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941.
942.
Martín-Zamora E Ferrete A Llera JM Muñoz JM Pappalardo RR Fernández R Lassaletta JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(23):6111-6129
Staudinger-like cycloadditions between chiral, non-racemic N,N-dialkylhydrazones 1 and functionalized ketenes constitute an efficient methodology for the stereoselective construction of the beta-lactam ring. The potential for fine tuning of the dialkylamino auxiliary structure, the availability of a high-yielding deprotection method for the release of the free azetidinones, and the high thermal and chemical stability of hydrazones as N-dialkylamino imines are highlighted as the key elements for the success of the strategy. This last aspect is of particular importance concerning generality: even hydrazones from easily enolizable aldehydes or from formaldehyde reacted to afford the corresponding cycloadducts with high chemical and stereochemical yields. The syntheses of the beta-amino-alpha-hydroxyacids (2R,3S)-phenylisoserine (42) and (2R,3S)-norstatin (45) were accomplished as illustrative examples of the synthetic utility of this procedure. A model system for the cycloaddition of g series auxiliaries was studied by ab initio computational methods. The collected results support a two-step mechanism through zwitterionic intermediates, and explain the observed absolute and relative stereochemistry in terms of the preferred outward cycloaddition to the Re face of the hydrazone. 相似文献
943.
Callaghan PL Fernández-Pacheco R Jasim N Lachaize S Marder TB Perutz RN Rivalta E Sabo-Etienne S 《Chemical communications (Cambridge, England)》2004,(2):242-243
Metal phosphine hydrides of type RuP(4)H(2) and RhP(3)H(3) react photochemically with HB(pin)(pin = pinacolate) to form metal boryl hydride complexes via 16-electron intermediates generated by H(2) loss; the second order rate constants for reaction of the intermediates with HB(pin) are even larger than those for reaction with Et(3)SiH. 相似文献
944.
945.
946.
Claudio?FernándezEmail author Georgi. D.?RaikovEmail author 《Annales Henri Poincare》2004,5(2):381-403
We consider the three-dimensional Schrödinger operators
and
where
, A is a magnetic potential generating a constant magnetic
field of strength
, and
where
decays fast enough at infinity. Then, A. Pushnitskis representation of the spectral shift function (SSF)
for the pair of operators
is well defined for energies
We study the behaviour of the associated representative of the equivalence class
determined by the SSF, in a neighbourhood of the Landau levels
Reducing our analysis to the study of the eigenvalue asymptotics for a family of
compact operators of Toeplitz type, we establish a relation between the type of the
singularities of the SSF at the Landau levels and the decay rate of V at infinity.
Communicated by Bernard HelfferSubmitted 23/09/03, accepted 15/01/04 相似文献
947.
We show that water constrained by vicinal hydrophobes undergoes a librational dynamics that lowers the dielectric susceptibility and induces a "redshift" of the relaxation frequency in the hydration shell. The results shed light on the way proteins enhance their intramolecular interactions as they fold or associate. 相似文献
948.
[reaction: see text] Lindlar semihydrogenation of a vitamin D type trienyne leads spontaneously to 9 alpha,19-methano-1 alpha,25-dihydroxyvitamin D3. The intermediate tetraene resulting from the reduction undergoes a rapid, stereoselective 8pi electron electrocyclization affording a novel steroid containing a linearly fused ABC (six-eight-six) 1,3,5-cyclooctatriene carbon framework. 相似文献
949.
Fernández M Padrón C Marconi L Ghini S Colombo R Sabatini AG Girotti S 《Journal of chromatography. A》2001,907(1-2):257-265
The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate as surfactant. The variation in MEKC surfactant composition results in changes in the coefficients of the correlation equation, which in turns leads to information on solute-solvent and solute-micelle interactions. Lithium perfluorooctanesulfonate is more dipolar and hydrogen bond acidic but less polarizable and hydrogen bond basic than lithium dodecyl sulfate. Therefore mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate cover a very wide range of polarity and hydrogen bond properties, which in turn results in important selectivity changes for analytes with different solute properties. 相似文献
950.
We have performed Monte Carlo simulations of interacting dipoles that relax through quantum tunneling. We aim to mimic tunneling experiments on crystals of magnetic clusters, such as Fe(8), at very low temperatures. Accordingly, we allow spin flips only if the corresponding energy change is less than some 2 delta h(hf). Time evolutions of the dipolar field distribution P(t)(H) are studied. As in experiments, a "hole" develops in P(t)(H). The half-width W of incipient holes of weakly polarized systems are, under certain conditions, simply related to delta h(hf). For k(B)T less than approximately 0.5 delta h(hf) and delta h(hf) smaller than approximately 1/10 of the half-width of the dipolar field distributions of disordered systems, W approximately equal to 0.75 delta h(hf). 相似文献