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991.
Nicolas DesroyRémi Le Roux Phannarath PhansavathLucia Chiummiento Carlo Bonini Jean-Pierre Genêt 《Tetrahedron letters》2003,44(9):1763-1766
The stereocontrolled synthesis of a C15C24 fragment of dolabelides is reported. The C19 and C21 hydroxyl-bearing stereocenters were installed using ruthenium-mediated asymmetric hydrogenations of cyclic hemiketal 4 and β-keto ester 7. The C25C30 portion of dolabelides was prepared as well by ring opening of chiral epoxy alcohol 12 to set up the C27 stereogenic center. 相似文献
992.
Barbaro P Ienco A Mealli C Peruzzini M Scherer OJ Schmitt G Vizza F Wolmershäuser G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(21):5196-5210
Thermal reaction of white phosphorus with [(triphos)RhH(3)] (1) in THF affords [(triphos)Rh(eta(1):eta(2)-P(4)H)] (2), triphos=MeC(CH(2)PPh(2))(3). Similar complexes [(triphos)Rh(eta(1):eta(2)-P(4)R)] (R=Me (7), Et (8), Ph (9)) also form at lower temperature by the reaction of P(4) and [(triphos)Rh(R)(eta(2)-C(2)H(4))] with elimination of ethene. In contrast, a double-insertion process follows the reaction of [(triphos)Rh(H)(eta(2)-C(2)H(4))] and P(4) to generate tetraphosphido ethyl complex 8. Compounds 2, 7, 8 and 9 are thermally unstable and eventually decompose into the cyclotriphosphorus complex [(triphos)Rh(eta(3)-P(3))] (3) plus other unidentified phosphorus-containing species. Otherwise, PH(3) or PH(2)R is generated in the presence of H(2). The formation of PH(3) and 3 is quantitative starting from the precursor 2. The electrophilic attack of MeOTf or HBF(4) on the P(4)R ligand in the complexes 2, 7-9 is regioselective, and yields a cationic product of formula [(triphos)Rh(eta(1):eta(2)-P(4)RR')](+). The direct attack on the substituted p-R phosphorus atom is demonstrated by crossing experiments. Complexes of the latter type have been isolated in the solid state for the combinations R=H and R'=Me (11) or R=Ph and R'=Me (12). The latter species, [(triphos)Rh(eta(1):eta(2)-P(4)PhMe')]OTf.2 CH(2)Cl(2) (OTf=triflate), has been characterised by X-ray methods. The geometry at the metal is better described as a trigonal bipyramidal than pseudo-octahedral. In fact, the P(4)RR' unit acts as a bidentate ligand with its exocyclic PR(2) donor group and the endocyclic, dihapto-coordinated Pdbond;P linkage. The latter group lies in the equatorial plane, in a similar way to a classic olefin ligand that is coordinated to a butterfly-shaped L(4)M fragment (M=d(8)). DFT calculations on a model of 2 and all possible protonated isomers confirm that double substitution at the exocyclic P-donor positions of the open P(4) unit is energetically favoured. A multinuclear and multidimensional NMR analysis confirms that this structure is maintained in solution for both the parent and the protonated compounds. 相似文献
993.
Prosperini S Pastori N Ghilardi A Clerici A Punta C 《Organic & biomolecular chemistry》2011,9(10):3759-3767
We report a new and fast domino synthesis of aminoalcohols under mild conditions. The free-radical reaction of aliphatic and aromatic amines with alcohol cosolvents is promoted by means of the TiCl(4)-Zn/t-BuOOH system. According to the proposed mechanism, the amine reacts with two molecules of alcohol in an electrophilic-nucleophilic cascade process. This procedure, if compared with the TiCl(3)/t-BuOOH-mediated protocol previously reported, appears to be more selective, of more general applicability and affords the desired products in higher yields. Besides, with the same catalytic system it was possible to promote the reaction of primary arylamines with two molecules of cyclic ether, leading to the formation of a wider range of functionalized aminoalcohols. 相似文献
994.
James C. W. Chien Carlo Carlini 《Journal of polymer science. Part A, Polymer chemistry》1985,23(5):1383-1393
Poly(dicyanoacetylene) (PDCA) has been synthesized and characterized. The pristine polymer has EPR g-value, linewidth, unpaired spin concentration, spin—spin relaxation time (T2), and room temperature dc conductivity (σRT) very similar to those of pristine cis-polyacetylene (PA), but shorter spin—lattice relaxation time (T1). Saturation doping with iodine has little effect on most EPR characteristics of the polymer except for a slight increase in T1. The doped PDCA has σRT value of only 5 X 10-9 (Ω cm)-1, indicating either low carrier concentration and/or carrier mobility. Partial cyclization of the nitrile groups by heating at 400°C of PDCA produces l-PDCA with significant increases in unpaired spin concentration and σRT but marginal effects on other properties. Saturation doping of l-PDCA with iodine increases σRT to 7 × 10-3 (Ω cm)-1 without appreciable changes in EPR characteristics. The dopants in both polymers can be removed by evacuation indicating only weak charge transfer interactions. The possible stereoelectronic contribution toward the property differences between the PDCA polymers and PA are discussed. 相似文献
995.
Chemistry of Free Cyclic Vicinal Tricarbonyl Compounds (‘1,2,3‐Triones’). Part 3. Polar and Redox Reactions of 1,2,3‐Triones with Enamines of Different Types – News on Oxonol Dyes, Radicals, and Biradicals The central C?O groups of cyclic 1,2,3‐triones possess outstanding electrophilic (electron‐pair‐accepting) as well as oxidizing (one‐electron‐accepting) properties. Thus, 1,2,3‐triones are chemically related to 1,2‐ and 1,4‐benzoquinones. Whereas polar reactions with carbanion‐like (electron rich) species give rise to nucleophilic addition reactions to C?O groups under exclusive C,C‐bond formation, SET (single‐electron transfer) or redox reactions effect a partial ‘carbonyl Umpolung’ via ketyl intermediates (C,C‐ and/or C,O‐bond formation). Here, we report on numerous reactions between electron‐rich, more‐ or less‐polar enamines with 5,5‐dimethylcyclohexane‐1,2,3‐trione ( 9a ) and 1H‐indene‐1,2,3‐trione ( 9b ). Various new derivatives of basic oxonol dyes were formed, including the first oxonol dye incorporating a 1,3‐dioxocyclohexyl moiety. A novel stable radical, 50 / 50′ , was obtained from 9b and 11a via addition, hydrolysis, and treatment with conc. H2SO4. Radical 50 / 50′ represents a vinylogous ‘monodehydroreductone’ and is, thus, related to monodehydroascorbic acid ( 143 ), to Russell's radical cation ( 144 ), to indigo ( 141 / 141′ ), and to quinhydrone. 相似文献
996.
997.
Raffaella Puliti Carlo Andrea Mattia 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(4):496-499
Ammonium N‐acetyl‐l ‐threoninate, NH4+·C6H10NO4?, and methylammonium N‐acetyl‐l ‐threoninate, CH6N+·C6H10NO4?, crystallize in the orthorhombic P212121 and monoclinic P21 space groups, respectively. The two crystals present the same packing features consisting of infinite ribbons of screw‐related N‐acetyl‐l ‐threoninate anions linked together through pairs of hydrogen bonds. The cations interconnect neighbouring ribbons of anions involving all the nitrogen‐H atoms in three‐dimensional networks of hydrogen bonds. The hydrogen‐bond patterns include asymmetric `three‐centred' systems. In both structures, the Thr side chain is in the favoured (g?g+) conformation. 相似文献
998.
999.
Giuliana Gorrasi Liberata Guadagno Concetta D'Aniello Carlo Naddeo Gennaro Romano Vittoria Vittoria 《Macromolecular Symposia》2003,203(1):285-294
Three syndiotactic polypropylene samples were crystallized under different conditions in order to obtain different polymorphs. A first sample was crystallized at high temperature, obtaining the helical form I; a second was crystallized from the melt at 0°C for many days obtaining the trans-planar mesophase; a third sample was obtained by solvent induced crystallization followed by annaeling of the trans-planar mesophase, leading to a mixture of both the helical forms I and II. In the dynamic-mechanical analysis the helical form I showed only one peak of tan δ corresponding to the amorphous glass transition. The other polymorphs also showed this transition centered at about the same temperature. Beside the peak corresponding to the Tg, the trans-planar mesophase was characterized by a peak appearing at 70°C, and the helical form II by a peak at 100°C. These peaks, unambiguously associated to transitions of the different forms, can be considered a distinctive evidence for the polymorphs obtained in different processing conditions. 相似文献
1000.
The behaviour in a magnetic field of the spin polarisation of triplet excitons in biphenyl-tetracyanobenzene is accounted for by the generation of the triplet state into a superposition of spin states. The molecular conformation in the excited singlet state is discussed. 相似文献