Progress toward a biomimetic synthesis of phomoidride B

[reaction: see text]. An intramolecular cyclization strategy for effecting a biomimetic synthesis of the core structure of the fungal secondary metabolites phomoidrides A and B is described. The cyclization substrate 20 is prepared in eight steps from dibromide 10. Treatment of 20 with triethylamine in acetonitrile results in a rapid cyclization to give 21 and 22 in good yield.     

Nature of the chemical bonding in D3h [MH3M]+ cations (M = Be,Mg)

Motivated by the particularly short metal-metal distance that has been predicted for the D_3h [BeH₃Be]⁺ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D_3h [MH₃M]⁺ cations (M = Be, Mg). CCSD(T)/cc-pVQZ calculations are used to determine optimized geometries for all of the various species, including those “capped” by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-center two-electron M─H─M bonding character instead of any significant direct metal-metal bonding.     

Guaiazulene-based phenolic radical scavengers: synthesis, properties, and EPR studies of their reaction with oxygen-centred radicals

A variety of phenolic derivatives 4, carrying the guaiazulene moiety, were prepared starting from guaiazulene. Compounds 4 react with oxygen-centred radicals exhibiting chromotropic behaviour. The radical scavenging power of these compounds was evaluated by different methods. Compounds 4 are less efficient than some of the most common radical scavengers but show quite selective behaviour towards different oxygen-centred radicals. A correlation is found between the antioxidant activity of the compounds 4 and the corresponding phenolic O-H bond dissociation energy. Some aspects of the reaction of the compounds 4 with oxygen-centred radicals were elucidated by EPR and DFT studies.     

Advanced Sunscreens: UV Absorbers Encapsulated in Sol-Gel Glass Microcapsules

Undesirable phototoxic and photoallergic reactions accompanying a justified increased use of sunscreen active ingredients within cosmetic products have encouraged the development of new products safer for human use. The sol-gel microencapsulation technology developed utilizes an interfacial polymerization process, allowing for the achievement of transparent silica glass microcapsules with sizes ranging between 0.3–3 microns and a characteristic core-shell structure. Within the sol-gel microcapsule structure a UV absorber core, constituting roughly 80% of the final product weight, is enclosed within a silica shell. These advanced sunscreen actives are then incorporated into a suitable cosmetic vehicle to achieve high Sun Protection Factors (SPF), while affording an improved safety profile, as the penetration of the UV absorbers is markedly reduced.     

Adsorption and diffusion of Mg, O, and O2 on the MgO(001) flat surface

A thorough investigation of the adsorption and diffusion of Mg, O, and O(2) on MgO(001) terraces is performed by first-principles calculations. The single Mg adatom weakly binds to surface oxygens, diffuses, and evaporates easily at room temperatures. Atomic O strongly binds to surface oxygens, forming peroxide groups. The diffusion of the O adatom is strongly influenced by the spin polarization, since energy barriers are significantly different for the singlet and triplet states. The crossing of the two Born-Oppenheimer surfaces corresponding to the distinct spin states is also analyzed. Although the O(2) molecule does not stick to the perfect surface, it chemisorbs on surface nonstoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. We show that the oxidation rate limiting factor in an O(2) atmosphere is the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers are used to estimate the Mg and O surface lifetimes and diffusion lengths, and some implications for the homoepitaxial growth of MgO are discussed.     

NMR and computational studies of the chemical reduction of [2.2]paracyclophane: formation of dianionic p-xylenyl oligomers

Reaction of [2.2]paracyclophane with K/Na alloy in THF gives a p-xylylenyl dianion together with its dimer and trimer which are relatively stable at low temperatures; at ambient temperatures further polymerization takes place.     

Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometric detection applied to the determination of pesticides in food extracts

The separation provided by conventional gas chromatography (1D-GC) can be significantly enhanced by using comprehensive two-dimensional GC (GC X GC) instead. Combination with mass spectrometric detection is desirable for unambiguous confirmation of target compounds and the provisional identification of unknowns. A GC X GC system using a cryogenic modulator was coupled to a time-of-flight mass spectrometric (TOF MS) detector. With the determination of pesticides in vegetable extracts as an example, it was demonstrated that GC X GC improves the separation dramatically. All 58 pesticides of interest could be identified using their full-scan mass spectra, which was not possible when using ID-GC-TOF MS. In addition, the high scan speed of the TOF MS allowed the deconvolution of compounds partly co-eluting in GC X GC.     

Biological weighting function of the UV-B-induced impairment of phototaxis in the freshwater ciliate Ophryoglena flava

We determined the biological weighting function (BWF) of the effect of UV radiation on phototaxis of the freshwater, histophagous ciliate Ophryoglena flava. Dose-effect curves were measured by exposing the cells to 12 different irradiation regimens obtained with two different levels of UV-B radiation and by using six filters with cutoff wavelengths ranging from 280 to 335 nm. The results show that there are significant damages to phototaxis at the doses used and that the effect increases when the cutoff is shifted toward short wavelengths. The data were used to calculate the BWF of phototaxis impairment by applying a nonlinear fit procedure. The BWF thus obtained decays exponentially with increasing wavelength in agreement with similar findings reported in the literature for other systems.     

Rhodium catalysed dehydrogenative borylation of vinylarenes and 1,1-disubstituted alkenes without sacrificial hydrogenation--a route to 1,1-disubstituted vinylboronates

The complex trans-[Rh(Cl)(CO)(PPh3)2] (1) is an efficient catalyst precursor for the dehydrogenative borylation of alkenes without consumption of half the alkene substrate by hydrogenation, giving useful vinylboronate esters including 1,1-disubstituted derviatives that cannot be made by alkyne hydroboration.