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2.
A subset S of the vertex set of a graph G is called acyclic if the subgraph it induces in G contains no cycles. S is called an acyclic dominating set of G if it is both acyclic and dominating. The minimum cardinality of an acyclic dominating set, denoted by γa(G), is called the acyclic domination number of G. Hedetniemi et al. [Acyclic domination, Discrete Math. 222 (2000) 151-165] introduced the concept of acyclic domination and posed the following open problem: if δ(G) is the minimum degree of G, is γa(G)?δ(G) for any graph whose diameter is two? In this paper, we provide a negative answer to this question by showing that for any positive k, there is a graph G with diameter two such that γa(G)-δ(G)?k. 相似文献
3.
Acyclic nitrones react with dimethyl acetylenedicarboxylate (DMAD) to give stable isoxazolines, from which the ones that contain electron-donating aromatic rings at the C3 position (R1) were shown to undergo unprecedented fragmentation at room temperature, giving the R1-aldehyde and inseparable product mixtures, probably due to the formation of highly reactive species such as iminocarbenes. Attempts to convert the isoxazolines to the corresponding stable azomethine ylides, by refluxing in toluene, again led to the same product mixtures as above (e.g., the room temperature decomposition). Isoxazolines when reacted with methoxide at room temperature afforded highly functionalised diastereomeric mixtures. Also, isoxazolines, when reacted with propylamine, gave the corresponding amides regioselectively, all of which were more stable than the parent isoxazolines. 相似文献
4.
Mariola Koszytkowska-Stawińska 《Tetrahedron letters》2004,45(28):5437-5440
N-Pivaloyloxymethyl amides and sulfonamides, readily available from N-alkylation of both amides and sulfonamides with commercial chloromethyl pivaloate, were converted into acyclic azanucleosides via a one-pot base silylation/nucleoside coupling procedure. 相似文献
5.
用邻硝基苯酚经过先桥接,还原,然后使生成的重氮盐分别与苯酚,取代苯酚和取代苯胺偶联的方法,合成了两类新型的双偶氮基于链有色冠醚,方法简便,产率高,在一定的酸性介质中,这类冠醚存在着铵醌互交异构。测定了它们在不同介质中的紫外可见光谱,并对最大吸收峰进行归属。 相似文献
6.
Madhuri Manpadi 《Tetrahedron letters》2005,46(26):4433-4437
Various substituted arylcuprates undergo stereocontrolled additions to a d-mannitol-derived γ-alkoxy-α,β-enoate with exclusive anti-selectivity. The method is well suited for the preparation of a broad range of biologically active Amaryllidaceae alkaloids and their aromatic analogues. A model accounting for the stereochemical outcome of this process is presented. 相似文献
7.
Summary The new concept of the resonance energy in conjugated hydrocarbons introduced by Jiang Y, Zhang H (1989) Theor Chim Acta 75:279 is further developed. This model is based on expansion of the -electron energy in terms of moments which are also equal to numbers of closed walks in a molecular graph. The reference system is established by counting only acyclic walks, i.e. those tracing only on acyclic subgraphs. Because acyclic walks could be counted only up to some finite length, the energy of the reference system has been evaluated by truncating higher terms in the expansion. In this paper a finite expression for the energy of the same reference system is derived, thus allowing its exact evaluation. The exact values differ significantly from the truncated ones. This difference, as well as the discrepancy between exact results and chemical experience, are discussed. 相似文献
8.
《Journal of Coordination Chemistry》2012,65(7):1157-1164
The amide-type acyclic polyether, 1,2-ethylenedioxy-bis(N-pyridin-2-ylmethyl-benzamide) (L) was synthesized and characterized by EA, IR and 1H NMR spectroscopy. The reaction of yttrium nitrate with L produced a rare chain structural coordination polymer {[Y(NO3)3L(H2O)]?·?(H2O)} n (1) by use of the flexible ligand L as the building blocks. The structure of 1 was determined by X-ray crystallography and thermal analysis. In 1, L, as a bidentate bridging spacer, binds two neighboring Y atoms to form a one-dimensional chain polymer. The chain polymer was further connected into the three-dimensional network by hydrogen bonding interactions. 相似文献
9.
Caffeine (1,3,7-trimethylxanthine) is a chemical substance associated with everyday human life. In order to recognize caffeine in water, six water-soluble acyclic phane compounds composed of three aromatic rings were examined as artificial receptors. 1H NMR titration experiments revealed that 6,6′-[1,3-phenylenebis(carbonylimino)]bis-1,3-naphthalenedisulfonate had the highest binding ability for caffeine, with a binding constant (Kb) of 127±5 M−1 at 300 K. While this phane compound also formed a complex with theophylline (1,3-dimethylxanthine) at around half the value of the binding constant for caffeine (Kb=64±4 M−1), it showed weak or little complexation for adenosine, guanosine, inosine, and their 5′-phosphates (sodium salts of adenylic acid, guanylic acid, and inosinic acid). 相似文献
10.
The Kováts retention indices of all 93 acyclic octenes on polydimethylsiloxane and squalane as stationary phases as well as their mass spectra were measured. The means of gas chromatography-mass spectrometry (GC-MS) were used for confirmation of GC identification as well as for mass spectrometric deconvolution of the majority of gas chromatographic unseparated isomeric octene peaks. The distinction between corresponding E and Z acyclic octenes, that is either difficult or even impossible by means of GC-MS, was obtained on the basis of larger temperature coefficients of retention indices for Z isomeric octenes than for corresponding E isomers. The retention data expressed as homomorphy factors were correlated with the degree of branching, position of double bond, and position of alkyl group with respect to the double bond of acyclic octenes, and the structure-retention relationships were formulated. The 81 acyclic octenes were identified in FCC gasoline. 相似文献