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781.
782.
783.
The two 1,3-butadienyl 2-propenyl sulfones and , 1,3,5-heptatriene synthons, have been transformed into the tri- and tetraenes - by alkylcuprate addition and Ramberg-Bäcklund SO2 extrusion. The reaction stereochemistry is discussed. 相似文献
784.
785.
Ferdinand Bohlmann Rupert Herrmann Helmut Schwarz Hans-Martin Schiebel Norbert Schróder 《Tetrahedron》1977,33(3):357-362
The investigation of some 2H-labelled compounds as well as the analysis of energetic properties (appearance potential measurement, kinetic energy release) reveal some unusual elimination of radicals (CH3., OH. resp. CHO.) from ortho substituted bencoic acid methyl esters. It can be shown that most of the reactions are induced by a novel hydrogen transfer to the carboxyl function. 相似文献
786.
787.
Hans-Heinrich Limbach Simone Baumgrtner Roland Franke Ferdinand Mnnle Gerd Scherer Gleb S. Denisov 《Molecules (Basel, Switzerland)》2021,26(14)
Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d8 and in CD2Cl2. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d8 and CD2Cl2, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD2Cl2. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell–Limbach tunneling model. 相似文献
788.
Goulnara Arzhantseva Pierre de la Harpe Delaram Kahrobaei Zoran Šunić 《Geometriae Dedicata》2007,124(1):5-26
The true prosoluble completion
of a group Γ is the inverse limit of the projective system of soluble quotients of Γ. Our purpose is to describe examples
and to point out some natural open problems. We discuss a question of Grothendieck for profinite completions and its analogue
for true prosoluble and true pronilpotent completions.
Goulnara Arzhantseva and Zoran Šunić were the authors of the Appendix. 相似文献
789.
The paper presents the theory of the discontinuous Galerkin finite element method for the space-time discretization of a linear
nonstationary convection-diffusion-reaction initial-boundary value problem. The discontinuous Galerkin method is applied separately
in space and time using, in general, different nonconforming space grids on different time levels and different polynomial
degrees p and q in space and time discretization, respectively. In the space discretization the nonsymmetric interior and boundary penalty
approximation of diffusion terms is used. The paper is concerned with the proof of error estimates in “L
2(L
2)”-and “
”-norms, where ɛ ⩾ 0 is the diffusion coefficient. Using special interpolation theorems for the space as well as time discretization,
we find that under some assumptions on the shape regularity of the meshes and a certain regularity of the exact solution,
the errors are of order O(h
p
+ τ
q
). The estimates hold true even in the hyperbolic case when ɛ = 0. 相似文献
790.
Given a Steiner triple system , we say that a cubic graph G is -colourable if its edges can be coloured by points of in such way that the colours of any three edges meeting at a vertex form a triple of . We prove that there is Steiner triple system of order 21 which is universal in the sense that every simple cubic graph is -colourable. This improves the result of Grannell et al. [J. Graph Theory 46 (2004), 15–24] who found a similar system of order 381. On the other hand, it is known that any universal Steiner triple
system must have order at least 13, and it has been conjectured that this bound is sharp (Holroyd and Škoviera [J. Combin.
Theory Ser. B 91 (2004), 57–66]).
Research partially supported by APVT, project 51-027604.
Research partially supported by VEGA, grant 1/3022/06. 相似文献