A new class of non-classical azapentalenes. Synthesis and c-13/n-15 NMR characterization of 2H-imidazo[1, 5-d]tetrazoles

The title compounds 6 (as well as the oxazoles 5 ) were obtained by thermolysis of 2-alkyl-5-(methoxycarbon-yldiazomethyl)tetrazoles 3 in benzonitrile. They were fully characterized by ¹H, ¹³C and ¹⁵N nmr spectroscopy.     

Porphotetramethenes with 1,3-alternate conformation of pyrrole rings from oxidative N-alkylation of porphyrin tetraphenols

Tetrapyrrole macrocycles, of which the pyrroles are connected by sp2 centers, were readily obtained from porphyrin tetraphenols by air oxidation under basic conditions, followed by N-alkylation. The degree of N-alkylation could be controlled and either di- or tetraalkylated derivatives are obtained in high yields.     

Atmospheric photooxidation of isoprene part II: The ozone-isoprene reaction

A series of experiments have been performed to study the ozone-isoprene reaction in a smog chamber by adding externally produced O₃ to the hydrocarbon in the dark. A chemical tracer, methyl cyclohexane, was added to probe the OH formation in the system. O(³P) formation was also examined using the known distribution of products that are unique to the O(³P)-isoprene reaction (part I). The results provide clear evidence that both OH and O(³P) are produced by the O₃-isoprene reaction directly in large quantities; about 0.68 ± 0.15 and 0.45 ± 0.20 per O₃-isoprene reaction, respectively. These additional radicals severely complicate the analysis of the O₃ reaction, hence, computer kinetic modeling was necessary to ascertain the products of the O₃ reaction itself, corrected for OH and O(³P) reactions. The product distribution, which differs dramatically from that published previously, is: 67 ± 9% methacrolein, 26 ± 6% methyl vinyl ketone, and 7 ± 3% propene, accounting for 100 ± 10% of the reacted isoprene. Applicability of these results to the gas-phase O₃ reaction with other unsaturated hydrocarbons is briefly discussed.     

A new synthesis of alkyl fluorides

Reduction of -fluorosulfides with sodium in alcohol results in the formation of fluoroalkanes.     

An extension of the small-slope approximation for rough surface scattering

Solutions are derived for scattering from a rough one-dimensional pressure-release surface in the form of a functional series in the surface slope. These solutions are obtained by solving an integral equation of the first kind for the surface potential to obtain a representation for the scattering amplitude. It is shown that the subsequent expansion of terms occurring in the scattering amplitude to obtain a functional series in the slope does not yield a unique result. The result obtained contains a free parameter that may be arbitrarily selected. Thus, this result is an extension or generalization of the small-slope approximation of Voronovich (1985 Sov. Phys.-JETP 62 65-70) that differs at second order in the slope from his result. It is also shown that the free parameter can be selected such that each term of the functional series is reciprocal and exhibits a limiting grazing angle dependence consistent with the requirements of flux conservation and the absence of boundary waves. A new formulation of the leading terms of the small-slope expansions is derived and is used to explore the conditions under which the two expansions reduce to the Kirchhoff approximation. Finally, a numerical example is presented to demonstrate that the extended approximation provides corrections that are important for near grazing scatter.     

Atmospheric photooxidation of isoprene part I: The hydroxyl radical and ground state atomic oxygen reactions

The OH reaction with isoprene is studied. Methyl nitrite photolysis experiments were carried out in an outdoor smog chamber in an attempt to identify as completely as possible OH-isoprene product distribution. Emphasis was placed on identification and quantification of oxygenated products. A Tenax-based cryo-trap thermal desorber used to trap, concentrate, and dry chamber samples for identification on a GC/MS is described. Analysis of the products revealed that O(³P) can form in reaction systems designed to study OH reactions that include high concentrations of NO, and consequently NO₂, hence, this reaction is also examined. The yields of methacrolein and methyl vinyl ketone are determined as 25 ± 3 and 35.5 ± 4%, respectively, with an additional 4 ± 2% as 3-methyl furan, totaling 65 ± 4%. These results, combined with those of previous studies allow 80% of isoprene's products to be explicitly identified, and the general structure of the remaining products to be ascertained. The O(³P) reaction produces 84 ± 8% epoxides, and 8 ± 3% species which result in production of HO₂, and subsequently, OH. A heretofore unidentified product of the O(³P) reaction, 2-methyl 2-butenal, is identified. The rate constant of the NO₂-isoprene reaction is measured.     

The chip firing game onn-cycles

Spencer introduced a chip firing game on a line, which was generalized by Björner, Lovasz and Shor to graphs. This paper characterizes those configurations which may be repeated in a game, and analyzes the game forn-cycles.This research was funded by NSERC operating grants.     

Optimal control for parabolic systems with competitive interactions

We consider a two-dimensional parabolic system with general competitive interactions as a two-player game with conflicting objectives and with controls on the inhomogencous (source) terms. We show the existence of an optimal solution of the game as the saddle point of a suitable objective functional.     

Enantioselective oxidation of di-tert-butyl disulfide with a vanadium catalyst: progress toward mechanism elucidation

The mechanism of the oxidation of di-tert-butyl disulfide (1) to the chiral thiosulfinate (2) by H(2)O(2) catalyzed by bis(acetylacetonato)oxovanadium and a chiral Schiff-base ligand (3) has been investigated. Techniques included (51)V NMR spectroscopy, solvent effects on reaction enantioselectivity, and the isolation and full characterization of a 2:1 ligand-to-vanadium catalyst precursor. A model for the dramatic solvent effect on the enantioselectivity of this reaction was developed, based on the identification of a competing nonselective oxidation pathway. From this model, strategies for limiting this competing pathway were developed.     

Tackling the chemical diversity of microbial nonulosonic acids – a universal large-scale survey approach

Nonulosonic acids, commonly referred to as sialic acids, are a highly important group of nine-carbon sugars common to all domains of life. They all share biosynthetic and structural features, but otherwise display a remarkable chemical diversity. In humans, sialic acids cover all cells which makes them important for processes such as cellular protection, immunity and brain development. On the other hand, sialic acids and other nonulosonic acids have been associated with pathological processes including cancer and viral infections. In prokaryotes, nonulosonic acids are commonly associated with pathogens, which developed through molecular mimicry a strategy to circumvent the host''s immune response. However, the remarkably large chemical diversity of prokaryotic nonulosonic acids challenges their discovery, and research on molecular characteristics essential for medical applications are often not feasible. Here, we demonstrate a novel, universal large-scale discovery approach that tackles the unmapped diversity of prokaryotic nonulosonic acids. Thereby, we utilize selective chemical labelling combined with a newly established mass spectrometric all-ion-reaction scanning approach to identify nonulosonic acids and other ulosonic acid-like sugars. In doing so, we provide a first molecular-level comparative study on the frequency and diversity across different phyla. We not only illustrate their surprisingly wide-spread occurrence in non-pathogenic species, but also provide evidence of potential higher carbon variants. Many biomedical studies rely on synthetic routes for sialic acids, which are highly demanding and often of low product yields. Our approach enables large-scale exploration for alternative sources of these highly important compounds.

A novel large-scale survey approach for microbial nonulosonic acids (sialic acids) including a first molecular level comparative study is presented.