Electron impact mass spectra of some enamines

The mass spectra of α,β-diarylaminocrotonic acid anilides have been determined and the fragmentation pathways are now discussed. The proposed pathways have been confirmed by deuterium labelling, the spectrum of the model compound, or the appropriate metastables. The detailed electron impact data are given.     

Theoretical Study of the Unimolecular Decomposition Mechanism of Chloromethanol

The decomposition pathways of chloromethanol have been studied by ab initio calculation. Equilibriums and transition states have been optimized at the UMP2(full)/6–31G(d) level. The single point energies have been obtained at higher level of G3 (MP2). Four transition states and eight reaction pathways have been revealed and the most favorable reaction to decomposition pathway is the 1, 2‐HCI elimination, which is consistent with the former scientist's conclusion.     

The fragmentation of the ions of nonan-4-one: A study by triple quadrupole mass spectrometry

The fragmentation pathways for the ions generated by electron impact from nonan-4-one have been studied using low energy collision induced dissociation in a triple quadrupole mass spectrometer. Over 400 fragmentation pathways have been identified. These results are compared with data from earlier ion kinetic energy spectrometry studies of nonan-4-one which employed metastable decompositions.     

A periodic DFT study of intramolecular isomerization reactions of toluene and xylenes catalyzed by acidic mordenite

A periodic density functional theory (DFT) study of the isomerization reactions of toluene and xylene catalyzed by acidic mordenite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed in detail. The use of periodic structure calculations allows consideration and analysis of zeolite electrostatic contributions and steric constraints that occur within zeolite micropores. Major differences in the details of protonation reaction pathways are found when periodic structures are used rather than small cluster models of the Br?nsted acidic site. Complex relationships are found between zeolite topology and reaction pathways.     

Theoretical studies on the mechanism of chlorination reaction of trichlorogermyl acrylic acid

The mechanism of the chlorination reaction of trichlorogermyl acrylic acid has been studied systematically using quantum chemistry methods. Geometries of reactants, transition states and products have been optimized at the B3LYP/6-311G(d,p) level. Vibrational frequencies, IR intensities and relative energies for various stationary points have been determined. The reaction pathways have been identified by intrinsic reaction coordinate (IRC) calculations. Theoretical analysis provides conclusive evidence that the process proceeds through two and three pathways for the first and second reaction steps, respectively.     

Synthesis, structure and stereochemistry of quinoline alkaloids from Choisya ternata

A range of seventeen quinoline alkaloids, involving several types of oxidations during their biosynthetic pathways, have been isolated from leaves of Choisya ternata. In addition to the nine known quinoline alkaloids, eight new members of the furoquinoline family, derived mainly from prenylation at C-5 (including two novel hydroperoxides), have been identified. The absolute configurations and enantiopurity values of all chiral quinoline alkaloids have been determined. One of the isolated alkaloids, 7-isopentenyloxy-gamma-fagarine, has been used as a precursor for the chemical asymmetric synthesis of the enantiopure alkaloids: evoxine, anhydroevoxine and evodine. The possible roles of oxygenase and other oxygen-atom-transfer enzymes, in the biosynthetic pathways of the C. ternata alkaloids, have been discussed.     

Photochromism of tetrahydroindolizines. Part XIV: synthesis of cis-fixed conjugated photochromic pyridazinopyrrolo[1,2-b]isoquinolines incorporating carbon-rich linkers

New carbon-rich photochromic tetrahydroindolizines (THIs) bearing dihydroisoquinoline derivatives as heterocyclic bases (region B) have been synthesized via different chemical and photochemical pathways. Three alternative pathways for the synthesis of the target photochromic THI-based pyridazinopyrrolo[1,2-b]isoquinolines via in situ trapping with hydrazine hydrate have been established. In order to obtain high product yields, different Sonogashira-mediated coupling reactions have been optimized. Low temperature multichannel UV–vis and flash photolysis techniques were used to detect the photochromic and kinetic properties of the synthesized system.     

The role of MAPK signal transduction pathways in the response to oxidative stress in the fungal pathogen Candida albicans: implications in virulence

In recent years, Mitogen-Activated Protein Kinase (MAPK) pathways have emerged as major regulators of cellular physiology. In the fungal pathogen Candida albicans, three different MAPK pathways have been characterized in the last years. The HOG pathway is mainly a stress response pathway that is activated in response to osmotic and oxidative stress and also participates regulating other pathways. The SVG pathway (or mediated by the Cek1 MAPK) is involved in cell wall formation under vegetative and filamentous growth, while the Mkc1-mediated pathway is involved in cell wall integrity. Oxidative stress is one of the types of stress that every fungal cell has to face during colonization of the host, where the cell encounters both hypoxia niches (i.e. gut) and high concentrations of reactive oxygen species (upon challenge with immune cells). Two pathways have been shown to be activated in response to oxidative stress: the HOG pathway and the MKC1-mediated pathway while the third, the Cek1 pathway is deactivated. The timing, kinetics, stimuli and functional responses generated upon oxidative stress differ among them; however, they have essential functional consequences that severely influence pathogenesis. MAPK pathways are, therefore, valuable targets to be explored in antifungal research.     

Pyrolysis of methyl tert-butyl ether (MTBE). 2. Theoretical study of decomposition pathways

The thermal decomposition pathways of MTBE have been investigated using the G3B3 method. On the basis of the experimental observation and theoretical calculation, the pyrolysis channels are provided, especially for primary pyrolysis reactions. The primary decomposition pathways include formation of methanol and isobutene, CH4 elimination, H2 elimination and C-H, C-C, C-O bond cleavage reactions. Among them, the formation channel of methanol and isobutene is the lowest energy pathway, which is in accordance with experimental observation. Furthermore, the secondary pyrolysis pathways have been calculated as well, including decomposition of tert-butyl radical, isobutene, methanol and acetone. The radicals play an important role in the formation of pyrolysis products, for example, tert-butyl radical and allyl radical are major precursors for the formation of allene and propyne. Although some isomers (isobutene and 1-butene, allene and propyne, acetone and propanal) are identified in our experiment, these isomerization reaction pathways occur merely at the high temperature due to their high activation energies. The theoretical calculation can explain the experimental results reported in part 1 and shed further light on the thermal decomposition pathways.     

Metabolic engineering of microorganisms for isoprenoid production

Isoprenoids are ubiquitous in nature and range from essential cell components to unique secondary metabolites. The two isoprenoid biosynthetic pathways have received much attention from a metabolic engineering standpoint, and significant advances have been made in increasing flux through these pathways. Engineering later steps in isoprenoid biosynthetic pathways, specifically those related to the functionalization of terpene backbones, is at an earlier stage of development, both in terms of gene discovery and heterologous expression. Here we review recent advances in the metabolic engineering of microbes for isoprenoid production as well as some novel approaches to gene discovery and expression.     

Gas‐phase dissociation of 1,4‐naphthoquinone derivative anions by electrospray ionization tandem mass spectrometry

Gas‐phase dissociation pathways of deprotonated 1,4‐naphthoquinone (NQ) derivatives have been investigated by electrospray ionization tandem mass spectrometry (ESI‐MS/MS). The major decomposition routes have been elucidated on the basis of quantum chemical calculations at the B3LYP/6‐31 + G(d,p) level. Deprotonation sites have been indicated by analysis of natural charges and gas‐phase acidity. NQ anions underwent an interesting reaction under collision‐induced dissociation conditions, which resulted in the radical elimination of the lateral chain, in contrast with the even‐electron rule. Possible pathways have been suggested, and their mechanisms have been elucidated on the basis of Gibbs energy and enthalpy values for the anions previously described at each pathway. Copyright © 2009 John Wiley & Sons, Ltd.     

Side products of methacrylic acid esterification with triethylene glycol methyl ether

Side reactions in the course of esterification of methacrylic acid with triethyleneglycol methyl ether in the presence of homogeneous acid catalyst have been studied. Chromato-mass spectrometry and authentic synthesis have revealed the chemical structure of the products; pathways of their formation have been suggested.     

Matrix-Free NALDI Mass Spectrometric Study of the Major and Minor Products of the Reaction of Imidazole with Bis(iodomethyl)tetramethyldisiloxane

The major and minor products of the reaction of imidazole with bis(iodomethyl)tetramethyldisiloxane have been identified, and their main fragmentation pathways have been determined, by matrix-free nano-assisted laser desorption/ionization mass spectrometry (NALDI TOF/TOF).     

Reductive demetallation of porphyrins: Evidence for peripheral and axial modes of reduction

Experiments with isotopically-labelled and stericaily-constrained porphyrins have been used to elucidate demetallation pathways for silver and copper porphyrins.     

On the generation of novel anticancer drugs by recombinant DNA technology: the use of combinatorial biosynthesis to produce novel drugs

Chemotherapeutic drugs for cancer treatment have been traditionally originated by the isolation of natural products from different environmental niches, by chemical synthesis or by a combination of both approaches thus generating semisynthetic drugs. In the last years, a number of gene clusters from several antitumor biosynthetic pathways, mainly produced by actinomycetes and belonging to the polyketides family, are being characterized. Genetic manipulation of these antitumor biosynthetic pathways will offer in the near future an alternative for the generation of novel antitumor derivatives and thus complementing current methods for obtaining novel anticancer drugs. Novel antitumor derivatives have been produced by targetted gene disruption and heterologous expression of single (or a few) gene(s) in another hosts or by combining genes from different, but structurally related, biosynthetic pathways ("combinatorial biosynthesis"). These strategies take advantage from the "relaxed substrate specificity" that characterize secondary metabolism enzymes.     

Zeolite-catalyzed three-phase hydration of α-pinene

In this paper, the kinetics, the product distributions and the corresponding reactions of some products of zeolite-catalyzed hydration of α-pinene have been studied. The results indicate: 1. α-Pinene is hydrated mainly to borneol, but isomerizations always accompany the hydration and camphene and limonene are the main olefines formed. 2. The disappearance of α-pinene follows the first-order kinetics, of which the rate constants, the activation energy as well as the activation entropies have been calculated. 3. A probable mechanism has been proposed, in which the hydration of α-pinene through two equilibrating adsorbed intermediates, which, via three pathways, lead to derivatives of fenchane, bornane and p-menthane respectively. The distributions of the three pathways have been examined and appear to be determined by the stability of the adsorbed intermediates themselves     

Kinetics and mechanism of the dehydration reaction of sarcosine to a bislactame through diacyclperoxide intermediate in strong acidic medium

The influence of substitution on the amine functional group of glycine in the permanganic oxidation of such an α‐amino acid in moderately concentrated sulfuric acid medium has been investigated. Reaction products analysis has revealed that contrary to the usual α‐amino acid oxidation product, which is an aldehyde species, a valuable compound, namely 1,4‐dimethylpiperazine‐2,5‐dione, has been obtained as the main product via a cheap, simple, efficient, and novel method. Sarcosine has been chosen as a substituted derivative of glycine, and the kinetics and mechanism of its permanganic oxidation have been investigated using a spectrophotometric technique. Conclusive evidence has proven delayed autocatalytic activity for Mn(II) in this reaction, analogous to some α‐amino acids. It has been revealed that such activity can show up when a certain concentration ratio of Mn(II) to sarcosine is built up in the medium, which we call the “critical ratio.” The magnitude of the latter ratio depends on the sulfuric acid concentration. Considering the “delayed autocatalytic behavior” of Mn(II) ions, rate equations satisfying observations for both catalytic and noncatalytic routes have been presented. The reaction shows first‐order dependence on permanganate ions and sarcosine concentrations in both catalytic and noncatalytic pathways, and apparent first‐order dependence on Mn²⁺ ions in catalytic pathways. The correspondence of pseudo‐order rate constants of the catalytic and noncatalytic pathways to Arrhenius and Eyring laws has verified “critical ratio” as well as “delayed autocatalytic behavior” concepts. The activation parameters associated with both pathways have been computed and discussed. Mechanisms for both catalytic and noncatalytic routes involving radical intermediates as well as a product having a diketopiperazine skeleton have been reported for the first time. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 689–703, 2009     

Determination de la structure d'halogenotriazoles-1,2,4 par spectrometrie de masse

The mass spectra of a substituted halo-1,2,4-triazoles series have been studied. The main fragmentation pathways have been determined and the utility in structure determinations of isomeric compounds is demonstrated.     

A possible construction of chemical reaction networks

The chemical reaction networks and the sequence networks represent the pathways of a complex chemical process. In order to study the pathways separately the systematization of the elementary processes included in the possible mechanism is inevitable.This systematization was realized by a special procedure based on linear algebraic methods and enabled us to select the corresponding processes from the possible mechanism. The efficiency of the procedure has been illustrated by its application to the liquid phase oxidation of ethylbenzene and the elementary processes have been selected using a computer program.     

Biomimetic oxidation of plant protecting agents

The suitability of two in vitro oxidation systems as chemical models for the biological degradation of plant protecting agents has been investigated. As representative herbicides diclofop, fenoxaprop, isoproturon, linuron and monolinuron have been oxidised by two systems, the Fentons' reagent and the ascorbic acid oxidation system (AAOS) and the results compared to those of the known metabolic pathways of these compounds. The herbicides have been oxidised by Fentons' reagent (hydroxy radicals). The main products were isolated by preparative scale HPLC and identified with (1)H-NMR and MS. Some of the products have been identified by comparing their retention times and UV/Vis-spectra to those of standard compounds. Several products known from biological degradation are also found after chemical oxidation, however, notable differences between the two pathways have been observed, for instance in the case of diclofop. Oxidation by the AAOS leads to comparable results. Reaction rates for the oxidation with the AAOS have been studied and compared with data known from degradation studies of the herbicides in soil. Compounds which are slowly degraded in soil are oxidised more slowly in the biomimetic process than those with a fast degradation in soil.