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The Grignard Reaction − Unraveling a Chemical Puzzle
2020
More than 100 years since its discovery, the mechanism of the Grignard reaction remains unresolved. Ambiguities arise from the concomitant presence of multiple organomagnesium species and the competing mechanisms involving either nucleophilic addition or the formation of radical intermediates. To shed light on this topic, quantum-chemical calculations and ab initio molecular dynamics simulations are used to study the reaction of CH3MgCl in tetrahydrofuran with acetaldehyde and fluorenone as prototypical reagents. All organomagnesium species coexisting in solution due to the Schlenk equilibrium are found to be competent reagents for the nucleophilic pathway. The range of activation energies …
THE KINETICS AND MECHANISM OF THE REACTION OF ONIUM CYCLOPENTADIENYLIDES WITH TETRAHALO-p-BENZOQUINONES
1989
Abstract The reaction of triphenylphosphonium cyclopentadienylide (1) with halogen-substituted p-benzoquinones (4) is shown to give a new class of dipolar (zwitterionic) dyes (5) containing phosphorus. The general structure of these molecules has been investigated by a combination of mass spectrometry and multinuclear (7H, 13C and 31P) nmr using the specialist techniques of DEFT spectroscopy, homonuclear (COSY) and heteronuclear 2-D nmr. In addition, stopped-flow (uv/visible) techniques have been used to study the kinetics of the reactions and hence demonstrate that the rate-limiting step is nucleophilic addition of the ylid nucleophile to the quinone, followed by a rapid loss of halide ion…
Synthesis of fluorinated 1,2,4-oxadiazin-6-ones through ANRORC rearrangement of 1,2,4-oxadiazoles
2009
Abstract The reaction of 3-ethoxycarbonyl-5-perfluoroalkyl-1,2,4-oxadiazoles with hydroxylamines has been investigated, evidencing the possibility of competitive reaction paths. Nucleophilic addition of hydroxylamine to the electrophilic C(5) of the 1,2,4-oxadiazole ring, followed by ring-opening and ring-closure with enlargement, leads to high yield and in very mild experimental conditions to the formation of 5-hydroxyamino-3-perfluoroalkyl-6 H -1,2,4-oxadiazin-6-ones, one of these presenting water gelation ability. In turn, reactions with N -methylhydroxylamine lead the exclusive formation of 4-perfluoroacylamino-2-methyl-2 H -1,2,5-oxadiazol-3-ones through the well known Boulton–Katritzk…
Addition of Nucleophiles to Fluorinated Michael Acceptors
2016
A series of nucleophiles, including primary and secondary amines, primary alcohols, and thiols, as well as diethyl malonate and nitromethane, were added to different fluorinated Michael acceptors including 2-fluoroalk-1-en-3-ones and 2-fluoro-1-phenylprop-2-en-1-one. The resulting beta-substituted afluoro ketones were isolated in 34-92 % yield, depending on the substrate and the nucleophile. The best yields were obtained with secondary amines and with p-methylthiophenol.
Assessment of Human Exposure to Deoxynivalenol, Ochratoxin A, Zearalenone and Their Metabolites Biomarker in Urine Samples Using LC-ESI-qTOF.
2021
Human are exposed to a wide range of mycotoxins through dietary food intake, including processed food. Even most of the mycotoxin exposure assessment studies are based on analysis of foodstuffs, and evaluation of dietary intake through food consumption patterns and human biomonitoring methods are rising as a reliable alternative to approach the individual exposures, overcoming the limitations of the indirect dietary assessment. In this study, human urine samples were analyzed, seeking the presence of deoxynivalenol (DON), ochratoxin A (OTA), zearalenone (ZEA), and their metabolites. For this purpose, 40 urine samples from female and male adult residents in the city of Valencia (Spain) were …
Multi-mycotoxin contamination of green tea infusion and dietary exposure assessment in Moroccan population
2021
Green tea infusion is one of the most widely drunk beverages worldwide due to its health benefits associated with microelements, essential oils, and polyphenols, etc. Several studies have reported that green tea is subjected to contamination by various toxigenic fungi. Thus, this work aims to investigate the co-occurrence of 15 mycotoxins [four aflatoxins (AFB1, AFB2, AFG1, AFG2), ochratoxin A (OTA), beauvericin (BEA), four enniatins (ENA, ENA1, ENB, ENB1), zearalenone (ZEN), alternariol (AOH), tentoxin (TENT), T-2 and HT-2 toxins] in green tea samples available in Morocco by liquid chromatography tandem mass spectrometry method. Analytical and consumption data were then used to assess the …
Occurrence of fourteen mycotoxins in tiger-nuts
2012
Abstract A previous developed matrix solid-phase dispersion (MSPD) extraction method was applied for the routine analysis of aflatoxins (AFs), ochratoxin A (OTA), fumonisins (FB 1 and FB 2 ), beauvericin (BEA), nivalenol (NIV), deoxynivalenol (DON), the toxin T-2 (T-2), toxin HT-2 (HT-2), diacetoxyscirpenol (DAS) and zearalenone (ZEN) in tiger-nuts by liquid chromatography–triple-quadrupole linear ion trap (HPLC–QTRAP ® ). The extraction solid support used was C 18 , while the elution solvent was acetonitrile/methanol (50/50, v/v) 1 mM ammonium formate. Using matrix-matched calibration, recoveries and repeatabilities were in the range 67–89% and 2–11% relative standard deviation (RSD), resp…
Simultaneous determination of mycotoxin in commercial coffee
2015
Abstract Mycotoxins are secondary metabolites produced by filamentous fungi that usually contaminate food products. Coffee is a natural product susceptible to mycotoxin contamination. The present study evaluates the presence of nivalenol, deoxynivalenol, T-2 and HT-2 Toxin, diacetoxyscirpenol, aflatoxin B 1 , aflatoxin B 2 , aflatoxin G 1 , aflatoxin G 2 , fumonisin B 1 , fumonisin B 2 , ochratoxin A, zearalenone, enniatin A, enniatin A 1 , enniatin B, enniatin B 1 , and beauvericin in coffee samples, using liquid chromatography tandem mass spectrometry (LC-MS/MS). The results show that zearalenone was not present in any sample. In the positive samples the contents of fumonisins ranged from…
Multimycotoxin Determination in Tunisian Farm Animal Feed
2019
Mycotoxins presence was evaluated in animal feed marketed in Tunisia for the first time ever. A QuEChERS method was performed to analyze the natural copresence of 22 mycotoxins (enniatins, beauvericin, ochratoxin A, aflatoxins, alternariol monomethyl ether, alternariol, tentoxin, zearalenone, deoxynivalenol, 3-acetyldeoxynivalenol, 15-acetyldeoxynivalenol, nivalenol, neosolaniol, diacetoxyscirpenol, T-2 toxin, and HT-2 toxin) in 122 Tunisian marketed feed samples, intended for poultry (n = 43), cattle (n = 35), rabbit (n = 12), sheep (n = 16), and horse (n = 16). Analytes detection and quantification were done using both liquid chromatography and gas chromatography coupled to tandem mass sp…
Multi-mycotoxin contamination of couscous semolina commercialized in Morocco.
2015
The multi-mycotoxin contamination of ninety-eight (98) couscous semolina samples collected from various areas in Morocco was investigated in this study. Samples were surveyed for the presence of 22 mycotoxins (four aflatoxins, ochratoxin A, diacetoxiscyrpenol (DAS), three fumonisins, beauvericin (BEA), deoxynivalenol (DON), 15-acetyl-deoxynivalenol (15-ADON), 3-acetyl-deoxynivalenol (3-ADON), nivalenol (NIV), sterigmatocystin (STG), zearalenone (ZEA), four enniatins, T-2 and HT-2 toxins). Results showed that 96 out of 98 total couscous samples (98%) were contaminated by at least one mycotoxin. Enniatin B (ENB), Enniatin B1 (ENB1), Enniatin A1 (ENA1) and zearalenone (ZEA) have shown the high…