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981.
982.
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Diastereoselective hydrogenation of homochiral fumaramides 1 derived from (2R)-Oppolzer’s sultam was observed by analysis of the 1H NMR spectra of the succinamide mixtures with de’s of 77–88%. Reduction of these succinamides using LiAlH4 gave the corresponding (2S)-butane-1,4-diols and established that addition of hydrogen takes place selectively on the re-face of fumaramides 1. The stereoselectivity was confirmed by estimating the ee’s from the 19F NMR spectra of the Mosher’s diesters of the diols. This methodology was applied to the synthesis of selected pyrrolidine natural products in homochiral form. 相似文献
985.
Let V be an n-dimensional vector space over Fq. Let Φ be a Hermitian form with respect to an automorphism σ with σ2 = 1. If σ = 1 assume that q is odd. Let be the arrangement of hyperplanes of V which are non-isotropic with respect to Φ, and let L be the intersection lattice of . We prove that the characteristic polynomial of L has n ? v roots 1, q,…, qn ? v? 1 where v is the Witt index of Φ. 相似文献
986.
L. T. Pearson Pierre E. J. Louis Robert G. Gilbert Donald H. Napper 《Journal of polymer science. Part A, Polymer chemistry》1991,29(4):515-523
The effects of 9-vinyl anthracene (VAn) on solution and emulsion polymerization of styrene are examined, to gain information about the mechanism of particle formation in emulsion polymerizations. Styrene solution polymerization in ethyl benzene is found to be inhibited by small amounts of VAn. In an emulsion system, the effects of VAn are found to depend on surfactant concentration [S]. With [S] = 0, addition of VAn engenders some inhibition and very little change in the steady-state polymerization rate. As [S] is increased from zero to above the critical micelle concentration (cmc), a slight increases in the induction period and an increasing retardation (reduction in the steady-state rate) are observed. Below the cmc, VAn has no significant effect on the particle number, but above the cmc it considerably increases the latter with concomitant appearance of very small (5–15 nm radius) particles. The data strongly support the supposition that the coagulative/homogeneous nucleation mechanism dominates particle formation for [S] below the cmc. Above the cmc, the data are compatible with both the micellar entry and coagulative/homogeneous mechanisms. 相似文献
987.
All the 1H n.m.r. parameters of the following derivatives of 1,6-dimethylbicyclo[4.1.0]heptane are determined: 3,4-dimethoxycarbonyl (3 diastereoisomers), cis- and trans-3-methoxycarbonyl-2,2,5,5-d4, cis- and trans-3-methyl-3-methoxycarbonyl; the cis- and trans-1,5,5-trimethylbicyclo[4.1.0]-3-heptanols are studied in the same way. The different chemical shifts are correlated with the aid of a collection of empiral increments. The conformational equilibria are determined from the vicinal coupling constants; the conformational free energies of the COOCH3 group are evaluated; the part played by gauche interactions is considered. 相似文献
988.
Chu Louis W. M. Lee Kai Fong 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1978,6(3):329-337
An investigation is made on the conditions for validity of the analytical results for the resistive electromagnetic ion cyclotron instability discussed in a recent paper by Lee and Luhmann. The regions of validity in wavenumber space are determined numerically for various conbinations of plasma density, magnetic field, and electron drift velocity. It is concluded that the analytical results are good approximations over a large region of wavenumber space. 相似文献
989.
990.