2,3,4,6-Tetra-O-acetyl-β-d-galaktopyranosylmercaptan und dessen Anwendung zur Synthese von β-d-Thiogalaktosiden

Zusammenfassung Es wird die Darstellung von 2,3,4,6-Tetra-O-acetyl--d-galaktopyranosylmercaptan und dessen Umwandlung in Natrium--d-galaktopyranosylmercaptid beschrieben. Diese beiden Substanzen lassen sich zu einer bequemen Darstellung von -d-Thiogalaktosiden benützen.     

Efficient preparation of a 1,3-diazidocyclitol as a versatile 2-deoxystreptamine precursor

A synthesis route toward 2-deoxystreptamine, a common structure in many of the clinically important aminoglycosides, is presented. Starting from p-benzoquinone and cyclopentadiene, 2-deoxystreptamine is synthesized with key steps involving Pd(0)-catalyzed rearrangement, a retro-Diels-Alder by flash vacuum thermolysis, and Yb(III)-directed regioselective epoxide opening. The obtained diazidocyclitol 17 is a suitable 2-deoxystreptamine precursor, conveniently protected for incorporation in new aminoglycoside entities.     

Microcomputer programming in basic for the evaluation of capillary gas chromatography in the analysis of pesticide residues. I. MATRIXCOMP--a program that facilitates the recognition of interfering peaks from the biological matrix

Chromatograms of pesticide residues in food include peaks produced by pesticides and matrix compounds. Pesticide peaks are recognized by means of relative retention times and response factors; two detectors are used and internal standard methods are applied. Chromatograms of reference samples for all types of food are stored as raw data in a reduced format, together with tables of all chromatographic data for the matrix compounds. MATRIXCOMP provides the analyst with the chromatograms of the actual sample and the reference in parallel on a visual display screen for visual comparison. Simultaneously, the relevant chromatographic data for the sample, the reference and the calibration tables are displayed on a second screen page in a condensed form.     

Thermal degradation observed with different injection techniques: Quantitative estimation by the use of thermolabile carbamate pesticides

An experiment has been designed to study the thermal degradation of thermolabile compounds caused by various injection techniques. The four carbamate pesticides aminocarb, bendiocarb, carbaryl, and dioxacarb decompose thermally into methylisocyanate and the corresponding phenol. The carbamets and the phenols arising from them were separated on a 25 m SE-54 fused silica column; all compounds exhibited sharp peak shape indicating that the degradation observed took place completely within the injector. When cold on-column injection was employed no thermal degradation was observed whereas with hot splitless injection at 220°C decomposition of the carbamates was almost complete. PTV injection was found to produce intermediate results. When packed with glass wool and operated with glass wool and operated with starting temperatures lower than the boiling point of the solvent, decomposition was found to be almost complete. Applying isothermal conditions at 140°C (30°C above the boiling point of toluene) aminocarb and bendiocarb underwent only slight decomposition while carbaryl and dioxacarb were about half degraded. Results from PTV injection with an empty insert resembled those obtained using cold on-column injection and in this mode the application of temperatures up to 200°C resulted in no visible degradation. This can be explained by the short residence time of the sample in the injector.     

Determination of triazine herbicides with GC/MS under EI, methane CI and ammonia CI conditions detecting positive and negative ions

Summary Mass spectra of 26 triazine herbicides including a few metabolites were measured with a Finnigan 4000 applying electron impact and chemical ionization using methane and ammonia as reactant gases monitoring both positive and negative ions. Chemical ionization was studied to find out possible advantages of ammonia over other reactant gases. Detection limits with SIM were measured with methane and ammonia. For most of the triazines ammonia was found to be superior to methane with respect to detection sensitivity.Part of this paper was presented at the Seventh International Congress of Pesticide Chemistry, Hamburg, August 5–10, 1990     

Evaluation of automated and manual hot-splitless,cold-splitless (PTV), and on-column injection technique using capillary gas chromatography for the analysis of organophosphorus pesticides

Three different injection modes were compared for the analysis of organophosphorus pesticides. Differing results were found for several compounds depending on their chemical structure and thermostability. The programmed temperature vaporizing injector used in this study seemed to be the most promising sampling device for pesticide residue analysis in biological samples.     

Synthesis,structure, and characterization of Cu4S10(4-methylpyridine)4

The title compound, Cu₄S₁₀(4-methylpyridine)₄ · 4-methylpyridine was prepared by three different reactions: the oxidation of copper power by sulfur and the reaction of copper (I) sulfide (or CuBr-SMe₂) with cues sulfur, both in the coordinating solvent, 4-methylpyridine. Red crystals of the compound obtained by layering with hexans were subjected to single crystal X-ray diffraction. The structure was refined toR=0.026 and R _w =0.036 in a space groupP1bar (No. 2), withZ=2,a=13.983 (2) A,b=15.384( 2) Å,c=9.660 (1) Å, = 93.87 (1)°,=93.38 ==(1)°,V=2037.9 (9) ų. The commpound has approximate S₄ symmetry and consists of two pentasuffide chains linking four Cu(I) ions, each with a coordinating 2-methylpyridine. The infrared spectrum was dominated by absorption due to coodinated 4-methylpyridine with several low-energy peaks attributable to S-S stretches, which were also observed by Raman spectroscopy. A featureless electronic absorption spectrum yielded a single peak in the mar ultraviolet upon computer enhancement (=334 nm, = 10,000), most likely an intraligand transition. Cyclic voltammetry indicates that the polysulfide complex undergoes irreversible oxidation and reduction at +0.04 and –0.34 V vs. SCR respectively, at NS K in 4-methylpyridine when swept at 20 mV/sec. The electrochemical behavior was unvaried even at sweep rates as high as 100 V/sec.     

Plasma-induced deposition of titanium nitride from TiCl4 in a direct current glow discharge: Control of the chlorine content and gas-phase nucleation

Titanium nitride thin films have been deposited from TiCl₄, N₂, and H₂, in a low-pressure glow discharge at temperatures of 500°C, which are compatible with most applications. Based on our earlier work on the effect of tire plasma parameters on the chemical pseudoequilibrium in this and in similar systems, the chlorine content could he reduced to less than 2 at %. Suppression of the positive column reduced the gas-phase reaction within the reactor volume and the concomitant homogenous nucleation Dense, vent hard films with golden color could he deposited on various substrates including silicon and steel.     

Lewis Acid-Promoted, Stereocontrolled, Gram Scale, Diels-Alder Cycloadditions of Electronically Matched 2-Pyrones and Vinyl Ethers: The Critical Importance of Molecular Sieves and the Temperature of Titanium Coordination with the Pyrone

(R)-(+)-Binol-titanium coordinates with commercial methyl 2-pyrone-3-carboxylate and promotes mild, highly enantiocontrolled Diels-Alder cycloadditions with electron-rich vinyl ether CH(2)=CHOCH(2)-1-naphthyl and vinyl silyl ether CH(2)=CHOSiMe(2)Bu-t leading to valuable 1alpha,25-dihydroxyvitamin D(3) (calcitriol) intermediate (-)-2. Unexpectedly, two subtle variables were found to be critical for obtaining reproducibly over 90% enantioselectivities in gram scale cycloadditions: (1) the moisture content (15-17% is best) of the molecular sieves used to prepare the binol-titanium complex according to the Mikami protocol and (2) the temperature (50 degrees C is best) at which the pyrone ester is mixed with the binol-titanium complex. Unsubstituted 2-pyrone undergoes ytterbium-promoted, high-pressure, regioselective, and stereoselective Diels-Alder cycloaddition with benzyl vinyl ether to form versatile bicyclic lactone (+/-)-4 on gram scale.