A novel glycosylation concept; microwave-assisted acetal-exchange type glycosylations from methyl glycosides as donors

Efficient microwave-assisted glycosylations from methyl glucopyranosides are described. We have discussed the effects of microwave irradiation on this unique glycoside exchanging reaction from view points such as amount of Lewis acid promoters and acceptors, hydroxyl protecting groups of methyl glucopyranosides donors for reactivity, and neighboring effect.     

Ion-exchanger colorimetry-VIII Microdetermination of copper in natural waters

A microdetermination method for copper by ion-exchanger colorimetry has been developed. The porphyrin alpha,beta,gamma,gd-tetrakis(4-N-methylpyridyl)porphine was used as the colour reagent. After the complexation at pH 4.5 and 40 degrees , the surplus ligand was converted into the protonated species by addition of sulphuric acid. The complex and the ligand were easily sorbed on the cation-exchange resin, Dowex 50W-X2-H(+) (100-200 mesh). The resin-phase absorbances of the complex species at 424 nm and of the resin background at 510 nm were used for the measurements. There were no interferences by the foreign ions expected to be present in natural waters, at up to 1000 times the concentration of copper. With a 1 -litre sample the detection limit is 0.072 mug/l. It is possible to determine copper at mug/l. or lower levels in natural waters.     

Ion-exchanger phase absorptiometry for trace analysis

Ion-exchanger phase absorptiometry is based on the direct measurement of the degree of light-absorption by an ion-exchange resin phase which has sorbed a sample component. Direct application of the method makes it possible to determine trace elements in natural water samples without preconcentration. In this paper, all systems hitherto developed are reviewed and the theoretical background of solid-phase absorptiometry is described.     

Structural, textural, and electronic properties of a nanosized mesoporous ZnxTi1-xO2-) solid solution prepared by a supercritical drying route

Mesoporous nanosized TiO2 and Zn(x)Ti(1-x)O(2-x) solid solution having a Zn content below 10 mol % with a particles size between 13 and 17 nm are prepared by a template-free sol-gel method followed by high-temperature supercritical drying in 2-propanol. The structural, textural, and electronic properties of the obtained nanomaterials are methodically investigated by using XRD, SEM, TEM, ED, HREM, EDX, ICP-OES, N(2) adsorption-desorption, Raman spectroscopy, and diffuse reflectance UV-vis spectroscopy. It is shown that the proposed synthesis technique leads to the formation of a Zn(x)Ti(1-x)O(2-x) solid solution based on the anatase crystal structure rather than a two-phase sample. High-resolution electron microscopy and electron diffraction indicate that the distribution of zinc atoms over the anatase structure does not lead to a considerable deformation of the crystal structure.     

Reaction of sulphoximides with diazomalonate in the presence of Cu-salt : A new synthesis and stereochemistry of optically active oxosulphonium ylides

Sulphoximides (Ia–Ie) were found to react with dimethyl diazomalonate (DDM) in the presence of a catalytic amount of Cu-salts affording the corresponding oxosulphonium ylides in moderate yields. The reaction did not proceed at all under irradiation of UV light. (?)-Methylphenyloxosulphonium bis(methoxycarbonyl)-methylide ((?)-IIb) was obtained from (+)-(S)-methylphenylsulphoximide ((+)-(S)-Ib) together with (?)-(S)-methyl phenyl sulphoxide ((?)-(S)-IIIb) by this reaction. Hydrolysis of (?)-IIb gave (+)-methylphenyloxosulphonium methoxycarbonylmethylide ((+)-IIf) which was converted to (?)-(S)-IIIb upon treatment with dibenzoylethylene. Stereochemical cycle starting from (+)-(S)-Ib to (?)-(S)-IIIb was established and the absolute configurations of both ylides, (?)-IIb and (+)-IIf were assigned as (R)-configuration. The stereochemical courses, namely from (+)-(S)-Ib or (?)-(S)-IIIb to (?)-(R)-IIb or (+)-(R)-IIf to (?)-(S)-IIIb were determined as retention processes. The optical purities of the oxosulphonium ylides obtained from both reactions, (+)-(S)-Ib→(?)-(R)-IIb and (?)-(S)-IIIb→(?)-(R)-IIb, were almost equal. These results indicate that the mechanism of the reaction of sulphoximides with carbenes (or carbenoids) involves the initial formation of the sulphoxides which react subsequently with carbenes to afford the final products.     

Quantitative analysis of photoelectron angular distribution of single-domain organic monolayer film: NTCDA on GeS(0 0 1)

Angle-resolved photoelectron spectra were measured for a single-domain monolayer film of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTCDA) prepared on a cleaved GeS(0 0 1) surface using synchrotron radiation. The observed photoelectron angular distributions were analyzed by a calculation using the single-scattering approximation combined with molecular orbital calculation. With the help of the low-energy electron diffraction pattern, the structure of the NTCDA monolayer on GeS(0 0 1) was estimated quantitatively.     

Determination of conjugated bile acids in human urine by high-performance liquid chromatography with chemiluminescence detection.

A qualitative and quantitative analysis of the conjugated 1 beta- and 6 alpha-hydroxy bile acids, including common bile acids, in human urine using high-performance liquid chromatography with chemiluminescence detection is described. After extraction of urine with C18 silica cartridges, the bile acids were separated into non-conjugated, glycine, taurine and sulphate fractions by ion-exchange chromatography on a lipophilic gel. Solvolysis of the sulphate was carried out by treatment with trifluoroethanol in acetone containing hydrochloric acid, and the liberated amino acid conjugates were fractionated again. The individual bile acids were separated on a reversed-phase C18 column (Bile Pak II), with detection by an immobilized 3 alpha-hydroxysteroid dehydrogenase enzyme reactor and chemiluminescence reaction of the generated NADH using 1-methoxy-5-methylphenazinium methylsulphate-isoluminol-microperoxidase system. The assay method showed the detection limits ranging from 8 to 250 pmol for the bile acids tested. Analysis of urine samples obtained from newborns, non-pregnant women and women in late pregnancy showed a large difference in bile acid composition and conjugation mode, suggesting that bile acid metabolism is different during fetal and neonatal periods.     

A practical synthesis of 4′-thioribonucleosides

A practical synthesis of 4′-thioribonucleosides starting from inexpensive l-arabinose is described. 1,4-Anhydro-2,3-O-isopropylidene-4-thioribitol, which was prepared by using a novel reductive ring-contraction reaction, was converted to the 5-O-silylated sulfoxides. The Pummerer-type thioglycosylation of the sulfoxides gave the 4′-thioribonucleosides stereoselectively.