The aza-xylylene Diels-Alder approach for the synthesis of naturally occurring 2-alkyl tetrahydroquinolines

The recently discovered intramolecular aza-xylylene Diels-Alder reaction, based on a 1,4-dehydrohalogenation reaction, was extended in terms of substrates and leaving groups allowing the assembly of tetrahydroquinolines in two synthetic steps. Intramolecular cleavage of a thiocarbamate using triphenylphosphine and tetrachloromethane (Appel conditions) to give chloromethyl phenylisocyanate has been presented for the first time. The synthetic feasibility of this process was demonstrated in the first total syntheses of the alkaloids rac-Angustureine and 1-methyl-2-propyltetrahydroquinoline.     

Design,synthesis, and evaluation of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-ribitylaminolumazines as inhibitors of riboflavin synthase and lumazine synthase

A series of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-D-ribityllumazine were synthesized as inhibitors of both Escherichia coli riboflavin synthase and Bacillus subtilis lumazine synthase. The compounds were designed to bind to both the ribitylpurine binding site and the phosphate binding site of lumazine synthase. In the carboxyalkyl series, maximum activity against both enzymes was observed with the 3'-carboxypropyl compound 22. Lengthening or shortening the chain linking the carboxyl group to the lumazine by one carbon resulted in decreased activity. In the phosphonoxyalkyl series, the 3'-phosphonoxypropyl compound 33 was more potent than the 4'-phosphonoxybutyl derivative 39 against lumazine synthase, but it was less potent against riboflavin synthase. Molecular modeling suggested that the terminal carboxyl group of 6-(3'-carboxypropyl)-7-oxo-8-D-ribityllumazine (22) may bind to the side chains of Arg127 and Lys135 of the enzyme. A hypothetical molecular model was also constructed for the binding of 6-(2'-carboxyethyl)-7-oxolumazine (15) in the active site of E. coli riboflavin synthase, which demonstrated that the active site could readily accommodate two molecules of the inhibitor.     

Long-lived triplet charge-separated state in naphthalenediimide based donor–acceptor systems

1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor–acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π–π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet–triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor–acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.

Ultrafast electron transfer from singlet and triplet excited states in equilibrium results in the population of both singlet and triplet charge-separated states.     

Proton to carbon-13 INEPT in solid-state NMR spectroscopy

A refocused INEPT through-bond coherence transfer technique is demonstrated for NMR of rigid organic solids and is shown to provide a valuable building block for the development of NMR correlation experiments in biological solids. The use of efficient proton homonuclear dipolar decoupling in combination with a direct spectral optimization procedure provides minimization of the transverse dephasing of coherences and leads to very efficient through-bond (1)H-(13)C INEPT transfer for crystalline organic compounds. Application of this technique to 2D heteronuclear correlation spectroscopy leads to up to a factor of 3 increase in sensitivity for a carbon-13 enriched sample in comparison to standard through-bond experiments and provides excellent selectivity for one-bond transfer. The method is demonstrated on a microcrystalline sample of the protein Crh (2 x 10.4 kDa).     

Identification of odor signature chemicals in cocaine using solid-phase microextraction-gas chromatography and detector-dog response to isolated compounds spiked on U.S. paper currency

Solid-phase microextraction (SPME) combined with gas chromatography (GC) is optimized and applied to the analysis of street-cocaine samples followed by the field-testing of isolated chemicals using certified detector dogs. SPME proves to be a very sensitive and rapid method for isolating odor chemicals from street-cocaine samples. SPME-GC and activated charcoal strip (ACS)-SPME-GC signature profile methods are developed for the detection and quantitation of cocaine-odor chemicals, including the optimization of controllable variables such as fiber chemistry, extraction time, and desorption time. The volatile odor chemicals in representative illicit cocaine samples are identified and quantitated by the ACS-SPME-GC signature profile method and direct injection. Field tests with drug detector dogs show methyl benzoate to be the dominant signature odor chemical along with cocaine on U.S. currency at a threshold level of approximately 1-10 microg when spiked or when 10 ng/s methyl benzoate is diffused from polymer bottles, which is required in order to initiate an alert. No other substance studied initiated consistent responses by the drug dogs. The results indicate that the microgram levels of cocaine that have been reported on circulated U.S. currency are insufficient to signal an alert from law-enforcement trained drug detector dogs.     

Reversed-phase high-performance liquid chromatography-thermospray mass spectrometry of radiation-induced decomposition products of thymine and thymidine.

High-performance liquid chromatography-thermospray mass spectrometry was applied to the analysis of various radiation-induced decomposition products of thymidine including N-(2-deoxy-beta-D-erythro-pentofuranosyl)formamide and the various diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine, 5-hydroxy-5,6-dihydrothymidine and 5,6-dihydrothymidine. This method combines high sensitivity and product resolution, rendering it particularly useful for monitoring the formation of radiation-induced base damage within DNA.     

The solvent extraction of Cu(II), Ni(II) and Co(II) with benzil mono(2-quinolyl)hydrazone

Benzil mono(2-quinolyl)hydrazone, BmQH, has been studied as an extracting agent for Cu(II), Ni(II) and Co(II). Though the uncomplexed ligand remains undissociated in the pH range 3.5-10, it can lose a proton on complexation with metals, owing to the electron-withdrawing effects of neighbouring groups. The dependence of degree of extraction on pH indicates that complexes of both Cu(2+) and Cu(OH)(+) are extracted. Cu(BmQH)(2) and Cu(OH)BmQH species are extracted into MIBK, and the Cu(OH)BmQH complex is extracted into benzene. In the vicinity of pH 5.5-6, extraction efficiencies greater than 95% can be achieved with both solvents. Both Ni(II) and Co(II) also show dependence of extraction on pH, but precipitation of both metals in the vicinity of pH 6 limits further studies.     

Detection of adsorption of Ru(II) and Os(II) polypyridyl complexes on gold and silver nanoparticles by single-photon counting emission measurements

The present study describes a new application of ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) and osmium(II) tris(bipyridine) (Os(bpy)3(2+)) as phosphorescent labels for the quantification of surface binding of molecules to gold and silver nanoparticles. The fraction of Ru(bpy)3(2+) and Os(bpy)3(2+) that is in solution can be distinguished from the surface-bound fraction by the relative lifetimes and integrated emission yields as determined by time-correlated single-photon counting (TCSPC) spectroscopy. Complementary steady-state measurements were carried out to confirm surface attachment of the phosphorescent label molecules. Although the emission of solutions of Ru(bpy)3(2+) and Os(bpy)3(2+) is quenched proportional to the concentration of 10 nm Au or 20 nm Ag nanoparticles, the quenching is static and not diffusional quenching observed in Stern-Volmer plots. The results demonstrate that time-resolved spectroscopy provides a rapid method for the measurement of surface binding of labeled molecules on metallic nanoparticles. While steady-state measurements require the preparation of a series of samples with varying quencher concentrations and a reference, the method described herein requires a single sample plus reference. The mechanism for phosphorescence quenching on Au and Ag nanoparticles is discussed in terms of energy and electron transfer theories.     

Heterobimetallische phosphanido-verbrückte Zweikern-Komplexe - Synthese von cis-rac-[(η-C5H4R)2Zr{μ-PH(2,4,6−iPr3C6H2)}2M(CO)4] (RMe,MCr,Mo; RH,MMo)

Heterobimetallic Phosphanido-bridged Dinuclear Complexes - Syntheses of cis-rac-[(η-C₅H₄R)₂Zr{μ-PH(2,4,6-ⁱPr₃C₆H₂)}₂M(CO)₄] (R?Me, M?Cr, Mo; R?H, M?Mo) The zirconocene bisphosphanido complexes [(η-C₅H₄R)₂Zr{PH(2,4,6-ⁱPr₃C₆H₂)}₂] (R?Me, H) react with [(NBD)M(CO)₄] (NBD?norbornadiene, M?Cr, Mo) to give only one diastereomer of the phosphanido-bridged heterobimetallic dinuclear complexes cis-rac-[(η-C₅H₄R)₂Zr{μ-PH(2,4,6-ⁱPr₃C₆H₂)}₂M(CO)₄] [R?Me, M?Cr ( 1 ), Mo ( 2 ); R?H, M?Mo ( 3 )]. However, no reaction was observed between [(η-C₅H₅)₂Zr{PH(2,4,6-^tBu₃ C₆H₂)}₂] and [Pt(PPh₃)₄]. 1—3 were characterised spectroscopically. For 1—3 , the presence of the racemic isomer was shown by NMR spectroscopy. No reaction was observed at room temperature for 3 and CS₂, (NO)BF₄, Me₃NO or PH(2,4,6-Me₃C₆H₂)₂. With Et₂AlH or PhC?CH decomposition of 3 was observed.     

Matched/Mismatched interaction of a cyclic hexapeptide with ion pairs containing chiral cations and chiral anions

The binding of a chiral quaternary ammonium ion to a cyclopeptide containing aromatic amino acid subunits is affected not only by the configuration of the cation but also by the configuration of the chiral counterion. Analysis of the binding equilibria shows that complex formation involves interaction of the whole ion pair with the host indicating that steric requirements of the anion influence complex geometry and stability.