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排序方式: 共有71条查询结果,搜索用时 15 毫秒
1.
Hui‐yi Lin Chung‐Hsiung Chen KarinC.S. ChenLiu Shoei‐Sheng Lee 《Helvetica chimica acta》2003,86(1):127-138
Six 14‐membered cyclopeptide alkaloids, i.e., ramosines A–C, mucronine J, and lotusines A and D, were isolated from the roots of Paliurus ramosissimus, and an additional four, hemsines A–D, from the roots of P. hemsleyanus. Among these, ramosines A–C ( 1, 5 , and 6 , resp.) and hemsines A and B ( 7 and 8 , resp.) are new bases of the amphibine‐B type, and hemsines C and D ( 9 and 10 , resp.) are new integerrine‐type alkaloids. Additionally, ramosine C ( 6 ) represents the first 14‐membered cyclopeptide alkaloid possessing a substitution (? OH) at C(13′). Their structural elucidations were based on spectral analysis and molecular‐modeling studies. Pronounced solvent effects in the 1H‐ and 13C‐NMR spectra of these two types of alkaloids were observed. 相似文献
2.
3.
4.
Susumu Tajima Yukiyoshi Nagai Osamu Sekiguchi Masao Fujishige Nozomu Uchida 《Journal of the American Society for Mass Spectrometry》1995,6(3):202-206
The spontaneous unimolecular dissociation reaction of methyl lactate (1) ionized by electron impact was investigated by a combination of mass-analyzed ion kinetic energy spectrometry and deuterium labeling. The metastable ions 1+· decompose in a variety of ways: four fragment peaks are observed at m/z 89, 76, 61, and 45, which correspond to the losses of ?H3, CO, CH3?O, and ?OOCH3, respectively. Double hydrogen atom transfer occurs in the third reaction. The source-generated m/z 61 ions decompose into oxygen-protonated methanols at m/z 33 ([CH3OH 2 + ]) by the loss of CO with double hydrogen atom migration. Both hydroxyl and methyne hydrogen atoms in 1 +· are present in the resultant protonated methanols. 相似文献
5.
A dammarane derivative dihydrojujubogenin-2-en-1-one, which possesses a skeleton and pharmacophore partially similar to those of Territrem B, a potent AChE inhibitor, was synthesized via three different paths. The anti-AChE activity of this compound and related analogue was measured. 相似文献
6.
We show a descent method for submodular function minimization based on an oracle for membership in base polyhedra. We assume
that for any submodular function f: ?→R on a distributive lattice ?⊆2
V
with ?,V∈? and f(?)=0 and for any vector x∈R
V
where V is a finite nonempty set, the membership oracle answers whether x belongs to the base polyhedron associated with f and that if the answer is NO, it also gives us a set Z∈? such that x(Z)>f(Z). Given a submodular function f, by invoking the membership oracle O(|V|2) times, the descent method finds a sequence of subsets Z
1,Z
2,···,Z
k
of V such that f(Z
1)>f(Z
2)>···>f(Z
k
)=min{f(Y) | Y∈?}, where k is O(|V|2). The method furnishes an alternative framework for submodular function minimization if combined with possible efficient
membership algorithms.
Received: September 9, 2001 / Accepted: October 15, 2001?Published online December 6, 2001 相似文献
7.
Epicatechin ( 3 ) and five oligomers ( 2 , 4 , 5 , 10 , 11 ) together with five other type compounds were isolated and characterized from n‐BuOH and EtOAc soluble fractions of the ethanol extract of the leaves of Machilus konishii Hayata. Their structures were elucidated by means of MS and NMR spectroscopic analyses. Dehydrodiepicatechin A ( 11 ) is the first occurrence as a natural product although it has been prepared from epicatechin. The stereochemistry of 11 is elucidated for the first time on the basis of 1D NOESY and chemical model analyses. Tiliroside ( 7 ) and kaempferol 3‐O‐α‐L‐(2″,4″‐di‐E‐p‐coumaroyl)‐rhamnopyranoside ( 8 ) showed moderate inhibitory activity against α‐glucosidase. 相似文献
8.
Ki Chul Park Masatsugu Fujishige Susumu Arai Morinobu Endo 《Journal of Physics and Chemistry of Solids》2008,69(10):2481-2486
High-speed agitation by a mixing blade has efficiently achieved the cutting of a large diameter (100-150 nm) of multi-walled carbon nanotubes. The cutting process is caused by an inter-collision of the nanotubes with high transfer energy. The collision-induced cutting allows for the shortening of the nanotubes without serious damage of the original graphitic layers due to the cutting effect being limited to the collision points. Furthermore, the operation under ambient atmosphere introduces oxygen-containing functional groups to the cut nanotubes. The estimated length distribution has indicated that high-speed agitation achieves a large cutting effect during a short duration of several minutes. 相似文献
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10.
Bisubmodular functions are a natural “directed”, or “signed”, extension of submodular functions with several applications.
Recently Fujishige and Iwata showed how to extend the Iwata, Fleischer, and Fujishige (IFF) algorithm for submodular function
minimization (SFM) to bisubmodular function minimization (BSFM). However, they were able to extend only the weakly polynomial
version of IFF to BSFM. Here we investigate the difficulty that prevented them from also extending the strongly polynomial
version of IFF to BSFM, and we show a way around the difficulty. This new method gives a somewhat simpler strongly polynomial
SFM algorithm, as well as the first combinatorial strongly polynomial algorithm for BSFM. This further leads to extending
Iwata’s fully combinatorial version of IFF to BSFM.
The research of S. T. McCormick was supported by an NSERC Operating Grant.
The research of S. Fujishige was supported by a Grant-in-Aid of the Ministry of Education, Culture, Science and Technology
of Japan. 相似文献