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491.
Kiyooka S Shahid KA Goto F Okazaki M Shuto Y 《The Journal of organic chemistry》2003,68(21):7967-7978
The segments C(1)-C(13) and C(15)-C(21) containing the 13 stereogenic centers required for the frame of (+)-discodermolide were synthesized in good to excellent enantio- and diastereoselectivities from a common racemic aldehyde, derived from 2-methyl-1,3-propanediol. The enantioselective aldol reactions of the racemic aldehyde with a silylketene acetal, derived from ethyl 2-bromopropionate, in the presence of chiral oxazaborolidinones, prepared in situ with N-p-toluenesulfonyl-(R)- and -(S)-valine and BH(3).THF, proceeded under kinetic control to give the stereotriads with a high degree of enantioselectivity. Enantioselective (chiral borane) and diastereoselective (BF(3).OEt(2) and TiCl(4)) aldol reactions with the silylketene acetal, coupled with diastereoselective radical debrominations (Bu(3)SnH, Et(3)B, with or without MgBr(2)), were used iteratively. This aldol reaction strategy for the construction of the polypropionate frame dramatically shortened the steps needed for the construction of the final segments. 相似文献
492.
AHMAD H. Badaruddin ARFAN Muhamma RAMA N. Hasan MALANA M. Aslam MAHBOOB Shahid 《中国化学》2007,25(1):102-104
6,8-Dimethoxy-3-[2-(4-methoxyphenyl)ethyl]isocoumarin was synthesized by condensation of 5,7-dimethoxyhompophthalic acid with 3-(4-methoxyphenyl)propanoyl chloride. The structure of the synthesized compound was confirmed by its mass spectrometric studies. The synthesized compound serves as a model for synthesis of DL-agrimonolide. 相似文献
493.
Shahid Anwar 《Journal of solid state chemistry》2008,181(5):997-1004
The structural variants and their coexistence across the antiferroelectric phase transition in Sr0.60Ca0.40TiO3 ceramic has been studied through transmission electron microscopy (TEM) at room temperature and ∼100 °C. A clear evidence of the presence of superlattice reflections, corresponding to the cell doubling along the c-axis of Pbnm (or b-axis along Pnma), occurring during paraelectric to antiferroelectric transition, has been obtained through selected area electron diffraction, convergent beam electron diffraction and lattice-resolution imaging. Coexistence of the Pbnm and Pbcm phases at room temperature has been observed and attributed to the strain/disorder-induced broadening of the first-order antiferroelectric phase transition. Drastic changes in the domain structure during Pbnm to Pbcm transformation have been observed. This clearly indicates that the antiferrodistortive transition responsible for the occurrence of the antiferroelectric phase is of completely different origin and it is not just an additional follow-up of the already-existing ordering due to a−a−c+ tilt schemes in the Pbnm domain. Thermal cycling studies on microstructural changes indicate some kind of memory mechanism, which retains the memory of the original a−a−c+ tilt schemes in the Pbnm phase. This has been attributed to the symmetry conforming short-range order (SC-SRO) of the point defects. 相似文献
494.
Shahid Adeel Khalid Mahmood Zia Muhammad Abdullah Mahwish Salman Mohammad Zuber 《Natural product research》2019,33(14):2060-2072
Natural products with therapeutic nature are nowadays warmly welcomed in textiles. Current study is concerned with isolation of tannin from neem bark (Azadirachta indica) and its application onto silk fabric. For improvement in shades, chemical & bio-mordants have been employed at optimal dyeing conditions. It is found that methanolic extract of pH 5 obtained 8 g dye powder after US Treatment for 30 min has given high color strength if used to dye irradiated silk for 65 min at 75 °C. Bio-mordanting using herbal plants has improved the color characteristics as compared to chemical mordants used. ISO standards for color fastness reveal that herbal based bio-mordants have improved the rating from good to excellent as compared to chemical mordants used. It is concluded that US treatment has not only isolate the colorant for neem bark under mild condition but also given excellent characteristic using bio mordants onto silk fabrics. 相似文献
495.
This article reports the homotopy solution for stagnation point flow of a non-Newtonian fluid.An incompressible second grade fluid impinges on the wall either orthogonally or obliquely.The resulting nonlinear problems have been solved by a homotopy analysis method(HAM).Convergence of the series solutions is checked.Such solutions are compared with the numerical solutions presented in a study [Int.J.Non-Linear Mech.43(2008) 941].Excellent agreement is noted between the numerical and series solutions. 相似文献
496.
Roshan Ahmad Mohammad Zia-ul-Haq Shahid Hameed Humaira Akhtar Helmut Duddeck 《Monatshefte für Chemie / Chemical Monthly》2000,131(4):393-400
Summary. A convenient procedure is reported for the preparation of benzofuro-annelated 2-phenyl-1,5-benzothiazepine derivatives by
oxidative cyclocondensation of phenolic β-diketones with o-aminothiophenol in DMSO. The regiochemistry of these compounds is proven by HMBC signals and the existence of a five-bond 19F,13C-2 coupling. Surprisingly, treatment with LiAlH4 at room temperature led to a double reduction under opening of the five-membered ring. Refluxing such solutions with a higher
amount of LiAlH4 gave rise to a further reduced derivative possessing the trans-configuration. All structures (regio- and stereochemistry) were assigned on the basis of NMR spectroscopic data.
Received November 22, 1999. Accepted December 21, 1999 相似文献
497.
Using ladder operators for the non-linear oscillator with position-dependent effective mass, realization of the dynamic group SU(1,1) is presented. Keeping in view the algebraic structure of the non-linear oscillator, coherent states are constructed using Barut-Girardello formalism and their basic properties are discussed. Furthermore, the statistical properties of these states are investigated by means of Mandel parameter and second order correlation function. Moreover, it is shown that in the harmonic limit, all the results obtained for the non-linear oscillator with spatially varying mass reduce to corresponding results of the linear oscillator with constant mass. 相似文献
498.
Total synthesis and selected biological activities of naturally occurring homalicine and (dl)-dihydrohomalicine are described. 相似文献
499.
500.
Studies undertaken on the fresh leaves of Carissa carandas have led to the isolation of four pentacyclic triterpenoids (1-4) including one new constituent carissin (1) and two hitherto unreported compounds 2 and 3. The structure of the new triterpenoid has been elucidated as 3beta-hydroxy-27-E-feruloyloxyurs-12-en-28-oic acid. Compelete assignment of the protons of 2 has also been made based on 2D NMR studies. 相似文献