to metals) is considered in fluoride metalates which crystallise from the (Al(OH)3, Cr(OH)3, FeF3, ZrF4, Ta2O5)-tren-HFaq·-ethanol systems (microwave heating at 190 °C during 1 h). The presence of (H3O)(H2O)6+ clusters or H3O+ cations, of isolated or associated H2O molecules, of (HF2)− and F− anions is evidenced. The thermal stability of the solids depends strongly on the nature of the hydrogen-bonded species associated with the preceding cations or anions and on the formation of water ribbons or layers. 相似文献
674.
Oliva V Mrabet B Baeta Neves MI Chehimi MM Benzarti K 《Journal of chromatography. A》2002,969(1-2):261-272
Two cement pastes, commonly used in concrete formulations, were characterised by IGC at 35-80 degrees C before and after coating with an epoxy resin and a hardener. The cements are mixtures of hydrates in various proportions, such as calcium silicate hydrate (CaO-SiO2-H2O) and calcium hydroxide Ca(OH)2. Apolar and polar probes were used to determine the dispersive and acid-base characteristics of the cement pastes. These materials have high surface energy as judged from the dispersive contribution to the surface free energy (gamma(s)d) values lying in the 50-70 mJ/m2 range at 60-80 degrees C. Examination of the specific interactions permitted to show that the cement pastes are strongly amphoteric species with a substantial predominant Lewis basicity that is in line with the basic pH of their aqueous suspensions. Following coating with an epoxy resin (DGEBA) and a hardener (triethylene tetramine), the surface energy of the cements decreases substantially with the mass loading of the organic material. The surface thermodynamic properties were also correlated with the surface chemical composition as determined by X-ray photoelectron spectroscopy. 相似文献
675.
V. P. Perevalov A. K. Kh. Karim A. V. Khrapov L. I. Baryshnenkova B. I. Stepanov 《Chemistry of Heterocyclic Compounds》1988,24(1):33-36
The bromination of 1,3,5-trimethyl-4-chloropyrazole with N-bromosuccinimide (NBS) in the presence of benzoyl peroxide leads to 5-bromomethyl-1,3-dimethyl-4-chloropyrazole, whereas 3,5-bis(bromomethyl)-1-methyl-4-chloropyrazole predominates in the case of a twofold excess of NBS. Products of subsequent substitution in the 3- and 5-bromomethyl groups of the 3,5-bis(bromomethyl) compound in a ratio of 31 were detected in small amounts; this is evidently associated with the syn orientation of the bromine atom in the 5-bromomethyl group, which hinders attack by the bromine radical.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 40–42, January, 1988. 相似文献
676.
(Use of a factorial design to study interferences in spectrophotometry.) Ozonation of aqueous solutions leads to the formation of hydrogen peroxide and oxalic acid. Hydrogen peroxide is determined spectrophotometrically by the classical method in which TiO·H2O22+ is formed, but oxalic acid interferes by forming a complex with titanium. The interference is studied by using a simple factorial design. The results, calculated by linear regression, show the existence of both translational and rotational effects on the absorbance signals. 相似文献
677.
Enamul Karim Kaushal Kishore Jai N. Vishwakarma 《Journal of heterocyclic chemistry》2003,40(5):901-903
A facile one‐pot synthesis of 5‐benzoyl‐6‐methylthio‐1,2,3,4‐tetrahydropyrimidines in good yields is reported. 相似文献
678.
679.
Abstract— Deionization of the purple membrane of Halobacterium halobium shifts the visible absorption maximum from 570 to 605 nm and inhibits proton transport. FTIR-difference-spectra of this blue membrane at 280 K reveal that the retinal chromophore adopts a 13 -cis and all -trans geometry in a light dependent ratio. In contrast to purple membrane the 13-cu isomer forms much faster in the dark. The all- trans component produces an L-intermediate which can be stabilized at 170 K. Spectral characteristics are similar to normal L. including comparable changes of internal aspartic acids of the opsin. However, stronger changes in the amide-I absorption are observed. IR bands of the chromo-protein states are assigned to retinal normal modes by the use of bacteriorhodopsin regenerated with'C-labeled retinals. 相似文献
680.
Ashley KM Raghavan D Douglas JF Karim A 《Langmuir : the ACS journal of surfaces and colloids》2005,21(21):9518-9523
Thin polymeric films are increasingly being utilized in diverse technological applications, and it is crucial to have a reliable method to characterize the stability of these films against dewetting. The parameter space that influences the dewetting of thin polymer films is wide (molecular mass, temperature, film thickness, substrate interaction) and a combinatorial method of investigation is suitable. We thus construct a combinatorial library of observations for polystyrene (PS) films cast on substrates having orthogonal temperature and surface energy gradients and perform a series of measurements for a range of molecular masses (1800 g/mol < M < 35 000 g/mol) and film thicknesses h (30 nm < h < 40 nm) to explore these primary parameter axes. We were able to obtain a near-universal scaling curve describing a wetting-dewetting transition line for polystyrene films of fixed thickness by introducing reduced temperature and surface energy variables dependent on M. Our observations also indicate that the apparent polymer surface tension gamma(p) becomes appreciably modified in thin polymer films from its bulk counterpart for films thinner than about 100-200 nm, so that bulk gamma(p) measurements cannot be used to estimate the stability of ultrathin films. Both of these observations are potentially fundamental for the control of thin film stability in applications where film dewetting can compromise film function. 相似文献