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1.
Stefan Pitsch Sebastian Wendeborn Ramanarayanan Krishnamurthy Armin Holzner Mark Minton Martin Bolli Christian Miculca Norbert Windhab Ronald Micura Michael Stanek Bernhard Jaun Albert Eschenmoser 《Helvetica chimica acta》2003,86(12):4270-4363
Pyranosyl‐RNA (‘p‐RNA’ ) is an oligonucleotide system isomeric to natural RNA and composed of the very same building blocks as RNA. Its generational, chemical, and informational properties are deemed to be those of an alternative nucleic acid system that could have been a candidate in Nature's evolutionary choice of the molecular basis of genetic function. We consider the study of the chemistry of p‐RNA as etiologically relevant in the sense that knowledge of its structural, chemical, and informational properties on the chemical level offers both a perspective and reference points for the recognition of specific structural assets of the RNA structure that made it the (supposedly) superior system among possible alternatives and, therefore, the system that became part of biology as we know it today. The paper describes the chemical synthesis of β‐d‐ (and L )‐ribopyranosyl‐(4′→2′)‐oligonucleotide sequences, presents a resume of their structural and chemical properties, and cautiously discusses what we may and may not have learned from the pyranosyl isomer of RNA with respect to the conundrum of RNA's origin. 相似文献
2.
3.
T. V. Ramakrishnan H. R. Krishnamurthy S. R. Hassan G. Venketeswara Pai 《Journal of Chemical Sciences》2003,115(5-6):767-774
Manganites of the LA1−x
Ca
x
MnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach
recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degeneratee
g
states at the Mn sites dynamically reorganize themselves into localised, JT polaronsl with exponentially small inter-site hopping, and band-like, nonpolaronic statesb, leading to anew 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment
of the model which explains quantitatively a wide variety of experimental results, including insulator-metal transitions and
CMR, in terms of the influence of physical conditions on the relative energies and occupation of thel andb states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and
spatial correlation is the key to manganite physics.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
4.
Rogelio P. Frutos Isabelle Gallou Diana Reeves Yibo Xu Dhileepkumar Krishnamurthy Chris H. Senanayake 《Tetrahedron letters》2005,46(48):8369-8372
Expedient and practical new methodology for the synthesis of substituted imidazoles was developed to provide a rapid access to a variety of 2-substituted, 1,2-disubstituted and 1,2,4-trisubstituted imidazoles by the direct CuCl-mediated reaction of nitriles with α-amino acetals in an intermolecular as well as intramolecular fashion. 相似文献
5.
Crouch R Boyer RD Johnson R Krishnamurthy K 《Magnetic resonance in chemistry : MRC》2004,42(3):301-307
Compensation of refocusing inefficiency in a gHMBC experiment by replacing the rectangular pi pulse with a pair of adiabatic pulses with synchronized inversion sweep (CRISIS) significantly improves the performance of the gHMBC experiment. The CRISIS-gHMBC experiment retains the pure absorptive shapes in F1 and hence results in better lineshape and higher resolution than the current versions of magnitude mode gHMBC spectra. When used as a broadband experiment, CRISIS-gHMBC, owing to better refocusing efficiency of the adiabatic pulse pairs, gives improved performance across the 13C spectral width. Moreover, it is shown that CRISIS-gHMBC is a robust and improved alternative and when used along with the IMPRESS (Improved Resolution using Symmetrically Shifted pulses) technique further increases the sensitivity and resolution without additional experimental time. The IMPRESS-CRISIS combination is demonstrated for broadband gHMBC and band-selective gHMBC experiments. The ICbs-gHMBC [IMPRESS-CRISIS-band-selective gHMBC] experiment is an attractive and better alternative to individual band-selective gHMBC. 相似文献
6.
The pentafluoropropionyl, (PFP) and trifluoroacetyl (TFA) esters of several naturally occurring and synthetically modified simple trichothecenes were synthesized in nanogram amounts and characterized. Optimum conditions for the gas chromatographic (GC) separation of these derivatives and their analysis by negative ion chemical ionization (NICI) mass spectrometric technique were determined. These perfluoroacyl derivatives under the NICI conditions undergo limited but characteristic fragmentations similar to the fragmentations of heptafluorobutyryl esters of trichothecenes under the same conditions. Characteristic ions for the specific detection and accurate quantification of these PFP and TFA derivatives were chosen. Preliminary results indicated that the PFP derivatives are better suited for the analysis of simple trichothecenes by GC-NICI-MS technique. Ultra trace (0.5-2.0 pg) amounts of these PFP derivatives were detected by the developed procedure. 相似文献
7.
S. Krishnamurthy 《Tetrahedron letters》1982,23(33):3315-3318
Formylation of functionalized primary aromatic and aliphatic amines with acetic formic anhydride (AFA) followed by borane:methyl sulfide reduction in the same pot affords the corresponding -methylamines in excellent isolated yields, uncontaminated bybis alkylation; the reaction sequence is applicable to even very weakly basic and sterically hindered amines. 相似文献
8.
Xiao-jun Wang Li Zhang Yibo Xu Dhileepkumar Krishnamurthy Chris H. Senanayake 《Tetrahedron letters》2004,45(38):7167-7170
A practical protocol for synthesis of 2-(N-substituted)-aminobenzimidazoles was developed. N-(2-Aminoaryl)thioureas undergo a CuCl-promoted intramolecular cyclization to give the corresponding 2-(N-substituted amino)benzimidazoles in good to excellent isolated yields. 相似文献
9.
Kasani Aparna Professor Setharampattu S. Krishnamurthy Munirathinam Nethaji 《无机化学与普通化学杂志》1995,621(11):1913-1921
Lanthanide nitrate complexes of diphosphazane dioxides Ph2P(O)N(Pri)P(O)Ph2 ( 1 ) and (PhO)2P(O)N(Me)P(O)(OPh)2 ( 2 ) have been synthesised and studied by conductometry, IR, multinuclear NMR spectroscopic methods and X-ray diffraction. Ligand 2 is accessible by two different methods, viz., by direct oxidation of the phosp(III)azane ligand or by starting from phosph(V)azane chloro precursor. The structure of 2 is confirmed by X-ray diffraction. Crystallographic data for 2 : Triclinic, Space group P1 , a = 10.078(1), b = 10.575(3), c = 12.364(4) Å, α = 75.70(2)°, α = 75.56(1)°, γ = 77.68(1)°, Z = 2, V = 1 220 Å3; structure refined to RF = 0.0459 on 3 495 data with F > 3σ(F). The diphosphazane dioxide ligand exhibits trans geometry in the solid state. The structure of a lanthanide complex, [Pr(NO3)3( 2 )2] ( 14 ) is also determined by X-ray diffraction. Crystallographic data for 14 : Trigonal, Space group P32, a = b = 15.710(2), c = 40.067(2) Å, Z = 6, V = 8 564 Å3; structure refined to RF = 0.0430 on 8 077 data with F > 5σ(F). The two diphosphazane dioxide ligands and the nitrate groups are coordinated to praseodymium in a bidentate chelate fashion. The geometry around the ten coordinated metal is distorted bicapped square antiprism. 相似文献
10.
S. S. Krishnamurthy P. Ramabrahmam A. R. Vasudeva Murthy R. A. Shaw M. Woods 《无机化学与普通化学杂志》1985,522(3):226-234
The reactions of pentachloro(2′, 2′, 2′-triphenylphosphazen-1′-yl)cyclotriphosphazene, N3P3Cl5(NPPh3), with primary and secondary amines have been investigated using diethyl ether, methyl cyanid or benzene as the solvent. The structures of the products obtained, N3P3Cl5minus;nRn(NPPh3) [n = 1, R = NHMe, NHBut, NMe2, NC5H10, NEt2; n = 2, R = NMe2, NC5H10, NEt2; n = 3, R = NMe2, NHBut; n = 5, R = NMe2] are elucidated by 1H and 31P NMR spectroscopy. The ? NPPh3 substituent exerts a pronounced geminal directing influence on incoming secondary amino nucleophiles; compounds containing a ≡ PCl(NPPh3) group are not formed at the bis and subsequent stages of chlorine replacement. The reactions that involve primary amines follow the pattern established for the analogous reactions of hexachlorocyclotriphosphazene. The effect of solvent and possible mechanism(s) are discussed. 相似文献