Untersuchung des ZerfallsBi^{212} (ThC)\xrightarrow{\beta }Po^{212} (ThC')\xrightarrow{\alpha }Pb^{208} (ThD) durch (γ, α)-Koinzidenzen

Theγ-spektrum of Po²¹² (ThC′) has been measured by (γ, α)-coincidences with a fast-slow coincidence apparatus. The results are in excellent agreement with the conversion lines found by theLatyshev group. The absoluteγ-intensities have also been determined in order to get spins and parities of the levels by calculating the absolute conversion coefficients. Theβ-intensity leading to the exited states of Po²¹²(ThC′) is estimated to be about one third of that measured byBurde andRozner by (β, α)-coincidences. Threeγ-lines measured byChinaglia andDemichelis by (γ, α)-coincidences do not agree with our results. The 2⁺ assignment of the first exited state is now well established, but for the other levels in the decay scheme there is still some uncertainty concerning spin and level assignment.     

Helical modes of instability in swirling flow between concentric cylinders

Zusammenfassung Die Stabilität einer reibungsfreien und schraubenartigen Strömung zwischen zwei konzentrischen Zylindern wird untersucht. Schranken für die Anfachungsgrösse und Fortpflanzungsgeschwindigkeit von nicht-rotationssymmetrischen Störungen sind abgeleitet und genaue Werte dieser Grössen für die Spezialfälle von Couette-Strömung zwischen einem ruhenden äusseren und einem drehenden inneren Zylinder mit zusätzlicher Axial-Geschwindigkeitskomponente, die lienare oder parabolische Funktionen der Radialrichtung sind, werden erhalten. Es ergibt sich, dass der Zusatz einer linear abhängenden axialen Geschwindigkeit destabilisierend wirkt, weil der Zusatz einer parabolischen Komponente stabilisierend ist. Die Maximalanfachungsgrössen befinden sich bei schraubenförmigen Störungen mit endlicher Steigung.     

The loss of a methyl radical and the retro diels—Alder reaction in the electron impact mass spectrometry of 2-cyclohexen-1-ol and related compounds

The electron impact mass spectra of 2-cyclohexen-1-ol and of several of its ²H and ¹³C labelled analogues show that the molecular ions lose a methyl radical by a completely different means from the mechanism described previously. Moreover, the retro Diels–Alder reaction also proceeds in a non-classical way; in addition to the elimination of an olefinic molecule from unrearranged molecular ions, a second more important route implies a formal 1,3 allylic rearrangement prior to the retro Diels–Alder reaction. The mass spectra of a series of alkyl substituted homologues show that the competition between the two processes is closely related to the size of the olefinic moiety that is expelled.     

Chromium-catalyzed pinacol-type cross-coupling: studies on stereoselectivity

A chromium-catalyzed pinacol-type cross-coupling reaction between alpha,beta-unsaturated carbonyl compounds and aldehydes is reported. Even sterically demanding substrates could be coupled to afford the corresponding pinacols in good yields. Systematic studies concerning the origin of the diastereoselectivities led to the proposal of a mechanism for this synthetically useful reaction. Acroleins with alpha-branched alkyl side chains were coupled to give the corresponding syn pinacols, while on the other hand, acroleins with less bulky substituents furnished the anti derivatives. The effects of both the substrates and the reagents on the diastereo- and enantioselectivities were investigated. An unexpected catalytic formation of cyclopropanols was found.     

Nitrosation of sugar oximes: preparation of 2-glycosyl-1-hydroxydiazene-2-oxides

Oximes of glucose, xylose, lactose, fructose, and mannose have been prepared. Nitrosation of the oximes of glucose, xylose, and lactose with NaNO2/HCl afforded 2-(beta-glycopyranosyl)-1-hydroxydiazene-2-oxides, which were isolated as salts 13, 22, and 28. Nitrosation of fructose oxime 29 furnished fructose, whereas nitrosation of mannose oxime 30 with NaNO2/HCl afforded the 1-hydroxy-2-(beta-D-mannopyranosyl)diazene-2-oxide 32, from which the p-anisidinium salt 31 and the sodium salt 33 were prepared. However, nitrosation of 30 with isopentyl nitrite in aqueous solutions of CsOH or KOH resulted in the formation of the 2-(alpha-D-mannofuranosyl)-1-hydroxydiazene-2-oxide salts 34 and 35, respectively. Methylation of the ammonium 2-(beta-D-glucopyranosyl)-1-hydroxydiazene-2-oxide 13 yielded the 1-methoxy compound, which was benzoylated to afford the tetra-O-benzoate 14a, the structure of which was confirmed by X-ray diffraction analysis. From the glucose O-methyloximes 15 and 16 the N-methoxy-N-nitroso-2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosylamine 18 was prepared. The structure of this compound was confirmed by X-ray diffraction analysis. Treatment of acetobromoglucose with cupferron furnished the 1-(2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyloxy)-2-phenyldiazene-2-oxide 20.