Branched Star-Type Polysilyllithium Compounds: The Effects of beta-Silyl Substitution and of Complexation on Their Molecular Structure

No Abstract     

Bestimmung kleiner Sauerstoffgehalte in Selen

Zusammenfassung Es wird eine Analysenmethode zur Bestimmung von Sauerstoff in Selen beschrieben. Die Abtrennung des Sauerstoffs geschieht über seine Umsetzung mit Schwefel; die Endanzeige des gebildeten Schwefeldioxids erfolgt colorimetrisch mit Parafuchsinchlorid. Mit dem Verfahren werden Gehalte bis 0,5 ppm (w)¹ erfaßt. Die relative Standardabweichung bei Analysen von SeO₂- und TeO₂-dotiertem Selen beträgt ±8%. Die Analysen von 5 Selenproben als Perlen ergaben Gehalte zwischen 4 und 11 ppm (w) Sauerstoff; Selengleichrichterschichten enthielten 1 ppm (w) Sauerstoff.

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Determination of small oxygen contents in selenium

An analysis method for the determination of oxygen in selenium is described. The separation of oxygen is accomplished through its reaction with sulphur. The final measurement of the sulphur dioxide formed is made colorimetrically with parafuchsine chloride. With this method contents as low as 0.5 ppm (w) can be determined. The relative standard deviation for analyses of selenium doped with SeO₂ and TeO₂ is ±8%. The analyses of 5 selenium samples having the form of beads resulted in oxygen contents between 4 and 11 ppm (w), whilst selenium rectifier layers contained 1 ppm (w) of oxygen.

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Die Verfasser bedanken sieh für das fördernde Interesse von Herrn Prof. Dr. W. Gebauhr und Herrn Dr. A. Spang. Zur Arbeit trugen wesentlich die anregenden Diskussionen mit Herrn J. Martin bei.     

Comment on: "Disentangling density and temperature effects in the viscous slowing down of glass forming liquids" [J. Chem. Phys. 120, 6135 (2004)

Recently, Tarjus et al. [G. Tarjus, D. Kivelson, S. Mossa, and C. Alba-Simionesco, J. Chem. Phys. 120, 6135 (2004)] concluded from a review of data for a variety of glass formers that the supercooled dynamics are almost invariably dominated by temperature T, rather than by density rho. By including additional published data into such a compilation, we show that for van der Waals molecular liquids, the dynamics near T(g) are in fact governed as much by density as by temperature. Moreover, relaxation times measured at various temperatures and pressures can be superimposed by plotting as a function rho(gamma)/T. This scaling form can arise from an assumed inverse power law for the intermolecular repulsive potential, with gamma a material constant. (c) 2004 American Institute of Physics.     

An improved synthesis of 3-deazaeytosine, 3-deazauracil, 3-deazacytidine,and 3-deazauridine

3-Dcazacytosine (4-amino-2-pyridone, 3 ), 3-doazauracil (4-hydroxy-2-pyridone, 5 ), 3-deaza-cytidine (4-amino-1-β-D-ribofuranosyl-2-pyridonc, 9 ), and 3-deazauridine (4-hydroxy-1-β-D-ribo-furanosyl-2-pyridone, 11 ) were prepared in high overall yields from 1-methoxy-1-buten-3-yne ( 1 ). Ethyl 3,5,5-triethoxy-3-pentenoate ( 2 ), obtained from acylatioti of 1 with diethyl carbonate and subsequent in situ conjugate addition of ethoxide, was cyelized with ammonia to provide 3 . Diazotization of 3 and subsequent in situ hydroxydediazotization afforded 5 . Nucleoside 9 was obtained from the stannic chloride-catalyzed condensation of bis-trimethylsilylated 3 and 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose ( 7 ), followed by ammonolysis of the blocking groups. Diazotization of 9 and subsequent in situ hydroxydediazotization afforded nucleosidc 11 .     

Synthesis of unsymmetrical 2,4-dialkylpyrazolo[1,5-a]-1,3,5-triazines

The reaction of 3-aminopyrazole with imidate esters such as ethyl acetimidate, gave N-(pyrazol-3-yl)acetamidine (1) rather than the isomeric 2-acetamidoyl-3-aminopyrazole. Ring closure of 1 with orthoesters such as ethyl propionimidate, afforded unsymmetrically substituted 2.4-dialkylpyrazolo[1,5-a]-1,3,5-triazines such as 4-ethyl-2-methylpyrazolo[1,5-a]-1,3,5-triazine (3). The structure of 1 was confirmed by several alternate syntheses. The unique feature of this two-step synthetic approach to the synthesis of pyrazolo[1,5-a]-1,3,5-triazines is that it is a convenient method of preparing fused triazines based on available pyrazoles rather than the less accessible dialkyltriazines.     

New photochromic 2H-1.4-oxazines and spiro-2H-1.4-oxazines

The synthesis and the photochemical behaviour of 4 new 2H-1.4-oxazines and 3 new spiro-2H-1.4-oxazines is described.     

Biosynthesis of δ-Jasmin Lactone ( = (Z)-Dec-7-eno-5-lactone) and (Z,Z)-Dodeca-6,9-dieno-4-lactone in the Yeast Sporobolomyces odorus

(all-Z)-(9,10,12,13,15,16-²H₆)Octadeca-9,12,15-trienoic acid ( = α-linolenic acid; D₆- 4 ) was synthesized to investigate the biochemical formation of linolenic-acid-derived aroma compounds in cultures of the yeast Sporobolomyces odorus, using an established gas chromatographic/mass spectrometric (GC/MS) method. Three compounds were identified as labeled: (Z)-dec-7-eno-5-lactone (δ-jasmin lactone), (Z,Z)-dodeca-6,9-dieno-4-lactone, and (2E,4Z)-hepta-2,4-dienoic acid. Both lactones were biosynthesized mostly under conservation of the initial configuration from their corresponding oxygenated linolenic-acid intermediates. The application of (13S,9Z,11E,15Z)-13-hydroxy(9,10,12,13,15, 16-²H₆)octadeca-9,11,15-trienoic acid (D₆- 7 ) as a OH-functionalized precursor of δ-jasmin lactone allowed to gain insight into the stereochemical course of the biosynthesis to both enantiomers of this lactone. In this experiment, 88.3% of the metabolized labeled precursor was transformed under retention of the original configuration of the (R)-enantiomer. This investigation is also a contribution to a better understanding of the C?C bond isomerization steps which took place during the β-oxidative degradation of the substrate.     

Pyrrolopyrimidine nucleosides VII. A study on electrophilic and nucleophilic substitution at position six of certain pyrrolo[2,3-d] pyrimidine nucleosides

A study involving the reactivity of the pyrrolo[2,3-d] pyrimidine ring system at position 6 with another exocyclic group (CN or -NH₂) already residing at C5 has established that hydrogen and bromine are susceptible to electrophilic and acid-catalyzed nucleophilic substitution, respect-tively. In one instance a strong nucleophile (hydrazine) gave nucleophilic substitution at position 6 which was followed by a reaction with the o-nitrile group to afford the tricyclic nucleoside 4,5-diamino-8-(β-D-ribofuranosyl)pyrazolo[3′, 4′ :5,4] pyrrolo[2,3-d] pyrimidine (4).