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131.
The enantiomers of the 2,3-dihydrobilatriene-abc1 have been separated by recycling chromatography on triacetylcellulose. Comparison of their chiroptical properties with those of amino acid substituted analogues, like3, reveals the chirality of center 3 to be of minor influence on the values. Stabilization of a definite helical conformation of these pigments seems to depend on this influence together with an appropriate dipole-dipole interaction. 相似文献
132.
Kizek R Masarik M Kramer KJ Potesil D Bailey M Howard JA Klejdus B Mikelova R Adam V Trnkova L Jelen F 《Analytical and bioanalytical chemistry》2005,381(6):1167-1178
The electroanalytical determination of avidin in solution, in a carbon paste, and in a transgenic maize extract was performed in acidic medium at a carbon paste electrode (CPE). The oxidative voltammetric signal resulting from the presence of tyrosine and tryptophan in avidin was observed using square-wave voltammetry. The process could be used to determine avidin concentrations up to 3 fM (100 amol in 3 l drop) in solution, 700 fM (174 fmol in 250 l solution) in an avidin-modified electrode, and 174 nM in a maize seed extract. In the case of the avidin-modified CPE, several parameters were studied in order to optimize the measurements, such as electrode accumulation time, composition of the avidin-modified CPE, and the elution time of avidin. In addition, the avidin-modified electrode was used to detect biotin in solution (the detection limit was 7.6 pmol in a 6 l drop) and to detect biotin in a pharmaceutical drug after various solvent extraction procedures. Comparable studies for the detection of biotin were developed using HPLC with diode array detection (HPLC-DAD) and flow injection analysis with electrochemical detection, which allowed biotin to be detected at levels as low as 614 pM and 6.6 nM, respectively. The effects of applied potential, acetonitrile content, and flow rate of the mobile phase on the FIA-ED signal were also studied. 相似文献
133.
The SINDO1 method is modified to include the calculation of molecular anions. Two versions of modifications are presented which are based on charge dependent orbital exponents. Calculated heats of formation and electron affinities are much improved compared to the standard version with fixed orbital exponents.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday 相似文献
134.
Halland N Alstrup Lie M Kjaersgaard A Marigo M Schiøtt B Jørgensen KA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(23):7083-7090
The mechanism for the 2,5-diphenylpyrrolidine-catalyzed enantioselective alpha-chlorination of aldehydes with electrophilic halogenation reagents has been investigated by using experimental and computational methods. These studies have led us to propose a mechanism for the reaction that proceeds through an initial N-chlorination of the chiral catalyst-substrate complex, followed by a 1,3-sigmatropic shift of the chlorine atom to the enamine carbon atom. The suggested reaction course is different from previously proposed mechanisms for organocatalytic enamine reactions, in which the carbon-electrophile bond is formed directly. Furthermore, the rate-determining step in the overall reaction was determined and the presence of nonlinear effects was probed. 相似文献
135.
A simple preparation of the chloronitrosylcarbonylmolybdenum(0) complexes Mo(NO)(CO)4(AlCl4) and MoCl(NO)(CO)2(PPh3)2 is described. The homogeneous system MoCl(NO)(CO)2(PPh3)2/RAlCl2 (R = Et, Me) represents a new highly active long-living catalyst for the metathesis of 2-pentene. 相似文献
136.
Hans Beat Bürgi Hermann Gehrer Peter Strickler Fritz Karl Winkler 《Helvetica chimica acta》1976,59(7):2558-2565
Crystals of the title compound are triclinic, a = 27.87 Å, b = 10.77 Å, c = 12.94 Å, α 73.1°, β 116.1°, γ 120.0°, space group P1 . The structure consists of octanuclear ions: Eight Cd(II) ions are found at the corners of a distorted cube, the center of the cube is occupied by an iodide, the twelve thioglycolate sulfur atoms bridge the twelve edges of the cube thereby forming a distorted icosahedron. Cadmium ions are either five or seven coordinate. The phase problem for this structure was solved using a combination of very high and very low E-values. 相似文献
137.
[reaction: see text] The "Southern Hemisphere" intermediate 2, used by Masamune and co-workers for their asymmetric total synthesis of bryostatin 7 (1), has been synthesized from (E)-1,4-hexadiene (11) by a 24-step pathway that has a longest linear sequence of only 20 steps. This is the shortest synthesis of 2 so far recorded, and moreover, it is fully stereocontrolled. 相似文献
138.
Battaglia TM Dunn EE Lilley MD Holloway J Dable BK Marquardt BJ Booksh KS 《The Analyst》2004,129(7):602-606
The development of a field portable fiber optic Raman system modified from commercially available components that can operate remotely on battery power and withstand the corrosive environment of the hydrothermal vents is discussed. The Raman system is designed for continuous monitoring in the deep-sea environment. A 785 nm diode laser was used in conjunction with a sapphire ball fiber optic Raman probe, single board computer, and a CCD detector. Using the system at ambient conditions the detection limits of SO(4)(2-), CO(3)(2-) and NO(3)(-) were determined to be approximately 0.11, 0.36 and 0.12 g l(-1) respectively. Mimicking the cold conditions of the sea floor by placing the equipment in a refrigerator yielded slightly worse detection limits of approximately 0.16 g l(-1) for SO(4)(-2) and 0.20 g l(-1) for NO(3)(-). Addition of minerals commonly found in vent fluid plumes also decreased the detection limits to approximately 0.33 and 0.34 g l(-1) respectively for SO(4)(-2) and NO(3)(-). 相似文献
139.
Jörg Senn-Bilfinger John R. Ferguson Keith W. Lumbard Karl Zech Peter J. Zimmermann 《Tetrahedron letters》2006,47(19):3321-3323
Glucuronide conjugates of Soraprazan (BY359), a potent novel anti-secretory drug (currently in Phase II clinical trials), were not directly accessible synthetically. This was due to the relative instability of Soraprazan under the harsh Lewis acid conditions employed in popular glucuronidation methodologies and a lack of reactivity under alternative, Koenigs-Knorr, coupling conditions. We have now devised a successful synthesis using the novel N-acetylated Soraprazan to access the required glucuronide metabolites on gram scale. Coupling of this novel aglycone with methyl 1-O-trichloroacetimidoyl-2,3,5-tri-O-isobutyryl-α-d-glucopyran-uronate in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) gave the protected glucuronide intermediates. A one-pot two-step deprotection involving hydrolysis of the ester functionalities and removal of the N-acetyl group with alkaline hydrazine delivered the title compounds in satisfactory yield. 相似文献
140.
Petra Hoffman Siegfried Hünig Leonhard Walz Karl Peters Hans-Georg von Schnering 《Tetrahedron》1995,51(48):13197-13216
Azo compounds 1 and 3 containing a CC-double bond in a parallel but distant position are quatemized by Me3OBF4 to 1,MeBF4 and 3,MeBF4, whereas MeI produces the cage compounds 2,HX and 4,HX. These [3+2] cycloadducts also are quantitatively formed from 1,MeBF4 and 3,MeBF4 with catalytic amounts of azo compounds. Intermolecular [3+2] cycloadditions occur with a mixture of DBH (5) or DBO (8), MeI and a variety of alkenes (→ HI) salts of (6, 7, 9 — 12). The intermediate azomethineimines, if stabilized by a fluorenylidene group, can be isolated (20, 22,24), but not, however, in the presence of a close parallel CC-bond (25 → 26). 相似文献