Singlet oxygen photooxygenation of furans : Isolation and reactions of (4+2)-cycloaddition products (unsaturated sec.-ozonides)

Tetraphenylporphin-photosensitized oxygenations of furan (19), 2-methylfuran (26), 2-ethylfuran (39), furfurylalcohol (24), 2-acetylfuran (40), 2-methoxyfuran (42), 2,5-dimethylfuran (30), furfural (25) and 5-methylfurfural (41) in non-polar aprotic solvents yield the corresponding monomeric unsaturated secondary ozonides due to a (4+2)-cycloaddition of singlet oxygen to these furans. With the exception of the ozonide derived from 25, the ozonides were isolated and characterized (¹H- and ¹³C-NMR spectra, etc.). In non-polar aprotic solvents, the ozonides derived from 19, 26 and 39 undergo thermal rearrangements to the corresponding cis-diepoxides and epoxylactones. Ozonide 31, derived from 30, however, dimerizes, only above about 60° is a cis-diepoxide formed from either 31 or its dimer. Rose bengal-photosensitized oxygenations of the furans in alcohols (MeOH, EtOH, i-PrOH) also produce the corresponding ozonides as the primary products of (4+2)-cycloadditions of singlet oxygen to these furans. However, reactions of the alcohols with the ozonides are too fast to allow the ozonides to be isolated. Instead, the same products are obtained as are isolated from reactions carried out by dissolving the ozonides in the alcohols. Depending on the structure of the ozonide, three pathways are available to ozonide/alcohol (ROH) interactions:(1) addition of ROH to yield alkoxy hydroperoxides; one out of several possible isomers is formed in a completely stereoselective and regiospecific reaction; (2) elimination of a bridgehead proton by ROH as a base, as observed with the ozonide derived from 19 to give hydroxy butenolide (78) in yields between 20 and 60%, and (3) ROH-attack on a carbonyl side-chain under elimination of the corresponding alkyl ester, as observed with furfural photooxygenation which yielded hydroxy butenolide (78) in high yields (95%). Interaction of ozonide 31 with tert.-butyl alcohol (t-BuOH) yields quantitatively cis-3-oxo-1-butenylacetate (81) by a Baeyer-Villiger-type rearrangement with vinyl group migration Hydrogenbonding between the alcohol and the peroxy group of the ozonides assist the heterolysis of the C—O bonds in the ozonides; the most stabilized cation develops. Front-attack of ROH on this cation explains the stereoselectivity as well as the regiospecificity of the alkoxy hydroperoxide formation; with a bulky alcohol like t-BuOH, ROH-attack on the cation is sterically hindered thus allowing a rearrangement to occur. 1,3-Dipolar cycloaddition of p-nitrophenyl azide to ozonide 31 proceeds stereoselectively to one of the isomers 87a/87b. Finally, kinetic results of furan photooxygenation in methanol show the following order of furan-reactivity towards singlet oxygen: 30 > 42 > 26 > 19 > 41 > 25, with absolute rate constants ranging from 1.8 × 10⁸ (with 30) to 8.4 × 1O⁴ M^-1P^-1 (with 25).     

A Hilbert space approach to maximum entropy reconstruction

We examine here the problem of reconstructing an X-ray attenuation function from measurements of its integrals. The approach that is taken is to maximize the difference of the entropy and the residual error in meeting the measurements. The solution of this optimization problem is constrained by requiring that the solution lie in a certain weakly compact subset of L², to be determined by physical information. We show that the constrained optimization problem is well-posed: there exists a unique solution (even when the measured data are inconsistent) and the solution depends continuously on the measurements. In the course of proving this, we show that the entropy functional is continuous on L². We further demonstrate that the solution of the optimization problem for a special case, must be piecewise constant.     

Oxygen ylides - I. : Reactions of carbenes with oxetane

The ylides generated from carbenes (:CH₂, :CHCO₂Et, :CHPh) and oxetane in the presence of methanol undergo Stevens rearrangement and protonation competitively, yielding tetrahydrofurans and 1,3-dialkoxycyclopropanes as major products.     

Determination of heavy metals (Pb, Cd, Cu, Zn, Cr) in sedimentary reference materials with IDMS: total concentration and aqua regia soluble portion

Summary The total concentration and the aqua regia soluble portion of traces of Pb, Cd, Cu, Zn, and Cr are determined with isotope dilution mass spectrometry (IDMS) in three different types of sediments using a thermal quadrupole instrument (THQ). The results are compared with those obtained by other methods during a certification project to establish these sediments (BCR 277, BCR 280, BCR 320) as standard reference materials. The IDMS (THQ) results are in good agreement with the preliminary certification values and the results of two other laboratories, which applied IDMS using magnetic sector field mass spectrometers. Significant differences between the different analytical methods were found for the total Pb concentration, which could be explained by adsorption and coprecipitation effects during the chemical pretreatment of samples. Using aqua regia instead of HF/HNO₃ for the decomposition of sediments, only 65%–95% of the total Pb and 60%–90% of the total Cr could be dissolved. On the other hand, only a small or negligible portion of Zn, Cd, and Cu could not be dissolved by aqua regia. The IDMS values for the aqua regia soluble Cr portion are significantly higher compared to the results of other methods. This can be explained by adsorption effects.

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Bestimmung von Schwermetallen (Pb, Cd, Cu, Zn, Cr) in sedimentären Standard-Referenzmaterialien mit massenspektrometrischer Isotopenverdünnungsanalyse: Totalgehalt sowie in Königswasser löslicher Anteil

     

Affinity precipitation of enzymes

Lactate dehydrogenase has been purified by precipitation with a bis-ligand. The precipitating agent in this case was Bis-NAD. This approach of affinity precipitation is also applicable to other enzymes.