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991.
Barrett AG Blaney F Campbell AD Hamprecht D Meyer T White AJ Witty D Williams DJ 《The Journal of organic chemistry》2002,67(9):2735-2750
The total syntheses of eight members of the palmarumycin family have been achieved, with identification of the absolute stereochemistry for three of these natural products. In addition, the ras-farnesyl transferase inhibitor (-)-preussomerin G has been synthesized, achieving the first enantioselective route for accessing this family of natural products. Highlights of the synthetic work include an asymmetric epoxidation of a cyclic enone in excellent yield and enantiomeric excess and a potentially biomimetic oxidative spirocyclization for the introduction of the bis-spiroketal array unique to the preussomerin natural products. 相似文献
992.
Syntheses and Structures of the Titanium(III) Siloxanes [Ti(OSiPh3)3(thf)2] and [Ti(OSiPh3)3(py)2] The new titaniumtrioxysilanes [Ti(OSiPh3)3(thf)2] ( 1 ) and [Ti(OSiPh3)3(py)2] ( 2 ) have been obtained from the reaction of titaniumtrichloride with LiOSiPh3 in the presence of the corresponding bases tetrahydrofurane (thf) and pyridine (py). From the crystal structures of both compounds it is evident that the titanium atoms are in the centres of trigonal‐bipyramidal coordination figures, with the donor atoms in axial positions. The compounds 1 and 2 have slightly different structures (mean values: 1 : Ti‐O(Si) 1.897(9), Ti‐O(C) 2.136(8) Å; 2 : Ti‐O 1.902(9), Ti‐N2.252(8) Å) and have a single absorption band in the visible region of the UV‐spectrum. The exchange of the thf‐ligands in 1 by pyridine (in high molar excess) seems to be hindered as deduced from UV‐spectroscopy. 相似文献
993.
Single crystals of Zr3Al3C5—a carbide previously reported with the formula ZrAlC2−x—were isolated from a sample prepared by reaction of ZrC with an excess of aluminum. The carbides ScAl3C3and UAl3C3were synthesized from the elemental components by arc-melting. The crystal structures of these three compounds were redetermined from four-circle X-ray diffractomter data. In the original structure determination of ZrAlC2−x, the metal positions were found to form close-packed layers in the space groupP63/mmc, while the carbon atoms were assumed to occupy 5/6 of the octahedral voids at random. The present structure determination in the space groupP63/mc(R=0.024 for 519 structure factors and 23 variable parameters) shows that all carbon positions are fully occupied and one has a trigonal bipyramidal aluminum coordination. The structures of ScAl3C3and UAl3C3also have originally been determined in the space groupP63/mmc. The present structure refinements in the space groupP63mc(ScAl3C3:R=0.031 for 282Fvalues and 16 variables; UAl3C3:R=0.029 for 217Fvalues and 16 variables) essentially confirms the structures with the exception of one aluminum site. In all of these structures the metal atoms are arranged in close-packed layers and together with the previously reported structure of U2Al3C4they form a homologous series with the general formulaT1+nAl3C3+n, wheren=0, 1, 2 for ScAl3C3, U2Al3C4, and Zr3Al3C5, respectively. The packing of the metal atoms is represented by the Zhdanov symbols (4)2, (5)2, and (6)2. The arrangement of the aluminum atoms is very similar to that of the binary carbide Al4C3, while the other metal atoms form a cubic stacking sequence, as it is found in the binary carbidesTC with NaCl type structure. 相似文献
994.
Ternary Chlorides with Trigonal-Bipyramidal Clusters: [M5(C2)]Cl9 (M = La? Pr) The chlorides [M5(C2)]Cl9 (M = La? Pr) are obtained by metallothermic reduction of the respective trichlorides MCl3 with caesium in the presence of the lanthanide metal and carbon in sealed niobium ampoules at 800°C. They contain trigonal-bipyramidal clusters [M5(C2)] crystallizing with the triclinic crystal system. Only seven of the nine edges of the trigonal bipyramids are brigded by chloride (Cli). Each cluster is surrounded by twelve terminal ligands (Cla) so that units of the composition [M5(C2)Cl7i]Cl12a have to be considered. These are connected not only via Cli–a and Cla–a–a bridges. Rather, Cla–a (one linear and one bent) and Cli–i bridges are also observed. 相似文献
995.
996.
Gansäuer A Lauterbach T Narayan S 《Angewandte Chemie (International ed. in English)》2003,42(45):5556-5573
Over the last few decades the use of radicals in synthesis has witnessed an explosive growth through introduction of efficient chain and electron-transfer reactions. Strained heterocycles, in particular, have emerged as a highly versatile and readily available class of radical precursors. The generation of carbinyl radicals of heterocycles has resulted in many elegant applications of heteroatom-centered radicals, such as beta fragmentations, cyclizations, and intramolecular hydrogen atom abstractions. Direct electron transfer to strained heterocycles has been realized through the use of arene radical anions. The method combines the virtues of radical and organometallic chemistry to yield useful functionalized organolithium compounds. Epoxides have been opened with high regioselectivity by titanocene(III) reagents in either stoichiometric or catalytic quantities to yield beta-titanoxy radicals. This development has resulted in many new applications in natural product synthesis. 相似文献
997.
998.
999.
Moritz Mickler Enrico Schleiff Prof. Dr. Thorsten Hugel Prof. Dr. 《Chemphyschem》2008,9(11):1503-1509
Novel single‐molecule techniques allow the observation of single‐molecular motors in real time under physiological conditions. This enables one to gain previously inaccessible information about the mechanics of molecular motors, especially their mechano‐chemical coupling. As an example, we discuss the DNA import motor of the bacteriophage ?29 and protein import into chloroplasts. In contrast to these highly developed biological molecular motors, artificial molecular motors are still at an early stage of development. Nevertheless, they already give a wealth of information. Our review focuses on how the investigation of artificial and biological molecular motors can mutually enrich each other. 相似文献
1000.
We calculate the heats of vaporisation for imidazolium‐based ionic liquids [Cnmim][NTf2] with n=1, 2, 4, 6, 8 by means of molecular dynamics (MD) simulations and discuss their behavior with respect to temperature and the alkyl chain length. We use a force field developed recently. The different cohesive energies contributing to the overall heats of vaporisations are discussed in detail. With increasing alkyl chain length, the Coulomb contribution to the heat of vaporisation remains constant at around 80 kJ mol?1, whereas the van der Waals interaction increases continuously. The calculated increase of about 4.7 kJ mol?1 per CH2‐group of the van der Waals contribution in the ionic liquid exactly coincides with the increase in the heats of vaporisation for n‐alcohols and n‐alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions. 相似文献