Opening of tartrate acetals using dialkylboron bromide: evidence for stereoselectivity downstream from ring fission

Johnson-type acetals derived from dimethyl tartrate give, after opening with Me(2)BBr and cuprate displacement, secondary alcohols with high diastereoselectivity (>30:1). The mechanism proposed for the induction of diastereoselectivity is downstream from the ring fission. It implies a direct participation of the Lewis acid as a source of nucleophile and the stereospecific transformation of the resulting bromo acetal through an invertive and temperature-dependent process. The acetals are prepared by reaction of the desired aldehyde with dimethyl tartrate. Removal of the auxiliary is accomplished through SmI(2) reduction or by an addition-elimination protocol using methoxide.     

Assay of TNP-470 and its two major metabolites in human plasma by high-performance liquid chromatography-mass spectrometry

In this study, a sensitive, specific assay for the determination of TNP-470 and its two major metabolites M-IV (also know as AGM-1883) and M-II in human plasma is reported. The assay involves liquid-liquid extraction of acidified plasma followed by reversed-phase high-performance liquid chromatography-atmospheric pressure chemical ionization-tandem mass spectrometry. A liquid-liquid extraction using an organic solvent mixture (methyl-tert-butyl-ether-hexane, 1:1, v/v) is used in place of solid-phase extraction because it provides consistent recoveries for all analytes, including the internal standard. Retention times for the analytes and internal standard are less than 7 min. Within- and between-day precision is < or = 6.5% and < or = 13.3% relative standard deviation, respectively, for the three analytes. The lower limits of quantitation are 0.25, 0.5, and 1.0 ng/mL for TNP-470, M-IV, and M-II, respectively.     

On the mechanism of the gif system for the oxidation of saturated hydrocarbons

The high selectivity of the Gif system for hydrocarbon oxidation is shown to depend upon the capture of $\text{tert.}$ radicals by pyridine; the mechanism for the secondary oxidation products has only a minor radical component as judged by competitive trapping.     

2D correlation analysis of the continuum in single molecule surface enhanced Raman spectroscopy

The role and the nature of the continuum in Surface Enhanced Raman Spectroscopy (SERS) are unclear. Here, two-dimensional (2D) covariance and correlation analysis is applied to single molecule SERS spectra on silver colloids with and without rhodamine 6G (native colloid). The resulting 2D covariance and correlation maps show that the sharp molecular Raman peaks from rhodamine 6G and the molecule responsible for the SERS peaks from the native colloid are correlated to different continua even though both continua are present in each data set. This suggests that two distinct active sites on the silver colloids produce the two different continua, and that each site has some molecular specificity.     

Regioselectivity in the palladium-catalyzed addition of carbon nucleophiles to carbocyclic derivatives.

The regioselectivity of Pd-catalyzed malonate additions and arylations to cycloalkenyl esters can be predicted by completing a stereochemical analysis of the Pd-pi-allyl complex. The Pd-catalyzed malonate additions which have the greatest degree of regioselectivity are in which substituents have a steric influence in blocking the incoming nucleophile. Cyclopentenyl substrates displayed lower regioselectivity than the cyclohexyl counterparts presumably due to increased planarity of the system. Arylations using tin and hypervalent silicon reagents were compared.     

Capillary zone electrophoresis with indirect UV detection: Determination of sodium dodecyl sulfate in simulated stream water

SDS (sodium dodecyl sulfate) has been quantitatively determined by capillary zone electrophoresis using a fused silica capillary and a 5 mM dihydroxybenzoic acid / sodium hydroxide buffer in 5% methanol solution at a pH of 8.1. The ion was detected by indirect UV absorption at 250 nm. Detection range was from 0.8 to 50 mg SDS/L. This rapid method requiring only small sample volumes was developed in support of an aquatic toxicology study in a simulated stream water and is applicable to waters containing common inorganic ions.     

Specific detection of Clostridium botulinum types A,B, E,and F using the polymerase chain reaction

Clostridium botulinum organisms generally produce 1 of 4 neurotoxin types (A, B, E, and F) associated with human illness. Neurotoxin type determination is important in identification of the bacterium. A polymerase chain reaction (PCR) method was developed to identify 24 h botulinal cultures as potential types A, B, E, and F neurotoxin producers as well as other clostridial species which also produce neurotoxins. Components of the PCR and amplification conditions were adjusted for optimal amplification of toxin gene target regions to enable simultaneous testing for types A, B, E, and F in separate tubes using a single thermal cycler. Each primer set was specific for its corresponding toxin type. A DNA extraction procedure was also included to remove inhibitory substances that may affect amplification. This procedure is rapid, sensitive, and specific for identification of toxigenic C. botulinum.     

Hydrogen fluoride analyzer for gases and aerosols

Many fire suppression agents are currently used, and the replacement candidates for these agents contain fluorine atoms. When these agents are used to extinguish a fire, large quantities of hydrogen fluoride gas can be produced from the thermal degradation of fluorinated organic compounds. A real-time analyzer has been developed to measure exposure levels of hydrogen fluoride gas and aerosols during fire suppression tests. A vacuum pump pulls air through a continuous denuder, where the toxic gas and aerosols are extracted from the air into an aqueous trapping solution. The trapping solution then passes through a flow cell, where a fluoride ion-selective electrode measures the fluoride ion concentration. A solenoid pump moves the trapping solution and calibration standards through the analyzer. Once calibrated, the analyzer can generate a concentration profile of hydrogen fluoride versus time. This hydrogen fluoride analyzer is portable and can be calibrated in about 5 min. It provides rapid response to hydrogen fluoride gas and aerosols, over a detection range from 1 to 5000 mg/m³.     

Determination of copper, iron, manganese and zinc in river and estuarine water by atom trapping-flame atomic absorption spectrometry

Summary A simple method is described for the atomic absorption (AA) determination of Cu, Fe, Mn and Zn in river and estuarine water using two atom trapping techniques: a water-cooled dual silica tube and a commercially available double-slotted quartz tube mounted in an air-acetylene flame. Rapid, accurate analyses can be achieved using continuous aspiration. The concentration detection limits were 0.9, 1.5 and 0.3 ng ml^–1 for Cu, Mn and Zn, respectively, using a 2 min in situ preconcentration time with the dual silica tube atom trap and 4.0, 12.1, 2.0 and 1.2 ng ml^–1 for Cu, Fe, Mn and Zn, respectively, using the double-slotted quartz tube. The relative standard deviations are of the order of 2.9–6.9% for both techniques. The accuracy was assessed by analyses of NRCC SLRS-2 riverine and SLEW-1 estuarine water reference materials. Basic performance characteristics are also given for Ag, Bi, Cd, In, Pb and Tl using the dual silica tube atom trap.