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31.
Bhat B Swayze EE Wheeler P Dimock S Perbost M Sanghvi YS 《The Journal of organic chemistry》1996,61(23):8186-8199
A highly convergent free radical coupling of alkyl iodides and oximes, mediated by bis(trimethylstannyl) benzopinacolate (8), has been utilized to prepare a series of dimeric nucleosides as mimics of natural nucleic acids. The systematic optimization of the reaction conditions allowed for the single-step conversion of the appropriate iodides and oximes into the 2'-deoxy dimers 9 in moderate to excellent yields. For example, the reaction of 3'-deoxy-3'-iodo-5'-(triphenylmethyl)thymidine (6a) with 3'-O-(tert-butyldiphenylsilyl)-5'-O-(methyleneimino)thymidine (7a) in the presence of 8 in degassed benzene gave an 81% yield of 3'-de(oxyphosphinico)-3'-(methyleneimino)-5'-O-(triphenylmethyl)thymidylyl-(3'-->5')-3'-O-(tert-butyldiphenylsilyl)thymidine (9a). Similarly prepared were dimers containing both pyrimidine (thymine, 5-methylcytosine) and purine (adenine, guanine) bases. The reaction was highly stereoselective, giving only a single dimeric species having the ribo-configuration of the newly introduced C-3'-branched methylene moiety. Also prepared were dimers 16, incorporating 2'-O-methyl ribonucleosides in both halves of the dimer. This required the synthesis of 3'-deoxy-3'-iodo-2'-O-methyl nucleosides 12 as well as 2'-O-methyl-5'-O-methyleneimino nucleosides 15. For example, 5'-O-(tert-butyldiphenylsilyl)-3'-deoxy-3'-iodo-2'-O-methyl-5-methyluridine (12e) was prepared in 80% yield by displacement of the corresponding triflate with Bu(4)NI. Also prepared were the suitably protected 3'-deoxy-3'-iodo adenosine and guanosine derivatives. Compounds 15 were prepared in high yield by a regioselective Mitsunobu reaction to give the corresponding 5'-O-phthalimido nucleosides 13, which were subsequently converted to the requisite oximes 15. In the 2'-O-methyl series, the pinacolate coupling reaction proceeded with efficiency equal to that observed for the 2'-deoxy series 9, but with slightly less stereoselectivity, giving predominantly the C-3'ribo products 16, contaminated with 5-25% of the epimeric material. Mixed base dimers containing both pyrimidine and purine bases at all possible positions, including purine-purine dimers were prepared. The hydroxylamine or methyleneimino (MI) backbone of several representative dimers so prepared was converted via methylation to give the corresponding methylenemethylimino (MMI)-linked compounds, which are novel phosphate surrogates for use in antisense oligonucleotides. 相似文献
32.
Akutsu H Akutsu-Sato A Turner SS Le Pevelen D Day P Laukhin V Klehe AK Singleton J Tocher DA Probert MR Howard JA 《Journal of the American Chemical Society》2002,124(42):12430-12431
Normal state conductivity and superconductivity together with bulk magnetic susceptibility and magnetization measurements have been measured for two molecular charge-transfer salts: beta' '-(ET)4[(H3O)Ga(C2O4)3]G (ET = bis(ethylenedithio)tetrathiafulvalene, G = pyridine for compound I and nitrobenzene for compound II). With the exception of the included guest molecules (G) the crystal structures are almost identical. Both show minima in their electrical transport at 130 K for I and at 160 K for II, but at lower temperatures their behaviors differ markedly. The resistance of I reaches a maximum at 50 K with a further small peak at 2 K and possible superconductivity only below 2 K, whereas that of II increases continuously down to 7.5 K, where an abrupt transition to a superconducting state occurs. 相似文献
33.
C.S. Dimbylow I.J. McColm C.M.P. Barton N.N. Greenwood G.E. Turner 《Journal of solid state chemistry》1974,10(2):128-136
Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze EuxWO3 is stable up to 1000°C when x ? 0.125 and in the region 0.085 ? x ? 0.125 the symmetry is cubic. A tetragonal bronze exists at x = 0.05, and an orthorhombic bronze with a structure closely related to the orthorhombic form of WO3 exists below x = 0.01. Mössbauer spectra at room temperature and at 80 K indicate that in all these phases the europium is highly ionized as Eu(III) with no electron localization to give (EuII) even at low values for x. The decomposition products of the bronzes have been established, and the Mössbauer parameters for the highly nonstoichiometric tungstates EuxWO4 were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for EuxWO4 was found to fit the data best. In conformity with the foregoing data, a sample of composition “Eu2W2O7” was found not be be a pyrochlore but to comprise a mixture of Eu6WO12, EuxWO4, and W. The phase relationships for the europium bronze system EuxWO3 are compared with those of other ionic bronzes NaxWO3, LixWO3, and AlxWO3. 相似文献
34.
Direct-current cyclic voltammetry is used to investigate the suitability of a ferrocene derivative as a mediator with galactose, glycolate and l-amino acid oxidases. The three enzymes coupled catalytically to ferrocene monocarboxylic acid exhibiting homogeneous second-order rate constants in the range 0.4 × 105 to 8.5 × 105 l mol?1 s?1. Enzyme electrodes which responded to d-galactose, glycolate or l-amino acids were constructed. The appropriate oxidase was retained behind a dialysis membrane at a carbon paste electrode containing the poorly soluble derivative 1,1′-dimethylferrocene. All the electrodes responded rapidly to millimolar concentrations of their respective substrates producing 95% of the steady-state current response in <2 min. This general method of biosensor construction should be widely applicable to oxidases and other oxidoreductase enzymes. 相似文献
35.
The elucidation of the structure of the cytotoxic marine sponge alkaloid pyrinodemin A by synthesis is described. Based on the 13C NMR spectra of three double bond positional isomers and the natural product, it is concluded the C14′-C15′ isomer best represents the true structure of pyrinodemin A. In addition, the structural assignment of pyrinodemin C is evaluated. 相似文献
36.
Inspired by the concept of multivalency and in pursuit of ever more intricate artificial molecular machines, we investigated the strict self-assembly of a triply threaded two-component superbundle, starting from a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core and a trifurcated trication wherein three bipyridinium units are linked 1,3,5 to a central benzenoid core. The result of the investigation was quite unexpected and surprising. It transpired that the rapid formation of a doubly threaded two-component complex was followed by an extremely slow conversion (a week at 253 K in CD3COCD3 to reach equilibrium) of this kinetically controlled product into a thermodynamically controlled one, namely a triply threaded two-component superbundle. This intriguing observation begs the question: are there instances in nature where multivalency is expressed as a kinetically controlled process, prior to an equilibrium state being reached, and if so, what are the biological implications, if any? 相似文献
37.
Several radical cyclisation reactions involving indoles are described. Most notably, we have shown that radical additions to C3 of an indole are frequently facile. A dichotomy in the course of radical cyclisation reactions to C2 of the indole has also been exposed wherein 6-endo-trig cyclisations are propagated by the loss of a hydrogen atom from C2 while 5-exo-trig cyclisations are propagated by hydrogen atom abstraction at C3 from tributyltin hydride. Cyclisations involving the addition of indolyl radical intermediates to arenes have also been demonstrated. 相似文献
38.
A new photoelectron spectroscopic method is described directed towards the measurement of the photoelectron spectra of trapped ions. An electrostatic retarding field electron energy analyser incorporating a channel plate is included with an ion trap in a strong magnetic field. The form of photoelectron spectrum is shown to be strongly influenced by a elastic scattering effects which can be enhanced by using an electron trapping potential well. The resultant spectra then have a particularly simple and useful form. 相似文献
39.
Using a modified photoelectron spectrometer the radiative lifetimes of some electronically excited diatomic and triatomic molecular ions have been determined. The method is based on measuring the time delay between the ejection of a photoelectron from a molecule and the emission of a photon from the resulting ion. 相似文献
40.
Wender PA Baryza JL Bennett CE Bi FC Brenner SE Clarke MO Horan JC Kan C Lacôte E Lippa B Nell PG Turner TM 《Journal of the American Chemical Society》2002,124(46):13648-13649
Macrocycle 1 is a new highly potent analogue of bryostatin 1, a promising anti-cancer agent currently in human clinical trials. In vitro, 1 displays picomolar affinity for PKC and exhibits over 100-fold greater potency than bryostatin 1 when tested against various human cancer cell lines. Macrocycle 1 can be generated in clinically required amounts by chemical synthesis in only 19 steps (LLS) and represents a new clinical lead for the treatment of cancer. 相似文献