Methionin als Vorläufer zur enantioselektiven Synthese α-verzweigter Vinylglycine und anderer Aminosäuren

Methionine as Precursor for the Enantioselective Synthesis of α-Branched Vinylglycines and of Other Amino Acids Methionine is converted by previously published methods into the diastereoisomerically pure 3-thiabutyl-substituted oxazolidinone ( 7 ) and imidazolidinones 5 and 6 . An X-ray crystal structure determination of cis-3-benzoyl-2-(tert-butyl)-4-(3-thiabutyl)oxazolidin-5-one ( 7 ) confirms the configurational assignments made by NOE-NMR measurements. Oxidation to sulfoxides and pyrolytic elimination produce vinyl-substituted heterocycles (see 19, 21 ). Diastereoselective alkylations of the enolate 14 from the imidazolidinone 5 and of the dienolate 23 from the vinyl derivative 19 give geminally alkyl- and/or vinyl-substituted heterocycles. Some of these products were hydrolyzed to free amino acids, such as (R)-2-methyl- ( 25a ) and (R)-2-ethyl-2-vinylglycine ( 25b ) (R)-2-methylhomoserine ( 27 ). Raney-Ni desulfurization of 5 and oxidative degradation of 19 lead to enantiomerically pure derivatives of α-aminobutyric acid (see 28 ) and of glycine (see 31 ), respectively.     

Au sujet des flots riemanniens sur le groupe nilpotent de Heisenberg

One proves that on the three dimensional nilpotent Lie group endowed with a left invariant riemannian metric, the only riemannian flows are the isometric ones.     

A Low‐Valent Molybdenum Nitride Complex: Reduction Promotes Carbonylation Chemistry

Toward nitrogen functionalization, reactive terminal transition metal nitrides with high d‐electron counts are of interest. A series of terminal Mo^IV nitride complexes were prepared within the context of exploring nitride/carbonyl coupling to cyanate. Reduction affords the first Mo^II nitrido complex, an early metal nitride with four valence d‐electrons. The binding mode of the para‐terphenyl diphosphine ancillary ligand changes to stabilize an electronic configuration with a high electron count and a formal M?N bond order of three. Even with an intact Mo≡N bond, this low‐valent nitrido complex proves to be highly reactive, readily undergoing N‐atom transfer upon addition of CO, releasing cyanate anion.     

Alkalischer Celluloseabbau in Abwesenheit einer wässerig-flüssigen Phase

Zusammenfassung Beim thermisch-alkalischen Abbau von Baumwoll-Cellulose in Abwesenheit einer wäßr.-flüss. Phase erfolgt rasch Kettenabbau mit jedoch geringem, von den Endgruppen ausgehenden Abschälen (1,8–2,4 Glucose-Einheiten pro Kettenspaltung). Während des Abbaus stehen die Logarithmen derDP _n-Werte und Reaktionszeiten in einem annähernd geradlinigen Verhältnis. Die Cellulosen erwiesen sich auch bei weit fortgeschrittenem Kettenabbau als kristallin.

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Alkaline degradation of cellulose in absence of an aqueous liquid phase

In thermal alkaline degradation of cotton cellulose in absence of an aqueous-liquid phase, rapid chain splitting takes place, however, with little peeling-off involving the end groups (1.8–2.4 glucose units per chain splitting). During degradation, the logarithms of theDP _n-values and reaction times exhibit an approximate straight-line relationship. The celluloses were found to be crystalline even at far advanced chain splitting.

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Mit 2 Abbildungen

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Kurzbericht über bereits vor einiger Zeit am Pulp and Paper Research Institute of Canada durchgeführte Untersuchungen.     

Volume and compressibility effects in the formation of metal-EDTA complexes

We used precise measurements of ultrasonic velocity and density to study the complexation of ethylendiaminetetraacetic acid (EDTA) with Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ at 25‡C and pH 12. From these measurements we obtained the changes in the molar concentration increment of the ultrasonic velocity δA, the apparent molar adiabatic compressibility δK_sΦ, and the apparent molar volume δV_Φ of complex formation. The hydration contributions δ(AV_h) to the volume effect of binding range from 39.6 to 46.6 cm³-mol^-1 while the hydration contribution to the adiabatic compressibility change in the binding, δ(δK_h), ranges from 103.9X 10^-4 to 131.1 X 10^-4 cm³-mol^-1-bar^-1. These data are interpreted in terms of dehydration of interacting molecules,i.e., transfer of water molecules from the hydration shells of cations and EDTA into the bulk water. The ratio δ(δV_h)/ δ(δV_h) is in the range 0.35 to 0.38 bar, indicating a dominant contribution from the dehydration of charged atomic groups in the volume and the compressibility effects of complex formation.