Stereoselective synthesis of polyhydroxylated pyrrolidines: a route to novel 3,5-bis(hydroxymethyl)pyrrolidines from 2-azabicyclo[2.2.1]hept-5-enes

An efficient preparation of racemic and chiral 2-functionalized-3,5-bis(hydroxymethyl)pyrrolidines is described. The method uses 2-azabicyclo[2.2.1]hept-5-enes, readily obtained from glyoxylates of aliphatic amines and cyclopentadiene, as starting material. The hydroxylation of the double bond followed by the oxidative cleavage of the six-membered ring and in situ reduction of the dialdehyde intermediate gives the title pyrrolidines.     

Slurry sampling graphite furnace atomic absorption spectrometry: determination of trace metals in mineral coal

A procedure for lead, cadmium and copper determination in coal samples based on slurry sampling using an atomic absorption spectrometer equipped with a transversely heated graphite tube atomizer is proposed. The slurries were prepared by weighing the samples directly into autosampler cups (5-30 mg) and adding a 1.5 ml aliquot of a diluent mixture of 5% v/v HNO(3), 0.05% Triton X-100 and 10% ethanol. The slurry was homogenized by manual stirring before measurement. Slurry homogenization using ultrasonic agitation was also investigated for comparison. The effect of particle size and the use of different diluent compositions on the slurry preparation were investigated. The temperature programmes were optimized on the basis of pyrolysis and atomization curves. Absorbance characteristics with and without the addition of a palladium-magnesium modifier were compared. The use of 0.05% m/v Pd and 0.03% m/v Mg was found satisfactory for stabilizing Cd and Pb. The calibration was performed with aqueous standards. In addition, a conventional acid digestion procedure was applied to verify the efficiency of the slurry sampling. Better recoveries of the analytes were obtained when the particle size was reduced to <37 mum. Several certified coal reference materials (BCR Nos. 40, 180, and 181) were analyzed, and good agreement was obtained between the results from the proposed slurry sampling method and the certificate values.     

Fate of benzoate paralytic shellfish poisoning toxins from Gymnodinium catenatum in shellfish and fish detected by pre-column oxidation and liquid chromatography with fluorescence detection

Several cultured strains of Gymnodinium catenatum isolated worldwide have been shown to produce important proportions of the recently discovered benzoate paralytic shellfish poisoning (PSP) toxins GC1 through GC3. These toxins pose a new challenge for the HPLC analysis of shellfish predating during blooms of this microalga because due to their hydrophobicity are retained along the C18 solid-phase extraction step employed to eliminate interferences. The production of GC toxins was confirmed in a clone of G.catenatum isolated from the Portuguese Northwest coast during the winter bloom of 2005, in addition to a clone from 1989 reported previously by other authors. The major peroxide oxidation products of GC1+2 and GC3 were, respectively, dcGTX2+3 and dcSTX. The search of benzoate analogues in bivalves contaminated during the winter 2005 bloom showed these analogues constituted a minor component of the N(1)-H containing toxins, as selectively detected by peroxide oxidation. While in G.catenatum GC1-3 were the major components after C1+2 and B1, in bivalves dcGTX2+3 and dcSTX were the major components after C1+2 and B1. Similar conclusions were later extended to more shellfish species naturally contaminated during the autumn bloom of 2007. In the gut content of sardines GC toxins were present, while in crabs predating upon shellfish, these were absent. A generalised conversion of GC toxins into decarbamoyl analogues was confirmed by in vitro incubations of bivalve's digestive glands with semi-purified GC toxins. This is the first report of widespread carbamoylase activity in shellfish, exclusively targeted at benzoate PSP analogues and that is heat-inactivated. Despite the high proportion of benzoate analogues produced by G.catenatum, analyses of bivalves contaminated with PSP toxins seem to be simplified due to the important conversion of benzoate into decarbamoyl analogues that occurs in bivalves. These last analogues are detected by common HPLC methods used for food protection.     

Amino acids as selective acylating agents: regioselective N-acylation of imidazolidin-4-one derivatives of the antimalarial drug primaquine

The acylation of bioactive primaquine-based imidazolidin-4-ones was studied using N^α-Boc-protected glycine as acylating agent. Two synthesis routes, eight different coupling methods and seven distinct solvents were compared. Mild carbodiimide-based couplings on high dielectric constant solvents such as DMF or MeCN increased acylation yields, whereas alcohols inhibited carbodiimide-mediated acylations to take place. Achievement of the synthetic goals was limited by the size of the imidazolidin-4-one ring substituents R¹, R² and R³, but resort to MW-assisted synthesis allowed overcoming such obstacle, though with very modest reaction yields. All reactions involving a Boc-protected amino acid were regioselective, independent of reaction conditions employed. In contrast, regioselective acetylation of the imidazolidin-4-ones could only be achieved by resort to very mild coupling procedures.     

Method development for the determination of lead in wine using electrothermal atomic absorption spectrometry comparing platform and filter furnace atomizers and different chemical modifiers

A method has been developed for the determination of lead in wine by electrothermal atomic absorption spectrometry without any sample preparation and calibration against aqueous standards, using 7.5 μg Pd as a chemical modifier. The results obtained for seven wines using the proposed method and an acid digestion procedure did not show any significant difference using a Student's t-test. Atomization in a transversally heated filter atomizer (THFA) was compared with atomization in a conventional transversally heated platform furnace. The former provided a 2.6-fold higher sensitivity, improving the characteristic mass from 34 to 12 pg and a 1.6-fold better limit of detection (0.3 μg L⁻¹ compared to 0.5 μg L⁻¹) for aqueous solutions using the same injection volume of 20 μL. However, the average precision, expressed as the relative standard deviation for the determination of lead in wine under routine conditions was improved from 4.6% with platform atomization to 0.6% in the THFA. The lead content found in seven arbitrarily chosen white and red wines, five from Brazil, one from Chile and one from Spain, ranged from 6 to 60 μg L⁻¹ Pb with an average content of 11.4 μg L⁻¹ Pb for the wines from South America.     

Tight frames generated by finite nonabelian groups

Let $\cal H$ be a Hilbert space of finite dimension d, such as the finite signals ? ₂(d) or a space of multivariate orthogonal polynomials, and n?≥?d. There is a finite number of tight frames of n vectors for $\cal H$ which can be obtained as the orbit of a single vector under the unitary action of an abelian group G (of symmetries of the frame). Each of these so called harmonic frames or geometrically uniform frames can be obtained from the character table of G in a simple way. These frames are used in signal processing and information theory. For a nonabelian group G there are in general uncountably many inequivalent tight frames of n vectors for $\cal H$ which can be obtained as such a G-orbit. However, by adding an additional natural symmetry condition (which automatically holds if G is abelian), we obtain a finite class of such frames which can be constructed from the character table of G in a similar fashion to the harmonic frames. This is done by identifying each G-orbit with an element of the group algebra ?G (via its Gramian), imposing the condition in the group algebra, and then describing the corresponding class of tight frames.     

Dirac and Majorana heavy neutrinos at LEP II

The possibility of detecting single heavy Dirac and/or Majorana neutrinos at LEP II is investigated for heavy neutrino masses in the range . We study the process as a clear signature for heavy neutrinos. Numerical estimates for cross sections and distributions for the signal and the background are calculated and a Monte Carlo reconstruction of final state particles after hadronization is presented. Received: 20 September 2001 / Published online: 12 November 2001     

Decomposition of Fluorinated Graphene under Heat Treatment

Fluorination modifies the electronic properties of graphene, and thus it can be used to provide material with on‐demand properties. However, the thermal stability of fluorinated graphene is crucial for any application in electronic devices. Herein, X‐ray photoelectron spectroscopy (XPS), temperature‐programmed desorption (TPD), and Raman spectroscopy were used to address the impact of the thermal treatment on fluorinated graphene. The annealing, at up to 700 K, caused gradual loss of fluorine and carbon, as was demonstrated by XPS. This loss was associated with broad desorption of CO and HF species, as monitored by TPD. The minor single desorption peak of CF species at 670 K is suggested to rationalize defect formation in the fluorinated graphene layer during the heating. However, fluorine removal from graphene was not complete, as some fraction of strongly bonded fluorine can persist despite heating to 1000 K. The role of intercalated H₂O and OH species in the defluorination process is emphasised.