Asymmetric aza Diels-Alder reactions of amino acid ester imines with Brassard's diene

Imines 1 obtained from aromatic, aliphatic or functionalized aldehydes and valine tert-butyl ester undergo Lewis acid catalyzed hetero Diels-Alder reactions with Brassard's diene 2. The cycloadducts are formed in good to high yields and with diastereomer ratios of 92:8 – 97:3. For the removal of the chiral auxiliary group a new method was developed whose principle consists in the conversion of the amino acid -C-atom into an acetalic center employing a Curtius rearrangement as the key step.     

Optimal gauge in approximate calculations of molecular magnetic properties

A variational procedure is suggested for determining optimal gauge parameters (such as gauge origin etc.) in calculations of molecular magnetic properties with a finite basis set.     

Nonlinear progressive waves in elastic materials

Summary Progressive harmonic waves in nonlinear elastic materials are generally considered as small disturbances superposed on a finite deformation. In this first approximation they are governed by essentially the same laws as in linear elasticity. In the present paper a second-order theory is developed which allows for nonlinear effects on progressive waves in a finitely deformed elastic material. The problem is investigated by means of a perturbation procedure using intrinsically two time scales. After providing the kinematical prerequisites a transport equation is derived which governs the distortion of the wave profile. A closedform solution is obtained for the case of a plane wave in a homogeneous medium. The influence of nonlinearity is closely related to the evolution of acceleration waves.

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Zusammenfassung Fortschreitende harmonische Wellen in nichtlinearen elastischen Stoffen werden meist als kleine Störungen betrachtet, die einer endlichen Verformung überlagert sind. In dieser ersten Näherung genügen sie im wesentlichen den gleichen Gesetzen wie in der linearen Elastizitätstheorie. In der vorliegenden Arbeit wird eine Theorie zweiter Ordnung entwickelt, die auch nichtlineare Effekte bei fortschreitenden Wellen in einem endlich verformten elastischen Material berücksichtigt. Das Problem wird mit Hilfe einer Störungsmethode behandelt, welche implizit zwei verschiedene Zeitmaßstäbe zuläßt. Nach Bereitstellung der kinematischen Hilfsmittel wird eine Transportgleichung hergeleitet, welche die Verzerrung des Wellenprofils beschreibt. Für ebene Wellen in einem homogenen Medium wird eine geschlossene Lösung angegeben. Der Einfluß der Nichtlinearität zeigt eine enge Verwandtschaft zur zeitlichen Entwicklung von Beschleunigungswellen.

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With 1 figure     

Darstellung,Isolierung und Charakterisierung gemischter Selenocyanatoäthylendiamin-Chrom(III)-Komplexe

Preparation, Isolation, and Characterization of Mixed Selenocyanatoethylenediamine Chromium(III) Complexes cis-[Cr(NCSe)₄(en)]^? and cis-[Cr(NCSe)₂(en)₂]⁺ are prepared for the first time. They are separated by column chromatography on alumina and characterized by the chromium content and the spectra.     

Quantitative differential thermal analysis of the reduction of uranyl fluoride

The enthalpy of the reduction of UO₂F₂ with hydrogen was obtained from quantitative DTA measurements with a linear heating rate and under isothermal conditions, and the thermodynamic data on UO₂F, formed as a stable intermediate in the reduction of UO₂F₂ to UO₂, are also presented. The advantages of isothermal DTA in the reduction of U₃O₈ to UO₂ could be demonstrated.