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1.
The total synthesis of the dimeric ellagitannin coriariin A is reported. The key reaction to access the dimeric framework was realized early in the synthesis pathway via a bis acylation reaction of a dehydrodigalloyl diacid with 2 equiv of a glucopyranose trichloroacetimidate. The glucose rings were subsequently functionalized, culminating in a double oxidative cyclization to form stereoselectively both (S)-HHDP ester units. This bis acylation strategy was also employed to prepare a gallotannin analogue of coriariin A whose earlier synthesis by orthoquinone dimerization was plagued by yield-limiting side reactions. 相似文献
2.
Lawlor A Reagan D McCullagh GD Gregorio PD Tartaglia P Dawson KA 《Physical review letters》2002,89(24):245503
We introduce an order parameter for dynamical arrest. Dynamically available volume (unoccupied space that is available to the motion of particles) is expressed as holes for the simple lattice models we study. Near the arrest transition the system is dilute in holes, so we expand dynamical quantities in a series of hole density. Unlike the situation when presented in particle density, all cases of simple models that we examine have a quadratic dependence of the diffusion constant on hole density. This observation implies that in certain regimes ideal dynamical arrest transitions may possess a hitherto unnoticed degree of universality. 相似文献
3.
Kudavalli JS Boyd DR Coyne D Keeffe JR Lawlor DA MacCormac AC More O'Ferrall RA Rao SN Sharma ND 《Organic letters》2010,12(23):5550-5553
Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a β-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ? HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation. 相似文献
4.
k-core percolation is an extension of the concept of classical percolation and is particularly relevant to understanding the resilience of complex networks under random damage. A new analytical formalism has been recently proposed to deal with heterogeneous k-cores, where each vertex is assigned a local threshold k(i). In this Letter we identify a binary mixture of heterogeneous k-cores which exhibits a tricritical point. We investigate the new scaling scenario and calculate the relevant critical exponents, by analytical and computational methods, for Erd?s-Rényi networks and 2D square lattices. 相似文献
5.
Lawlor DA Kudavalli JS MacCormac AC Coyne DA Boyd DR O'Ferrall RA 《Journal of the American Chemical Society》2011,133(49):19718-19728
Cis- and trans-1,2-dihydrodiol isomers of benzene undergo acid-catalyzed dehydration to form phenol. In principle the isomeric substrates react through a common β-hydroxybenzenium (cyclohexadienyl) carbocation. Notwithstanding, the isomers show a large difference in reactivity, k(cis)/k(trans) = 4500. This difference is reduced to k(cis)/k(trans) = 440 and 50 for the 1,2-dihydrodiols of naphthalene and 9,10-dihydrodiols of phenanthrene, respectively, and to 6.9 for the dihydrodiols of the nonaromatic 7,8-double bond of acenaphthylene. Because the difference in stabilities of cis- and trans-dihydrodiols should be no more than 2-3-fold, these results imply a high cis stereoselectivity for nucleophilic trapping of a β-hydroxyarenium cation by water in the reverse of the carbocation-forming reaction. This is confirmed by studies of the 10-hydroxy-9-phenanthrenium ion generated from aqueous solvolyses of the trans-9,10-bromohydrin derivative of phenanthrene and the monotrichloroacetate ester of the phenanthrene cis-9,10-dihydrodiol. The cis stereoselectivity of forward and reverse reactions is explained by the formation (in the "forward" reaction) of different conformations of carbocation from cis- and trans-dihydrodiol reactants with respectively β-C-H and β-C-OH bonds in pseudoaxial positions with respect to the charge center of the carbocation optimal for hyperconjugation. Formation of different conformations is constrained by departure of the (protonated) OH leaving group from a pseudoaxial position. The difference in stability of the carbocations is suggested to stem (a) from the greater hyperconjugative ability of a C-H than a C-OH bond and (b) from enhanced conjugation arising from the stabilizing influence of an aromatic ring in the no-bond resonance structures representing the hyperconjugation (C(6)H(6)OH(+) ? C(6)H(5)OH H(+)). This is consistent with an earlier suggestion by Mulliken and a demonstration by Schleyer that the benzenium ion is subject to hyperconjugative aromatic stabilization. It is proposed that, in analogy with the terms homoconjugation and homoaromaticity, arenium ions should be considered as "hyperaromatic". 相似文献
6.
Glass-forming liquids have been extensively studied in recent decades, but there is still no theory that fully describes these systems, and the diversity of treatments is in itself a barrier to understanding. Here we introduce a new simple model that (possessing both liquid-crystal and glass transition) unifies different approaches, producing most of the phenomena associated with real glasses, without loss of the simplicity that theorists require. Within the model we calculate energy relaxation, nonexponential slowing phenomena, the Kauzmann temperature, and other classical signatures. Moreover, the model reproduces a subdiffusive exponent observed in experiments of dense systems. The simplicity of the model allows us to identify the microscopic origin of glassification, leaving open the possibility for theorists to make further progress. 相似文献
7.
SCF geometry optimizations, using double-zeta basis sets with polarization functions, either as d-orbitals or bond functions, were performed on ONH3, ONF3, and OCF 3 ? . The bonding in these molecules is discussed with the help of orbital density plots and Mulliken population analyses. ONF3 can be explained as donor-acceptor product of O with NF3. For OCF 3 ? , diffuse functions have also been added to the basis set. The CF distance is predicted to be 1.39 Å, in qualitative agreement with the known infrared spectrum. With the best basis set, the NF distance of ONF3 is too short by 0.08 Å at the SCF level. A geometry optimization by the configuration-interaction method gives the NF distance as well as the ON distance in excellent agreement with experimental results. 相似文献
8.
Gary Lawlor 《Mathematical Intelligencer》1998,20(1):29-31
Conclusion Finally, we compare the trianglesT
i with the isosceles triangles which would be obtained by slicing the circle by the same procedure. Since they all have the
same short base length and the same angle opposite the base, the isosceles triangles composing the circle have more area than
the trianglesT
i. Because the latter triangles cover all ofR
1 (perhaps even with overlap and/or extension beyondR
1), we see that the area of the quarter-circle is greater than that ofR
1, and thus the circle's entire area is greater than that of our original regionR. 相似文献
9.
Gary Lawlor 《Inventiones Mathematicae》1989,95(2):437-446
We complete the proof of the angle conjecture on when a pair of orientedm-planes is area-minimizing. The nonzero sum (oriented union)P
1+P
2 is area-minimizing if and only if the characterizing angles between them satisfy the inequality
相似文献
10.
Denise Halverson Gary Lawlor 《Calculus of Variations and Partial Differential Equations》2006,25(2):257-273
An unbounded portion of the catenoid and a class of subsurfaces of Scherk's singly periodic surface defined over nonconvex
domains are shown to be area-minimizing.
Mathematics Subject Classification (1991) Primary 49Q05 Secondary 53A10, 30C45 相似文献
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