Enantionselektive Synthese des L-Threonins

An Enantionselective Synthesis of L-Threonine An enantioselective synthesis of L-threonine ( 1 ) is described. Racemic ethyl 2-acetamido-3-oxobutyrate ( 6 ) was synthesized from ethyl acetoacetate ( 2 ) [4][5] and then transformed to the epimeric optically active alcohols 7a and 7b by microbiological reduction with Saccharomyces rouxii. The Mixture 7a/7b could be converted to 1 by slightly modified, known Methods in a yield of ca. 52% with respect to 7a/7b .     

Über die Oxydation von Manganoxidhydroxid

The oxidation of γ-MnOOH (manganite) in oxygen and its disproportionation in HNO₃ lead topotactically to β-MnO₂. The oxidation of synthetic α-MnOOH (groutite) in oxygen depends on its cristallite size; finely divided crystals oxidise rapidly to Mn₅O₈ which usually is stable but yields β-MnO₂ by further oxidation. Larger crystals of disperse synthetic α-MnOOH are topotactically transformed to γ-MnO₂. In HNO₃ α-MnOOH disproportionates into γ-MnO₂ and Mn²⁺. Though strictly topotactical, the reaction α-MnOOH → γ-MnO₂ is not single-phase as might be expected. The discontinuity in the function: JAHN -TELLER distortion vs. reaction rate, may simply be interpreted as the crosspoint of two different functions attributed to the crystal species α-MnOOH and γ-MnO₂, respectively. This distortion confirms the presence of Mn³⁺ in manganite and nsutite. The wide variety of possible X ray powder patterns of the phase γ-MnO₂ is explained by the superposition of, (i) cristallite size broadening, (ii) intergrowth structure effects on the profile, and (iii) BRAGG angle shifts due to substitution of part of Mn⁴⁺ by Mn³⁺.     

Der Einfluss eines Phosphatglases auf das Frequenzverhalten vonn-Inversionsschichten auf Silizium

Ohne Zusammenfassung     

Disorder-induced losses in planar photonic crystals

The phenomenological approach introduced by Benisty [Appl. Phys. Lett. 76, 532 (2000)] to model out-of-plane radiation losses in planar photonic crystals with a low vertical refractive index contrast is extended to the case of in-plane disorder. The model is experimentally validated by means of optical measurements on GaAs-based structures. For the present fabrication techniques the disorder-induced contribution is found to be negligible compared with the other loss mechanisms.     

On-line systems for continuous water and gas isotope ratio measurements

New continuous on-line techniques for water and air extracted from ice cores are developed. Water isotope ratio determination on any of the water phases (water vapour, water, ice) is of great relevance in different research fields, such as climate and paleoclimate studies, geological surveys, and hydrological studies. The conventional techniques for water isotopes are available in different layouts but all of them are rather time-consuming. Here we report new fast on-line techniques that process water as well as ice samples. The analysis time is only approximately 5 min per sample which includes equilibration and processing. Measurement precision and accuracy are better than 0.1 per thousand and 1 per thousand for delta18O and deltaD, respectively, comparable to conventional techniques. The new on-line techniques are able to analyze a wide range of aqueous samples. This allows, for the first time, to make continuous isotope measurements on ice cores. Similarly, continuous and fast analysis of aqueous samples can be of great value for hydrological, geological and perhaps medical applications.Furthermore, a new technique for the on-line analysis of air isotopes extracted from ice cores is developed. This technique allows rapid analyses with high resolution of the main air components nitrogen, oxygen, and argon. Measurement precision is comparable to precisions obtained by conventional techniques. It is now possible to measure delta15N and delta18O(atm) over entire ice cores helping to synchronize chronologies, to assess gas age-ice age differences, and to calibrate the paleothermometry for rapid temperature changes. This new on-line air extraction and analyzing technique complements the water methods in an ideal way as it separates the air from the melt-water of an ice sample. The remaining water waste flux can directly be analyzed by the water methods.     

Effect of an External Electric Field on the Decay Constant of 99Tcm in Halogen Complexes

A positive change Δλ/λ of the decay constant of ⁹⁹Tc^m has been observed on the application of an external electric field in K₂TcX₆ (X = F, Cl, Br, I). For its investigation the Tc-complexes were embedded as small conglomerates or more or less as isolated single molecules in a powder of high dielectric constant, which was then compressed into a thin layer of about 0.2 mm and inserted between two capacitor plates. The change Δλ/λ depends on the strength of the chemical bond of ⁹⁹Tc^m. As a first approximation Δλ/λ was assumed to be linearly dependent upon the electric field. The experimental results with an external field of 10₄ V/cm are: A relationship between Δλ/λ and the electro-negativities of the ligand atoms could be established. Difficulties encountered in the theoretical calculation of Δλ/λ are discussed.     

Lempert mappings and symplectic forms

We use Lempert's version of Riemann mapping to construct non-equivalent symplectic forms on an ellipsoid in .

     

Synthese von Astaxanthin aus β-Jonon. I. Erschliessung der enantiomeren C15-Wittigsalze durch chemische und mikrobiologische Racematspaltung von (±)-3-Acetoxy-4-oxo-β-jonon

Synthesis of Astaxanthin from β-Ionone. I. A Route to the Enantiomeric C₁₅-Wittig Salts by Chemical and Microbial Resolution of (±)-3-Acetoxy-4-oxo-β-ionone Racemic 3-acetoxy-4-oxo-β-ionone ( 10 ) was synthesized from the industrially accessible intermediate β-ionone ( 5 ). Resolution of 10 into its enantiomers was achieved via the corresponding diastereomeric camphanates and by microbial resolution. Site-selective alkylation of racemic and of optically pure 3-acyloxy-4-oxo-β-ionones with vinyl magnesium chloride at ?70° furnished the corresponding 3-acyloxy-4-oxo-9-vinyl-β-ionols which could be transformed to the Wittig salts 1, 3 and 4 , respectively, following known procedures [1].