Synthesis and reactivity of a series of [14]azaannulenes

Using a metal template reaction, a series of [14]azaannulenes formed from a β-diketone and an aromatic diamine in the presence of nickel(II) have been synthesized. Attempted cyclizations with bulky β-diketones and certain substituted diamines showed that the cyclization is sensitive to steric effects. At least one of the nickel complexes readily reacts with a series of nucleophilic reagents to give methine-substituted products.     

Diastereoselective intermolecular Pauson-Khand reactions of chiral cyclopropenes

In this Letter, it is demonstrated that the unusual reactivity of cyclopropenes can increase the scope and utility of intermolecular Pauson-Khand reactions. The well-defined chiral environment of cyclopropenes has a powerful influence on the diastereoselectivity of the reactions and leads to the production of a single cyclopentenone in each of the described cases. The cyclopropane ring strongly influences the stereochemistry of reactions at the enone, and the three-membered ring can subsequently be cleaved under mild conditions. [reaction: see text]     

H+, Na+ and K+ modulated lanthanide luminescent switching of Tb(III) based cyclen aromatic diaza-crown ether conjugates in water

The cyclen based aromatic diaza-15-crown-5 and 18-crown-6 ether conjugates 1Tb-4Tb were designed as luminescent switches for sodium and potassium where the delayed Tb(III) emission, occurring as line-like emission bands between 490-622 nm, was 'switched on' upon recognition of these ions in pH 7.4 buffered water solution.     

Synthesis of ansamycins: An approach to the naphthoquinone portion of the rifamycins and streptovaricins

Naphthoquinone $\text{1}$, a fully functionalized model for intermediates in ansamycin synthesis, was prepared by the intramolecular Claisen condensation of benzofuran $\text{9}$ followed by oxidation of the novel tricyclic compound $\text{10}$.     

Copper coupling of 1-chloro-2-iodo- and 1,2-di-iodo-perfluorocycloalkenes

Reductive coupling of 1-chloro-2-iodoperfluorocyclobutene (I), cyclopentene (III) and cyclohexene (VI) has been carried out with copper powder and a trace amount of dimethylformamide to give the corresponding 2,2′-dichloroperfluoro-(bi-1-cycloalken-1-yl) derivatives in 69%, 36% and 73% yield, respectively. A reduced by-product, 2H,2′H-dodecafluoro-(bi-1-cyclopenten-1-yl) was also obtained in the copper coupling of (III). Coupling of 1,2-di-iodoperfluorocyclobutene (IX) and cyclopentene (XII) under similar conditions gave good yields of cyclic trimers and tetramers. Mass spectra, infrared and ¹⁹F NMR data are discussed.     

Electron impact and chemical ionization mass spectra of some unsaturated anomeric C-glycosides

Electron impact mass spectra at 70 eV electron energy and chemical ionization mass spectra with ammonia as the reagent gas are reported for certain unsaturated C-glycosides. Comparisons are made between the mass spectra of anomeric pairs of these glycosides.     

Liquid chromatographic determination of diminazene diaceturate (Berenil) in raw bovine milk

A simple liquid chromatographic (LC) method is presented for the determination of diminazene (DZ) in raw bovine milk. DZ is extracted from raw milk by chilled aqueous centrifugation and is isolated from milk components on a cyano solid-phase extraction column. DZ is eluted by using a methanol-ion pairing reagent. A Phenomenex LUNA CN column and an acetonitrile-buffered mobile phase with a counter ion are used for gradient LC. The LC effluent is monitored at a detection wavelength of 372 nm by using a deuterium lamp. Under the parameters described, the retention time of DZ is 8-10 min with a peak area response of 6.5 mAU/ng. The method demonstrated excellent precision over all levels tested (25-400 ppb) with an overall average recovery of 90.4 +/- 14.5%. The method is applicable to the monitoring of milk for DZ residues at the 25 ppb level with a limit of quantitation of 10 ppb.     

Identification of selenium species in urine by ion-pairing HPLC–ICP–MS using laboratory-synthesized standards

This study focused on the detection/identification of possible selenium metabolites in human urine. Organoselenium compounds not commercially unavailable were synthesized and characterized by electrospray mass spectrometry. Separation of selenomethionine, methylselenomethionine, trimethylselonium, selenoethionine, and selenoadenosylmethionine was achieved by ion-pairing HPLC with a mobile phase of 2 mmol L^–1 hexanesulfonic acid, 0.4% acetic acid, 0.2% triethanolamine (pH 2.5), and 5% methanol. The column effluent was introduced on-line to inductively coupled plasma–mass spectrometry for selenium-specific detection (⁷⁷Se and ⁷⁸Se). For selenium speciation in urine, solid-phase extraction was carried out using C₁₈ cartridges modified with hexanesulfonic acid. Selective retention of cationic species was observed from acidified urine (perchloric acid, pH 2.0). After elution with methanol, evaporation, and dissolution in the mobile phase, the sample was introduced to the HPLC–ICP–MS system and the chromatographic peaks were assigned by adding standards. The species identified in urine were selenomethionine, trimethylselonium ion, and selenoadenosylmethionine. The last species was detected for the first time and our results suggest that selenomethionine might enter the metabolic pathway of its sulfur analog in the activated methylation cycle.Kazimierz Wrobel and Katarzyna Wrobel are on the leave from the Institute of Scientific Research, University of Guanajuato, L. de Retana No. 5, 36000 Guanajuato, Gto., Mexico     

Pulsed fluorescence measurements of trapped molecular ions with zero background detection

Sensitive methods have been developed to measure laser-induced fluorescence from trapped ions by reducing the detection of background scattering to zero levels during the laser excitation pulse. The laser beam diameter has been reduced to approximately 150 microm to eliminate scattering on trap apertures and the resulting laser-ion interaction is limited to a volume of approximately 10(-5) cm which is approximately 0.03-0.15 of the total ion cloud volume depending on experimental conditions. The detection optics collected fluorescence only from within the solid angle defined by laser-ion interaction volume. Rhodamine 640 and Alexa Fluor 350 ions, commonly used as fluorescence resonance energy transfer (FRET) fluorophores, were generated in the gas phase by using electrospray ionization and injected into a radiofrequency Paul trap where they were stored and exposed to Nd:YAG laser pulses at 532 and 355 nm for times up to 10 m. Fluorescence emitted by these ions was investigated for several trap q(z) values and ion cloud temperatures. Analysis of photon statistics indicated an average of approximately 10 photons were incident on the PMT detector per 15 ns pulse for approximately 10(3) trapped ions in the interaction volume. Fluorescence measurements displayed a dependence on trapped ion number which were consistent with calculations of the space charge limited ion density. To investigate the quantitative capability of these fluorescence techniques, the laser-induced fragmentation of trapped Alexa Fluor 350 ions was measured and compared with a rate equation model of the dynamics. Decay of the fluorescence signal as well as the parent ion number compared closely with quantitative predictions of the photofragmentation model.     

A new tri-orthogonal strategy for peptide cyclization

A solid phase tri-orthogonal protection/cleavage strategy that uses acidic, basic, and neutral conditions is described. Strategically protected alpha-azido-gamma-9-fluorenylmethyl-L-glutamate (1) and alpha-azido-epsilon-N-Fmoc-L-lysine (2) were incorporated into growing peptides on Wang resin using a novel azide protection strategy. These residues, separated by 1-3 monomers, were deprotected at the side chains and cyclized via lactam formation. The N-terminus was further functionalized to extend the chain. This method represents a straightforward protocol for peptide cyclization on solid support.