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81.
Kai Wu Liang‐Liang Wu Cong‐Ying Zhou Chi‐Ming Che 《Angewandte Chemie (International ed. in English)》2020,59(37):16202-16208
A transition‐metal‐free C(sp2)?C(sp2) bond formation reaction by the cross‐coupling of diazo quinones with catechol boronic esters was developed. With this protocol, a variety of biaryls and alkenyl phenols were obtained in good to high yields under mild conditions. The reaction tolerates various functionalities and is applicable to the derivatization of pharmaceuticals and natural products. The synthetic utility of the method was demonstrated by the short synthesis of multi‐substituted triphenylenes and three bioactive natural products, honokiol, moracin M, and stemofuran A. Mechanistic studies and density functional theory (DFT) calculations revealed that the reaction involves attack of the boronic ester by a singlet quinone carbene followed by a 1,2‐rearrangement through a stepwise mechanism. 相似文献
82.
83.
Che CM Mao Z Miskowski VM Tse MC Chan CK Cheung KK Phillips DL Leung KH 《Angewandte Chemie (International ed. in English)》2000,39(22):4084-4088
84.
L. Chełmicki J. Leciejewicz A. Zygmunt 《Journal of Physics and Chemistry of Solids》1985,46(5):529-538
Neutron diffraction and magnetization measurements indicate that, at low temperatures, long-range magnetic order is present in UCO2Si2, UNi2Si2, UCu2Si2, UNi2Ge2, and UCo2Ge2. UCo2Si2 and UNi2Ge2 are simple collinear antiferromagnets of +-+- type, UCu2Si2 a simple collinear ferromagnet. In UNi2Si2, a magnetic phase transition from a LSW type structure to collinear antiferromagnetism of +-+- type was found, while in UCu2Ge2, the antiferromagnetic structure of ++-- transforms into collinear ferromagnetism. Crystal structure and magnetic parameters are given. No magnetic moment on transition metal ions was found within the accuracy of a powder neutron diffraction experiment. The stability of particular magnetic ordering schemes is discussed in terms of an isotropic RKKY mechanism. 相似文献
85.
Communiols A-D (1-4), new tetrahydrofuran and bis-tetrahydrofuran derivatives, have been isolated from cultures of the coprophilous fungus Podospora communis, and identified by spectroscopic methods. 相似文献
86.
A new 1,4-phenanthraquinone was isolated from the roots of Menispermum dauricum. The structure was elucidated as 7-hydroxy-3,6-dimethoxy-1,4-phenanthraquinone on the basis of spectral evidence. 相似文献
87.
[reaction: see text] A ruthenium porphyrin-catalyzed stereoselective intramolecular carbenoid C[bond]H insertion is described. Using [Ru(II)(TTP)(CO)] as catalyst, aryl tosylhydrazones are converted to 2,3-dihydrobenzofurans, 2,3-dihydroindoles, and beta-lactams in good yields and remarkable cis selectivity (up to 99%). Enantioselective synthesis of 2,3-dihydrobenzofurans is also achieved with [Ru(II)(D(4)-Por*)(CO)] as catalyst, and up to 96% ee is attained. 相似文献
88.
In this paper we study three-color Ramsey numbers. Let K
i,j
denote a complete i by j bipartite graph. We shall show that (i) for any connected graphs G
1, G
2 and G
3, if r(G
1, G
2)≥s(G
3), then r(G
1, G
2, G
3)≥(r(G
1, G
2)−1)(χ(G
3)−1)+s(G
3), where s(G
3) is the chromatic surplus of G
3; (ii) (k+m−2)(n−1)+1≤r(K
1,k
, K
1,m
, K
n
)≤ (k+m−1)(n−1)+1, and if k or m is odd, the second inequality becomes an equality; (iii) for any fixed m≥k≥2, there is a constant c such that r(K
k,m
, K
k,m
, K
n
)≤c(n/logn), and r(C
2m
, C
2m
, K
n
)≤c(n/logn)
m/(m−1)
for sufficiently large n.
Received: July 25, 2000 Final version received: July 30, 2002
RID="*"
ID="*" Partially supported by RGC, Hong Kong; FRG, Hong Kong Baptist University; and by NSFC, the scientific foundations of
education ministry of China, and the foundations of Jiangsu Province
Acknowledgments. The authors are grateful to the referee for his valuable comments.
AMS 2000 MSC: 05C55 相似文献
89.
Stanislaw Dzwigaj Lorenzo Stievano Michel Che 《Journal of Physics and Chemistry of Solids》2007,68(10):1885-1891
Fe-containing SiBEA zeolites were prepared by a two-step postsynthesis method: creation of vacant T-sites by dealumination of tetraethylammonium BEA zeolite with nitric acid and then impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO3)3. X-ray diffraction shows that iron is incorporated in SiBEA at lattice sites. The presence of Fe in its oxidation state +3 and at isolated tetrahedral sites for low metal content, was demonstrated by diffuse reflectance UV-vis, EPR and Mössbauer spectroscopy. For high iron content, diffuse reflectance UV-vis and Mössbauer spectra revealed the additional presence of extra-lattice FeOx oligomers and superparamagnetic Fe-oxyhydroxide. Mössbauer spectroscopy identified superparamagnetic Fe-oxyhydroxide as the main phase when basic conditions are used for the preparation. 相似文献