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151.
Michael-Addition Reaction of Malononitrile with a,β-Unsaturated Cycloketones Catalyzed by KF/Al_2O_3
The utility of fluoride salts as potential base in a va-riety of synthetic reactions has been recognized in re-cent years.1 However, low solubility of fluoride salts in ordinary solvents hampers their wide applications in organic synthesis. On the other hand, there has been increasing use of inorganic solid supports as catalysts for many years2 resulting in higher selectivity, milder reaction conditions and easier work-up. Especially po-tassium fluoride-coated alumina (KF-alumina) has been a … 相似文献
152.
Many natural products contain carbohydrate moieties that contribute to their biological activity. Manipulation of the carbohydrate domain of natural products through multiple glycosylations to identify new derivatives with novel biological activities has been a difficult and impractical process. We report a practical one‐pot enzymatic approach with regeneration of cosubstrates to synthesize analogues of vancomycin that contain an N‐alkyl glucosamine, which exhibited marked improvement in antibiotic activity against a vancomycin‐resistant strain of Enterococcus. 相似文献
153.
A new synthetic method for the manufacture of glutaric dialdehyde is investigated. Glutaric dialdehyde was prepared by the
addition-hydrolysis reaction of benzimidazolium salt with saturated dihalide as the di-Grignard reagent. The yield of glutaric
dialdehyde by this method can reach 73%. Both infrared spectra and melting point of the compound were consistent with those
reported earlier.
Translated from Journal of Northwest University (Natural Science Edition), 2005, 35(2) (in Chinese) 相似文献
154.
The peeling of a d-dimensional set of points is usually performed with successive calls to a convex hull algorithm; the optimal worst-case convex hull algorithm, known to have an O(n˙ Log (n)) execution time, may give an O(n˙n˙ Log (n)) to peel all the set; an O(n˙n) convex hull algorithm, m being the number of extremal points, is shown to peel every set with an O(n-n) time, and proved to be optimal; an implementation of this algorithm is given for planar sets and spatial sets, but the latter give only an approximate O(n˙n) performance. 相似文献
155.
Victor Pambuccian 《Mathematical Logic Quarterly》1992,38(1):345-348
We proved in the first part [1] that plane geometry over Pythagorean fields is axiomatizable by quantifier-free axioms in a language with three individual constants, one binary and three ternary operation symbols. In this paper we prove that two of these operation symbols are superfluous. 相似文献
156.
We consider two problems: given a collection of n fat objects in a fixed dimension, (1) ( packing) find the maximum subcollection of pairwise disjoint objects, and (2) ( piercing) find the minimum point set that intersects every object. Recently, Erlebach, Jansen, and Seidel gave a polynomial-time approximation scheme (PTAS) for the packing problem, based on a shifted hierarchical subdivision method. Using shifted quadtrees, we describe a similar algorithm for packing but with a smaller time bound. Erlebach et al.'s algorithm requires polynomial space. We describe a different algorithm, based on geometric separators, that requires only linear space. This algorithm can also be applied to piercing, yielding the first PTAS for that problem. 相似文献
157.
Denis Prat Françoise Benedetti Lahlou Nait Bouda Gilles Franc Girard 《Tetrahedron letters》2004,45(4):765-768
An industrial synthesis of 11β-aryl-estrone derivatives is described, based on the 1,4-addition of the aryl side-chain, as a cuprate, on to a mixture of allylic 5(10) alpha and beta epoxides, followed by hydrolysis and subsequent aromatization. 相似文献
158.
LIU Hui-Min ZHANG Wei XIN Ying ZHANG Wen-Qin② 《结构化学》2004,23(5):532-536
1 INTRODUCTION We are presently involved in the synthesis and photodimerization of the analogues of trans-2-sty- rylbenzoxazole (BOEP), which have been widely reported as fluorescent whiteners and remedies[1]. Some structures of BOEP derivatives and thei… 相似文献
159.
Michelina Soccio Lara Finelli Nadia Lotti Valentina Siracusa Tiberio A. Ezquerra Andrea Munari 《Journal of Polymer Science.Polymer Physics》2007,45(13):1694-1703
Poly(butylene naphthalate) (PBN), poly(diethylene naphthalate) (PDEN), and poly(thiodiethylene naphthalate) (PTDEN) were synthesized and characterized in terms of chemical structure and molecular weight. The polyesters were examined by TGA, DSC, and DMTA. All the polymers showed a good thermal stability, even though depending on chemical structure. At room temperature they appeared as semicrystalline materials; the effect of the introduction along the PBN polymer chain of ether oxygen atoms or sulfur ones was a lowering in the Tg value, a decrement of Tm, and a decrease of the crystallization rate. Changing in chemical structure also affects the main α absorption associated with the glass transition which moves to lower temperature and whose energetic requirements decrease. The results were explained as due to the presence of highly flexible C? S? C or C? O? C bonds in the polymeric chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1694–1703, 2007 相似文献
160.
Elvira Vidović Doris Klee Hartwig Höcker 《Journal of polymer science. Part A, Polymer chemistry》2007,45(19):4536-4544
Poly(D ,L ‐lactide) and poly(D ,L ‐lactide‐co‐glycolide) with various composition and with one methacrylate and one carboxylate end group were synthesized and grafted onto poly(vinyl alcohol) (PVA) via the carboxylate group. The graft copolymers were crosslinked via the methacrylate groups using a free radical initiator. The polymer networks were characterized by means of NMR and studied qualitatively by means of IR spectroscopy. The influence of the glycolide content in the polyester grafts and of the number of ester units in the grafts on thermal properties and swellability were studied as well. The high swellability in water is characteristic of all hydrogels. Differential scanning calorimetry (DSC) showed a single glass transition temperature that occurs in the range between 51 and 69 °C. Thermogravimetric analysis (TGA) of the networks showed the main loss in weight in the temperature range between 290 and 370 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4536–4544, 2007 相似文献