Synthese monodisperser linearer und cyclischer Oligomere der (R)-3-Hydroxybuttersäure mit bis zu 128 Einheiten

Monodisperse Linear and Cyclic Oligo[(R)-3-hydroxybutanoates] Containing up to 128 Monomeric Units Using benzyl ester/(tert-butyl)diphenylsilyl ether protection, (COCl)₂/pyridine esterification conditions, and a fragment-coupling strategy (with H₂/Pd-C debenzylation and HF · pyridine desilylation), linear oligomers of (R)-3-hydroxybutanoic acid (3-HB) containing up to 128 3-HB building blocks (mol. weight > 11 000 Da) are assembled (Schemes 1,2,5, and 6). In contrast to the previously employed protecting-group combination, and due to the low-temperature esterifying conditions, this procedure leads to monodisperse oligomers: all steps occur without loss of single 3-HB units. The product oligomers with two, one, and no terminal protecting groups (mostly prepared in multi-gram amounts) are characterized by all standard spectroscopic methods, especially by mass spectroscopy (Figs. 2 and 3), by their optical activity, and by elemental analyses. Cyclization of the oligo[(R)-3-hydroxybutanoic acids] with up to 32 3-HB units, using thiopyridine activation and CuBr₂ for the ring closure, produces oligolides consisting of up to 128 ring atoms (Scheme 7). Mixed oligolides containing 3-HB and (R)-3-hydroxypentanoic units are prepared from the corresponding linear trimers, using Yamaguchi's method for the ring closure (Scheme 8 and Fig.4 (X-ray crystal structures of two folded conformers)). Comparisons of melting points (Table 1), of [α] values (Tables 2 and 3), of ¹H-NMR coupling constants (Table 3), and of molecular volume/hydroxyalkanoate unit (Table 4) of linear and cyclic oligomer derivatives and of the high-molecular-weigh polymer show that the monodisperse oligomers appear to be surprisingly good models for the polymer. Besides this insight, our synthesis is supplying the samples to further test the role of P(3-HB) (ca. 140 units) as a component of complexes forming channels through cell-wall phospholipid bilayers.     

Stir bar sorptive extraction based on restricted access material for the direct extraction of caffeine and metabolites in biological fluids

A biocompatible stir bar sorptive extraction (SBSE) device was prepared using an alkyl-diol-silica (ADS) restricted access material (RAM) as the SBSE coating. The RAM-SBSE bar was able to simultaneously fractionate the protein component from a biological sample, while directly extracting caffeine and its metabolites, overcoming the present disadvantages of direct sampling in biological matrices by SBSE, such as fouling of the extraction coating by proteins. Desorption of the analytes was performed by stirring the bar in a water/ACN mixture (3/1, v/v) and subsequently reconcentrating the sample solution in water to enable HPLC-UV analysis to be performed. The limit of detection, based on a signal to noise ratio of 3, for caffeine was 25 ng/mL in plasma. The method was confirmed to be linear over the range of 0.5-100 microg/mL of caffeine with an average linear coefficient (R2) value of 0.9981. The injection repeatability and intra-assay precision of the method were evaluated over ten injections, resulting in a %RSD of approximately 8%. The RAM-SBSE device was robust (>50 extraction in plasma without significant signal loss) and simple to use, providing many direct extractions and subsequent determination of caffeine and its metabolites in biological fluids. In contrast to existing sample preparation methods for the analysis of caffeine and selected metabolites in biological fluids, this feasibility study using a biocompatible SBSE approach was advantageous in terms of simplifying the sample preparation procedures.     

Perfluoralkyljod-verbindungen: das trifluormethyljod-bis-trifluoracetat CF3J(OCOCF3)2

CF₃I(OCOCF₃)₂ is formed from the reactions of CF₃IF₂ or CF₃IO with trifluoroacetic anhydride at low temperature. During the reaction of CF₃IF₂ with (CF₃CO)₂O CF₃IF(OCOCF₃) is formed as an intermediate product. The preparation, the ¹⁹F-nmr spectra and the thermal decomposition are described.     

In situ observations of water production and distribution in an operating H2/O2 PEM fuel cell assembly using 1H NMR microscopy

Proton NMR imaging was used to investigate in situ the distribution of water in a polymer electrolyte membrane fuel cell operating on H2 and O2. In a single experiment, water was monitored in the gas flow channels, the membrane electrode assembly, and in the membrane surrounding the catalysts. Radial gradient diffusion removes water from the catalysts into the surrounding membrane. This research demonstrates the strength of 1H NMR microscopy as an aid for designing fuel cells to optimize water management.     

Diastereoselectivity of the mercuration of acyclic allylic alcohols

The diastereoselectivity of the mercuration of acyclic alkenes $\text{4}$ can be reversed by changing the substituent in the allylic position; with alcohols the erythro isomers $\text{5}$ and with esters or hemiacetals the threo isomers $\text{6}$ and $\text{8}$ are formed predominantly (Table I).     

Fractional revivals in the rovibrational motion of I2

Motivated by pump-probe experiments of I(2) in a room-temperature sample, the detection of fractional revivals is investigated using full-dimensional quantum wave packet calculations. It is shown that the structures observed in the pump-probe signal depend sensitively on the probe parameters employed and that the observed signal reflects a particular phase effect between fractional revivals.     

Untersuchungen über ternäre Chalkogenide. XXIV. Zur Struktur des TlGaSe2

On Ternary Chalcogenides. XXIV. The Structure of TlGaSe₂ TlGaSe₂ is monoclinic with a = 10.772(3), b = 10.771(5), c = 15.636(8) Å, ß = 100.6(3)°, Z = 16, space group Cc. The refinement yielded an R value of 0.082 for 1366 unique observed reflections (MoKα). The compound crystallizes with a layer structure with two anion layers perpendicular to c* in the unit cell. The structural motive of the layers are large corner-linked Ga₄Se₁₀- tetraeder consisting of four corner-linked GaSe₄ tetrahedra. Two adjacent layers are turned relative to each other by 90°. They are kept together by Tl¹⁺ ions, which are situated on straight lines parallel to the edges of the Ga₄Se₁₀ groups. They are surrounded by six selenium atoms forming trigonal-prismatic TlSe₆ polyhedra. 4₂ screw-axes within the layers are the reason for a strongly pronounced pseudo tetragonal symmetry. Different possibilities of stacking cause a onedimensional disorder along the c* direction. The relations between the TlGaSe₂ type and the TlSe type and also the high-pressure modifications of some compounds with the TlGaSe₂ structure are discussed.     

Synthese und thermische Eigenschaften von molekularen Clusterverbindungen zusammengesetzt aus Elementen der Gruppen 11‐13‐16

Syntheses and Thermal Properties of Cluster Molecules, formed from Groups 11‐13‐16 Elements In the presence of PPh₃, CuX (X = Cl, CH₃COO) or AgOC(O)C₆H₅ and GaCl₃ react in THF with S(SiMe₃)₂ or Se(SiMe₃)₂ to yield [Cu₆Ga₈Cl₄S₁₃(PPh₃)₆] ( 1 ), [Cu₆Ga₈Cl₄Se₁₃(PPh₃)₆] ( 2 ), [Ag₆Ga₈Cl₄S₁₃(PPh₃)₆] ( 4 ) and [Ag₆Ga₈Cl₄Se₁₃(PPh₃)₆] ( 5 ). The use of PⁿPr₂Ph instead of PPh₃ and subsequent layering with n‐hexane leads to the formation of the cluster [Cu₆Ga₈Cl₄Se₁₃(PⁿPr₂Ph)₁₂] ( 3a , 3b ). Reaction of CuCl, GaCl₃ and PⁿPr₃ with Se(SiMe₃)₂ in THF results in the crystallisation of the ionic cluster (HPⁿPr₃)₂[Cu₂Ga₄Cl₄Se₆(PⁿPr₃)₄] ( 6 ). The structures of 1 — 6 were determined by X‐ray single crystal structure analysis. Thermogravimetric measurements of the cluster molecules and powder diffraction patterns of the remaining powders reveal the potential use of them as single source precursor compounds for the synthesis of the related ternary solid state materials.     

Zur Reaktion des Germaniums mit Germaniumtetrachlorid und zur Existenz der Oxidchloride GeOCl2 und GeOCl6 im Gleichgewicht

On the Reaction of Germanium with Germanium Tetrachloride and on the Existence of the Oxidechlorides GeOCl₂ and GeOCl₆ at the Equilibrium The above equilibrium was measured in a Gauge-manometer and the enthalpy of formation and the entropy for GeCl₂,g were derived from these results. From total pressure measurements and transport experiments of GeO₂ with GeCl₄ follows, that the oxidechlorides Ge₂OCl₆ and GeOCl₂ are not present in measurable concentrations, that means the enthalpy of formation of GeOCl₂ must be higher than ?327 kJ · mol^?1.     

Enantioselektive Synthese von Allyl-, Propargyl- und 4-En-2-inyl-aminen durch 1,2-Addition von Organocer-Reagenzien an chirale Aldimine

Enantioselective Synthesis of Allyl-, Propargyl-, and 4-En-2-ynyl-amines via 1,2-Addition of Organocerium Reagents to Chiral Aldehyde Imines (E)- and (Z)-Allyl-, propargyl-, and 4-en-2-ynyl-amines 5 and 14 , useful bifunctional building blocks and of pharmaceutical interest, are synthesized in high enantiomeric purity (e.e. ≥ 97%). Key step is the diastereoselective 1,2-addition (d.e. 86 to ≥ 98%) of organocerium reagents to chiral α,β-unsaturated aldehyde imines 3 or 8 to produce adduct amines 4 and 9 (Schemes 1 and 4, resp.). The propargylamine 9 is a substrate for Pd-catalyzed coupling with alkenyl halides to produce the enynylamine 11a and the thienyl-substituted alkynylamine 11b . The chiral auxiliary (S,S)- 2 is removed from 4 and 11 in 3 steps affording the title compounds 5 and 14 . Diastereoisomer enrichment of the hydrochloride of 6 by crystallization is possible.