Chiral calixarene derived from resorcinol. Part 4: Diastereoselective closure of the oxazine ring

A method for the preparation of a single diastereomer of the oxazine derivative of calixresorc[4]arene 3 has been developed. Lewis acids were employed for opening of the oxazine ring.     

Stereoselective E/Z photoisomerization of oxazolidinone functionalized enecarbamates: direct and triplet sensitized irradiation

Oxazolidinone-functionalized enecarbamates undergo diastereoselective E/Z photoisomerization upon direct and triplet sensitized irradiations with chiral/achiral sensitizers, showing that the enhanced product diastereoselectivity depends on the solvent and temperature.     

Die potentiometrische Bestimmung von Jod und jod-freisetzenden Oxydationsmitteln

Zusammenfassung Zwischen dem Redoxpotential und dem log der Jodkonzentration jodidhaltiger wäßriger Lösungen besteht unabhängig vom pH-Wert über 3–4 Zehnerpotenzen ein weitgehend linearer Zusammenhang, der eine einfache und genaue quantitative Bestimmung von Jod ermöglicht. Da unter diesen Bedingungen sehr viele Oxydationsmittel äußerst rasch und quantitativ unter Bildung von Jod (bzw. Trijodid) reagieren, kann die Methode sehr vielseitig angewandt werden (z. B. zur Bestimmung von Chlor, Brom, O- und N-Halogenverbindungen, Ozon u. a.).Die Leistungsfähigkeit der Methode ist vor allem gekennzeichnet durch die rasche Redoxpotentialeinstellung in jodidhalügen Jodlösungen, die absolute Konstanz der als Meßelektrode dienenden Platinelektrode, die unter sorgfältiger Einhaltung der gewählten Meßbedingungen (Temperatur, Jodidkonzentration, Gesamtionenstärke) eine Nacheichung praktisch nicht notwendig macht, sowie die Möglichkeit, mit den gleichen Reagenzien den Konzentrationsbereich 10^–1–2,5×10^–8 M/l zu vermessen. Mit den relativen Standardabweichungen 0,7–1,2% im Bereich 10^–3–10^–5 M/l, 1,5–1,7% bei 10^–6 M/l bzw. 4,4–9,1% bei 10^–7 M/l stellt die potentiometrische Jodbestimmung eines der genauesten analytischen Verfahren im Bereich niederer Konzentrationen dar, das zudem durch einen äußerst geringen meßtechnischen Aufwand gekennzeichnet ist.

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The potentiometric determination of iodine and iodine releasing oxidants

Summary Between the redox potential and the log of the iodine concentration of aqueous iodine solutions (pH<9) containing an excess amount of iodide exists independent of the pH-value over a concentration range of 3–4 powers a nearly linear relation, which enables a simple and precise determination of iodine. Since under these conditions a great many oxidants react very fast and quantitatively forming iodine (resp. tri-iodide) the method can be used very versatile (e. g. for the determination of chlorine, bromine, O- and N-halogene compounds, ozone). The efficiency of the method is characterized above all by the fast adjustment of the redox potential in iodine solutions containing iodide, the constancy of the Pt-electrode, by which carefully observing the choosen measuring conditions (temperature, iodide concentration, total ionic strength) a recalibration is virtually unnecessary, and the possibility to measure with the very same reagents the concentration range 10^–1–2.5×10^–8 M/l. With relative standard deviations of 0.7–1.2% in the range 10^–3–10^–5 M/l, 1.5–1.7% at 10^–6 M/l resp. 4.4–9.1% at 10^–7 M/l the potentiometric determination of iodine, though requiring only a very simple instrumental fitting out, represents one of the most precise analytical methods at low concentration levels.

     

Conformationally controlled (entropy effects), stereoselective vibrational quenching of singlet oxygen in the oxidative cleavage of oxazolidinone-functionalized enecarbamates through solvent and temperature variations

On photooxygenation (methylene blue as sensitizer) of E/Z enecarbamates, equipped with the oxazolidinone chiral auxiliary, the oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the choice of the alkene configuration; the efficacy of stereocontrol may be tuned by appropriate solvent and temperature conditions. Highlighted is the finding that the formation of the preferred MDB enantiomer (R or S) depends for the E isomer on the chosen solvent and temperature, but not for the corresponding Z isomer. The activation parameters for the various solvents disclose that differential entropy effects (ΔΔS^‡) dominate the conformationally more flexible E diastereomers. As mechanistic rationale for this unprecedented conformationally imposed stereochemical behavior, we propose the competitive action of stereoselective vibrational quenching of the attacking singlet oxygen by the enecarbamate versus sterically controlled stereoselective oxidative cleavage of its double bond.     

On the origin of geminal regioselectivity in the ene reaction of singlet oxygen with substituted alkenes

The geminal regioselectivity observed in the ene reaction between singlet oxygen and alkenes with anion-stabilizing groups is rationalized on the basis of a perepoxide intermediate, in which in analogy to the nucleophilic attack on protonated epoxides, the perepoxide is opened preferentially at the C---O bond weakened by the substituent.