Zur Berechnung des Polykondensationsgleichgewichtes der KieselsÄuren

Zusammenfassung Die in einer früheren Arbeit gemachte Annahme, da\ ein echtes thermodynamisches Gleichgewicht zwischen den einzelnen Kondensationsstufen der Kiesels?ure existiert, wird im Hinblick auf die theoretischen Konsequenzen für die insgesamt im Gleichgewicht in L?sung zu findende Kiesels?uremenge untersucht. Es wird für p_H < 12 eine Formel für die „Gesamtl?slichkeit der Kiesels?ure“ entwickelt, die unter plausiblen Annahmen gut mit bisher vorliegenden experimentellen Erfahrungen in übereinstimmung gebracht werden kann.     

4-Aminobicyclo[2.2.2]octanone Derivatives with Antiplasmodial and Antitrypanosomal Activities

Summary. 4-Aminobicyclo[2.2.2]octanones were converted to their N-oxides and to 4-aminobicyclo[2.2.2]octanes. Furthermore, the 6,7-bis-(4-methoxyphenyl) analogues were synthesized. All products were screened for their activities against Trypanosoma b. rhodesiense and Plasmodium falciparum. The pharmacological results were compared with those of formerly tested bicyclo[2.2.2]octanones and bicyclo[2.2.2]octanols. Structure-activity relationships are discussed.Received April 8, 2003; accepted April 14, 2003 Published online September 25, 2003     

4-Aminobicyclo[2.2.2]octanone Derivatives with Antiprotozoal Activities

Summary.  4-Aminobicyclo[2.2.2]octanones which were prepared in one-pot-reactions from benzylidene acetone and dialkylammonium rhodanides were reduced stereoselectively to their corresponding alcohols. The activities of the bicyclic compounds against causative organisms of tropical deseases were examined. The 4-aminobicyclo[2.2.2]octan-2-ols were in general more active against Trypanosoma b. rhodesiense and Plasmodium falciparum than the corresponding keto compounds. Corresponding author. E-mail: robert.weis@uni-graz.at Received November 27, 2002; accepted December 2, 2002 Published online May 2, 2003     

Intramolecular easily polarizable charged and non-charged hydrogen bonds: IR continua and hydrogen bond length

Substances with intramolecular, easily polarizable hydrogen bonds of different length are compared. Long hydrogen bonds cause IR continua which show band-like structures in the region 2800–1800 cm^?1, extending with weaker intensity toward smaller wave-numbers over the whole region studied. Medium length bonds cause continua beginning at 3200 cm^?1 and extending over the whole region studied. Very short, easily polarizable hydrogen bonds, such as the bond in the HAuCl₄ salt of NN'-tetramethyl-o-xylildiamine di-N-oxide, cause continua with very great intensity in the region 1500–850 cm^?1. Comparison of the experimental with calculated continua shows very good agreement with regard to wavenumber regions in which easily polarizable hydrogen bonds of different length cause IR continua.By comparing all these intramolecular, easily polarizable hydrogen bonds causing IR continua it is shown that the charge of the hydrogen bonds is not the decisive property for the occurrence of the continua but rather the shape of the proton potentials. Continua may occur only with hydrogen bonds with double minima or broad flat proton potential.     

A Stereoselective Synthesis of (±)-cis-γ-Irone

(±)-cis-γ-Irone( 1 ), a main constitutent of natural iris oil, has been stereoselectively synthesized from methyl (2E)-3 -[(2,2,4-trimethyl-3-cyclohexen-1-yl)methoxy]-2-propenoate (3) (6 steps, overall yield 14%). The cis-configuration as the exocyclic position of the double bond of 1 were secured by the thermal ene reaction of the β-(alkenyloxy)acrylate 3 yielding the 3-oxabicyclo [3,3,1] nonane derivative 5 .     

Comparison between capillary electrophoresis and ion-chromatography for analysis of inorganic anions

Separation and detection of some selected inorganic anions with capillary electrophoresis are shown. The anions are separated in microbore capillaries (25 m* 0.2 m) and detected with a UV-detector. Results are compared with the method of ion exchange chromatography. In consideration of the most important physical and chemical parameters an easy kind of computer simulation for such electropherograms was developed.To get optimal results of separation and UV-detection in capillary electrophoresis some parameters of the device HPE 100, i.e. loading time in the electrokinetic sample injection mode and the running voltage are varied. The behaviour of absorption in the UV region of the chosen anions as well as the influence of pH values in retention behaviour are investigated. There is a simple way to calculate the electrophoretic mobilities from known retention times. Approximate limits of detection for all anions and for each technique are given.     

ansa-Metallocene derivatives XXXIII. 2-Dimethylamino-substituted bis-indenyl zirconium dichloride complexes with and without a dimethylsilyl bridge: syntheses, crystal structures and properties in propene polymerization catalysis

Bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (2-(CH₃)₂N-C₉H₆)₂ZrCl₂, and dimethylsilyl-bridged bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (CH₃)₂Si(2-(CH₃)₂N-C₉H₅)₂ZrCl₂, were prepared by reaction of the corresponding ligand lithium salts with ZrCl₄ in toluene. Diffractometric structure determinations reveal C₂-symmetric complex geometries for both complexes. An increased electron density at the Zr center of the dimethylamino-substituted complexes is indicated by reduction potentials which are 0.3–0.4 V more negative than those of their unsubstituted analogs. When activated with methyl aluminoxane in toluene solution, (CH₃)₂Si(2-(CH₃)₂N-C₉H₅)₂ZrCl₂ catalyzes the polymerization of propene to polymers with a microstructure comparable with that of polymers produced with other Me₂Si-bridged bis(indenyl)ZrCl₂ complexes, but with a substantially increased fraction of i-propyl end groups derived from alkyl exchange between Zr-polymer and Al---Me species.     

Internally contracted multiconfiguration-reference configuration interaction calculations for excited states

Summary The calculation of electronically excited states with the internally contracted multiconfiguration-reference configuration interaction (CMRCI) method is discussed. A straightforward method, in which contracted functions for all states are included in the basis, is shown to be very accurate and stable even in cases of narrow avoided crossings. However, the expense strongly increases with the number of states. A new method is proposed, which employs different contracted basis sets for each state, and in which eigensolutions of the Hamiltonian are found using an approximate projection operator technique. The computational effort for this method scales only linearly with the number of states. The two methods are compared for various applications.Dedicated in honor of Prof. Klaus Ruedenberg     

Beiträge zur Chemie der Pyrrolpigmente, 19. Mitt.: Die elektrochemische Oxidation von Pyrromethenonen und Pyrromethenen (Gallenpigment-Partialstrukturen)

The electrochemical oxidation of arylmethylene-pyrrolinones, pyrromethenones and pyrromethenes as representative bile pigment partial structure models was investigated by means of a rotating disc platinum electrode using acetonitrile as the solvent. Two different oxidation reactions were found. The first reaction being a reversible one-electron oxidation with compounds of the arylmethylene-pyrrolinone series and pyrromethenones which are unsubstituted in position 5 of the pyrrole ring. A two step reaction (the first one reversible, the second irreversible) on the other hand was found to be typical for pyrromethenones bearing a methyl group in this position.Through protonation the first step is at a higher potential, whereas the second one is lowered and becomes reversible. The resulting oxidation pattern can be interpreted analogous to the oxidation of hydroquinones in aprotic solvents.The geometrical isomers of a pyrromethenone were oxidized at appr. the same potential, but there is a strong dependence of the potential of the first oxidation step on the substitution: a higher degree of alkylation favours oxidation by lowering the oxidation potential.

---

---

Mit 1 Abbildung     

Practical asymmetric enzymatic reduction through discovery of a dehydrogenase-compatible biphasic reaction media

[reaction: see text] An enzyme-compatible biphasic reaction media for the asymmetric biocatalytic reduction of ketones with in situ cofactor regeneration has been developed. In this biphasic reaction media, which is advantageous for reactions at higher substrate concentrations, both enzymes (alcohol dehydrogenase and FDH from Candida boidinii) remain stable. The reductions with poorly water-soluble ketones were carried out at substrate concentrations of 10-200 mM, and the optically active (S)-alcohols were formed with moderate to good conversions and with up to >99% ee.