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81.
Raymond N. Castle Robert Douglas Thompson N. Kent Dalley Stanley H. Simonsen Steven B. Larson 《Journal of heterocyclic chemistry》1981,18(8):1551-1555
The nucleoside, 5-amino-6-chloro-4-nitro-2-β-D-ribofuranosyl-2H-pyridazin-3-one ( 4 ), was synthesized as part of a study to prepare potential chemotherapeutic compounds. An X-ray crystal structure study of the compound was initiated in order to substantiate its formula and to examine its conformation and absolute configuration. The structure was determined using direct methods and refined to an R of 0.020 (Rw = 0.024). The compound has a glycosyl torsion angle of 71.2°. The structure contains three intermolecular hydrogen bonds and one intramolecular hydrogen bond. 相似文献
82.
83.
84.
The scalar wave equation between the inner and the outer horizon in the Schwarzschild–de Sitter geometry is solved numerically, and the spatial variations of the field amplitude, as well as of the potential, are shown graphically. By generalizing the "tortoise" coordinate x known from Schwarzschild theory to the SdS system we first transfer the wave equation to a convenient form in which the potential V is written as a function of x. We then show how a useful "tangent" approximation can be introduced which leads to a simple, analytically invertible, relation between x and the radius r. We concentrate on two limiting cases. The first case is when the two horizons are close to each other, the so-called Nariai black hole, and the second case is when the horizons are far apart. Reflection and transmission coefficients are worked out on the basis of a replacement of the real barrier V(x) by a square barrier. 相似文献
85.
86.
Enantioselective Organocatalytic Cascade Approach to Different Classes of Benzofused Acetals
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Bruno Matos Paz Dr. Lydia Klier Line Næsborg Vibeke Henriette Lauridsen Prof. Dr. Frank Jensen Prof. Dr. Karl Anker Jørgensen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(47):16810-16818
A novel enantioselective organocatalytic strategy is presented for the synthesis of tetrahydrofurobenzofuran and methanobenzodioxepine natural product core structures. The strategy is based on a pair of divergent reaction pathways in which hydroxyarenes react with γ‐keto‐α,β‐unsaturated aldehydes, catalyzed by a chiral secondary amine. One reaction pathway, which leads to chiral 5,5‐fused acetals with two stereocenters—the tetrahydrofurobenzofuran scaffolds—proceeds in moderate yields and up to 96 % ee. The other reaction pathway provides 5,6‐bridged methanobenzodioxepine scaffolds with three stereocenters in moderate to good yields and up to 95 % ee. The reaction is remarkable as it can proceed with catalyst loadings as low as 0.25 mol %, providing one of the highest known turnover numbers in iminium ion catalysis. Furthermore, the hemiacetal tetrahydrofurobenzofuran can undergo functionalizations including reduction, oxidation, and allylation. Finally, the effects involved in the substrate control for the divergent pathways, based on both experimental and computational studies, have been investigated. A model involving steric, electronic and stereoelectronic interactions is discussed to rationalize the observed selectivities. 相似文献
87.
Counterions Control Whether Self‐Assembly Leads to Formation of Stable and Well‐Defined Unilamellar Nanotubes or Nanoribbons and Nanorods
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Dr. Dong Shi Christian Schwall George Sfintes Dr. Erling Thyrhaug Dr. Peter Hammershøj Dr. Marite Cárdenas Dr. Jens B. Simonsen Dr. Bo W. Laursen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(23):6853-6856
Self‐assembly of the amphiphilic π‐conjugated carbenium ion ATOTA‐1+ in aqueous solution selectively leads to discrete and highly stable nanotubes or nanoribbons and nanorods, depending on the nature of the counterion (Cl? vs. PF6?, respectively). The nanotubes formed by the Cl? salt illustrate an exceptional example of a structural well‐defined (29±2 nm in outer diameter) unilamellar tubular morphology featuring π‐conjugated functionality and high stability and flexibility, in aqueous solution. 相似文献
88.
Growing interest has been devoted in the last few years to the synthesis of optically active β-halogenoethyl sulphoxides1 and sulphilimines2 via chiral discrimination. In this context we have developed a synthesis of optically active isopropyl vinylphenylphosphinates 1 via kinetic resolution of isopropyl β-halogenoethylphenylphosphinates 2(a-c) by chiral bases (3–6) and we have studied the influence of the resolving agent and of the leaving group on the kinetic resolution. 相似文献
89.
90.
M. J. Musmar Gary E. Martin Robert T. Gampe Vincent M. Lynch Stanley H. Simonsen Milton L. Lee Marvin L. Tedjamulia Raymond N. Castle 《Journal of heterocyclic chemistry》1985,22(2):545-553
1-Methylphenanthro[3,4-b]thiophene was prepared by the photocyclization of 1-(4′-methyl-2′-thienyl)-2-(2″-naphthyl)ethene. The 1H and 13C-nmr spectra were assigned using two-dimensional 1H/13C heteronuclear chemical shift correlation and relayed coherence transfer (RELAY) experiments. From nuclear Overhauser difference spectra, the H11-C1 methyl-H intramolecular distance was determined to be 2.10 Å. The molecule crystallized from chloroform in the monoclinic system, space group P21/c. A total of 3536 unique reflections were measured and the structure was solved by direct methods and refined to a final R = 0.049. The molecule is helical with both chiral forms observed in the crystal. The H11-C1 methyl-H distance in the crystal was 2.12(3)Å in excellent agreement with the distance measured in solution by NOE techniques. 相似文献