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排序方式: 共有163条查询结果,搜索用时 15 毫秒
91.
Mukund K. Gurjar Dandepally Srinivasa Reddy Mohan M. Bhadbhade Rajesh G. Gonnade 《Tetrahedron》2004,60(45):10269-10275
This paper describes an efficient synthetic route for novel bicyclic nucleosides. The stereochemistry of the targeted bicyclic nucleosides was successfully achieved by vinylogous Reformatsky reaction and ring closing metathesis reaction on a carbohydrate backbone. 相似文献
92.
Several (E)- and (Z)-ω-formyl-5-oxa-2-substituted-α,β-unsaturated esters ( 6Za-6Zc; 6Ea-6Ec; 6Eb′-6Ec′; 6Zb′-6Zc′ ) were prepared in high yields. They were subjected to cyclization of their intramolecular ketyl radicals induced by either samarium(II) iodide or tributyltin hydride. These two reagents gave distinct stereoselectivities. More bicylic lactone than monocyclic hydroxy-ester was formed when tributyltin hydride was used, but the opposite stereoselectivity was obtained when samarium(II) iodide was used. The effect of double-bond configuration, heteroatom and substituent on stereoselectivity is described. 相似文献
93.
Sharp bounds of the zeroth-order general Randi? index of bicyclic graphs with given pendent vertices
Let G be a simple connected graph and α be a given real number. The zeroth-order general Randi? index of 0Rα(G) is defined as ∑v∈V(G)[dG(v)]α, where dG(v) denotes the degree of the vertex v of G. In this paper, for any α(≠0,1), we give sharp bounds of the zeroth-order general Randi? index 0Rα of all bicyclic graphs with n vertices and k pendent vertices. 相似文献
94.
Víctor Jiménez López 《代数通讯》2013,41(1):63-76
Hirano studied the quasi-Armendariz property of rings, and then this concept was generalized by some authors, defining quasi-Armendariz property for skew polynomial rings and monoid rings. In this article, we consider unified approach to the quasi-Armendariz property of skew power series rings and skew polynomial rings by considering the quasi-Armendariz condition in mixed extension ring [R; I][x; σ], introducing the concept of so-called (σ, I)-quasi Armendariz ring, where R is an associative ring equipped with an endomorphism σ and I is an σ-stable ideal of R. We study the ring-theoretical properties of (σ, I)-quasi Armendariz rings, and we obtain various necessary or sufficient conditions for a ring to be (σ, I)-quasi Armendariz. Constructing various examples, we classify how the (σ, I)-quasi Armendariz property behaves under various ring extensions. Furthermore, we show that a number of interesting properties of an (σ, I)-quasi Armendariz ring R such as reflexive and quasi-Baer property transfer to its mixed extension ring and vice versa. In this way, we extend the well-known results about quasi-Armendariz property in ordinary polynomial rings and skew polynomial rings for this class of mixed extensions. We pay also a particular attention to quasi-Gaussian rings. 相似文献
95.
Chun Yang Zhendi WangBruce P. Hollebone Carl E. BrownMike Landriault 《Journal of chromatography. A》2009,1216(20):4475-4484
This study presents a quantitative gas chromatography–mass spectrometry analysis of bicyclic sesquiterpanes (BSs) in numerous crude oils and refined petroleum products including light and mid-range distillate fuels, residual fuels, and lubricating oils collected from various sources. Ten commonly recognized bicyclic sesquiterpanes were determined in all the studied crude oils and diesel range fuels with principal dominance of BS3 (C15H28), BS5 (C15H28) and BS10 (C16H30), while they were generally not detected or in trace in light fuel oils like gasoline and kerosene and most lubricating oils. Laboratory distillation of crude oils demonstrated that sesquiterpanes were highly enriched in the medium distillation fractions of ∼180 to 481 °C and were generally absent or very low in the light distillation fraction (boiling point to ∼180 °C) and the heavy residual fraction (>481 °C). The effect of evaporative weathering on a series of diagnostic ratios of sesquiterpanes, n-alkanes, and biomarkers was evaluated with two suites of weathered oil samples. The change of abundance of sesquiterpanes was used to determine the extent of weathering of artificially evaporated crude oils and diesel. In addition to the pentacyclic biomarker C29 and C30 αβ-hopane, C15 and C16 sesquiterpanes might be alternative internal marker compounds to provide a direct way to estimate the depletion of oils, particularly diesels, in oil spill investigations. These findings may offer potential applications for both oil identification and oil-source correlation in cases where the tri- to pentacyclic biomarkers are absent due to refining or environmental weathering of oils. 相似文献
96.
Jeanne Alladoum 《Tetrahedron letters》2009,50(6):617-4405
Diastereoselective aza-annulation of seven-membered β-enaminolactones 2 gives access to bicyclic heterocyles 5. Fragmentation of molecule 5a with lithium methoxide affords cis or trans bicyclic lactams 8 with excellent stereoselectivities. 相似文献
97.
On the Laplacian coefficients of bicyclic graphs 总被引:1,自引:0,他引:1
Let G be a graph of order n and let be the characteristic polynomial of its Laplacian matrix. Generalizing the approach in [D. Stevanovi?, A. Ili?, On the Laplacian coefficients of unicyclic graphs, Linear Algebra and its Applications 430 (2009) 2290-2300.] on graph transformations, we show that among all bicyclic graphs of order n, the kth coefficient ck is smallest when the graph is Bn (obtained from C4 by adding one edge connecting two non-adjacent vertices and adding n−4 pendent vertices attached to the vertex of degree 3). 相似文献
98.
Mariana Pazos Bruno González Leopoldo Suescun Gustavo Seoane Ignacio Carrera 《Tetrahedron letters》2017,58(23):2182-2185
The enantioselective preparation of three protected β-amino-γ-hydroxyesters from benzoic acid is described. The employed synthetic methodology involves the ipso, ortho cis-dihydroxylation of benzoic acid by the mutant strain Ralstonia eutropha B9, followed by a selective halonium induced beta lactamization. Modification of this novel β-lactam structure by the appropriate sequence of reactions allows for the selective preparation of the aforementioned β-amino-γ-hydroxyesters in a diastereodivergent manner. The overall transformation results in a selective formal aminohydroxylation of the diene moiety of the initial cis-cyclohexadienediol. The synthesized products are important building blocks and will allow for the selective preparation of aminoacids, inosamines and alkaloids from benzoic acid. 相似文献
99.
[(HBNPEt3)4NPEt3]3+ – a Bicyclic Penta(phosphorano)borazinium Trication [(HBNPEt3)4NPEt3]Br3 ( 1 ) has been prepared by the reaction of Me3SiNPEt3 with [HBBr2(SMe2)] in dichloromethane solution at 20°C. According to 11B‐NMR and 31P‐NMR spectroscopy and to a crystal structure determination the cation of 1 has the structure of a bicyclic penta(phosphorano)borazinium‐trication. 1 _.3CH2Cl2: Space group P212121, Z = 8, lattice dimensions at –83°C: a = 2124.8(1), b = 2307.6(1), c = 2323,1(1) pm, R1 = 0.0733. 相似文献
100.