Detection and characterization of twenty‐eight isomers of fumonisin B1 (FB1) mycotoxin in a solid rice culture infected with Fusarium verticillioides by reversed‐phase high‐performance liquid chromatography/electrospray ionization time‐of‐flight and ion trap mass spectrometry

Fumonisin mycotoxins which are hazardous to humans and animals were produced in a Fusarium verticillioides‐infected solid rice culture. To decrease the possibility of the formation of artifacts, the fumonisins were analysed by reversed‐phase high‐performance liquid chromatography/electrospray ionization time‐of‐flight (RP‐HPLC/ESI‐TOFMS) and ion trap mass spectrometry (RP‐HPLC/ESI‐ITMS) immediately after the extraction of the culture material, without any further sample clean‐up. The fumonisin isomers were separated by using a flat gradient on a special, high‐coverage C₁₈, narrow‐bore HPLC column (YMC‐Pack J'sphere ODS H80) suggested for the separation of structural isomers by the manufacturer. Exact mass measurements (TOFMS) of the protonated molecules and extraction of the ion chromatogram corresponding to the empirical formula (C₃₄H₅₉NO₁₅) of FB₁ toxins led to the identification of 29 peaks and shoulders, including those of FB₁. The FB₁ toxin and 28 of its isomers were also detected by ITMS after separation with RP‐HPLC. The characteristic m/z values of the product ions, including the backbones obtained by ITMS², undoubtedly indicated the structures of the FB₁ isomers for 28 peaks and shoulders. In the MS² spectra of the protonated molecules of the FB₁ isomers, with some exceptions, 15 characteristic product ions including the hydrocarbon backbone at m/z 299 were observed. The abundance ratio of the cation at m/z 299 ranged up to 5.8%. The relative quantities of the isomers found in the sample extract were expressed as percentages of the FB₁ content (0.001–0.579%). The total amount of the 28 FB₁ isomers was 2.803% of the quantity of FB₁ that is important from the aspect of food and feed safety. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of fumonisins B1 and B2 in Portuguese maize and maize-based samples by HPLC with fluorescence detection

Fumonisins B₁ (FB₁) and fumonisin B₂ (FB₂) are the main members of a family of mycotoxins produced by Fusarium verticillioides, Fusarium proliferatum, and other fungi species of the section Liseola. The present work shows the results of comparative studies using two different procedures for the analysis of fumonisins in maize and maize-based samples. The studied analytical methods involve extraction with methanol/water, dilution with PBS, and clean-up through immunoaffinity columns. Two reagents (o-phthaldialdehyde and naphthalene-2,3-dicarboxaldehyde) were studied for formation of fluorescent derivatives. The separation and identification were carried out by high-performance liquid chromatography with fluorescence detection. The optimized method for analysis of fumonisins in maize involved extraction with methanol/water (80:20), clean-up with an immunoaffinity column, and derivatization with naphthalene-2,3-dicarboxaldehyde (NDA). The limit of detection was 20 μg kg⁻¹ for FB₁ and 15 μg kg⁻¹ for FB₂. Recoveries of FB₁ and FB₂ ranged from 79% to 99.6% for maize fortified at 150 μg kg⁻¹ and 200 μg kg⁻¹, respectively, with within-day RSDs of 3.0 and 2.7%. The proposed method was applied to 31 samples, and the presence of fumonisins was found in 14 samples at concentrations ranging from 113 to 2,026 μg kg⁻¹. The estimated daily intake of fumonisins was 0.14 μg kg⁻¹ body weight per day.     

A single extraction method for the analysis by liquid chromatography/tandem mass spectrometry of fumonisins and biomarkers of disrupted sphingolipid metabolism in tissues of maize seedlings

The fungus Fusarium verticillioides is a pathogen of many plants and produces fumonisins. In addition to their well-studied animal toxicoses, these toxins contribute to the development of maize seedling disease in susceptible maize varieties. Fumonisin disruption of sphingolipid biosynthesis occurs during pathogenesis. An extraction method was developed for the simultaneous analysis of fumonisins B₁ (FB₁), B₂ (FB₂) and B₃ (FB₃), free sphingoid bases and sphingoid base 1-phosphates in maize tissues by liquid chromatography/linear ion trap tandem mass spectrometry. The method involved a single extraction using 1:1 acetonitrile:water + 5% formic acid (1 ml per 10 mg tissue). Mean recoveries ranged from approximately 50 to 99 percent, and limits of detection ranged from 10 fg μl⁻¹ to 6900 fg μl⁻¹. To test the efficacy of the method, seeds of a susceptible maize line were inoculated with a pathogenic, fumonisin-producing strain of F. verticillioides. The seedlings were then harvested, and fumonisin content, as well as sphingoid bases and their 1-phosphates, were measured in the leaf and root tissues. Fumonisin accumulation was significantly greater in leaf one compared to leaves two and three. While FB₁, FB₂, and FB₃ were detected in root tissues, FB₁ was preferentially accumulated in leaf tissues. Accumulation of sphingoid bases and their 1-phosphates was evident in roots and leaves of seedlings grown from inoculated seed, with the level of accumulation being similar in leaves 1, 2 and 3. The method developed was effective, fast, and sensitive for use in simultaneously measuring fumonisin in tissues and their effects on sphingolipid metabolite biomarkers of disease. The method should be useful for screening maize cultivars for susceptibility to F. verticillioides-induced seedling diseases. Figure Lesion with chromatography     

Incidence of Fusarium Mycotoxins in Wheat and Maize from Albania

In this study, ten Fusarium toxins were analysed in wheat and maize commodities from Albania. In total, 71 samples of wheat and 45 samples of maize were collected from different producing regions. The analytical procedure consisted of a simple one-step sample extraction followed by the determination of toxins using liquid chromatography coupled with tandem mass spectrometry. Fusarium toxins were found in 23% of the analysed wheat samples and in 78% of maize samples. In maize samples, most often fumonisins B₁ (FB1) and B₂ (FB2) were found. They were present in 76% of samples. They were detected in all positive samples except in one with concentrations ranging from 59.9 to 16,970 μg/kg. The sum of FB1 and FB2 exceeded the EU maximum permitted level (4000 μg/kg) in 31% of maize samples. In wheat samples, the only detected Fusarium mycotoxin was deoxynivalenol (DON), present in 23% of samples. In one sample with the concentration of 1916 μg/kg, the EU maximum permitted level (1250 μg/kg) was exceeded. This is the first report on the presence of Fusarium toxins in wheat and maize grains cultivated in Albania.     

Development of a multi‐mycotoxin liquid chromatography/tandem mass spectrometry method for sweet pepper analysis

A multi‐mycotoxin method was developed for the simultaneous determination of trichothecenes (nivalenol, deoxynivalenol, 3‐acetyldeoxynivalenol, 15‐acetyldeoxynivalenol, neosolaniol, fusarenon‐X, diacetoxyscirpenol, HT‐2 toxin, T‐2 toxin), aflatoxins (aflatoxin‐B₁, aflatoxin‐B₂, aflatoxin‐G₁ and aflatoxin‐G₂), Alternaria toxins (alternariol, alternariol methyl ether and altenuene), fumonisins (fumonisin‐B₁, fumonisin‐B₂ and fumonisin‐B₃), ochratoxin A, zearalenone, beauvericin and sterigmatocystin in sweet pepper. Sweet pepper was extracted with ethyl acetate/formic acid (99:1, v/v). After splitting up the extract, two‐thirds of the extract was cleaned up using an aminopropyl column followed by an octadecyl column. The remaining part was cleaned up using a strong anion‐exchange column. After recombination of both cleaned parts of the sample extract, the combined solvents were evaporated and the residue was dissolved in mobile phase; 20 µL was injected into the chromatographic system, so only one run was used to separate and detect the mycotoxins in positive electrospray ionization using selected reaction monitoring. The samples were analyzed with a Micromass Quattro Micro triple quadrupole mass spectrometer (Waters, Milford, MA, USA). The mobile phase consisted of variable mixtures of water and methanol, 1% acetic acid and 5 mM ammonium acetate. The limits of detection of the multi‐mycotoxin method varied from 0.32 µg.kg^?1 to 42.48 µg.kg^?1. The multi‐mycotoxin liquid chromatography/tandem mass spectrometry (LC/MS/MS) method fulfilled the method performance criteria required by the Commission Regulation (EC) No 401/2006. Sweet peppers inoculated by Fusarium species were analyzed using the developed method. Beauvericin (9–484 µg.kg^?1) and fumonisins (fumonisin‐B₁ up to 4330 µg.kg^?1, fumonisin‐B₂ up to 4900 µg.kg^?1, and fumonisin‐B₃ up to 299 µg.kg^?1) were detected. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of molecularly imprinted polymers to determine B1, B2, and B3 fumonisins in cereal products

The aim of this study was to develop an analytical method for qualitative and quantitative determination of the B₁, B_2, and B₃ fumonisins in cereal products. A LC coupled to an IT‐MS was used as the analytical instrument. The AFFINIMIP FumoZON Molecularly Imprinted Polymer SPE cartridges (Polyintell) were used to isolate fumonisins from the analyzed samples and the clean‐up step. Statistical parameters evaluated in some validation experiments were as follows: mean recovery 95–106%, precision <17% (expressed as recovery RSD). The developed method was used to determine fumonisins in 49 cereals (42 maize‐based and seven wheat‐based products). In most cases, concentrations of the studied compounds found in the analyzed samples were low. The highest total concentration of the B₁, B₂, and B₃ fumonisins was found in maize flour samples (range, 26–1102 μg/kg, mean 498 μg/kg).     

The diastereoisomers methyl 5‐(S)‐[2‐(R)/(S)‐methoxycarbonyl)‐2,3,4,5‐tetrahydropyrrol‐1‐ylcarbonyl]‐1‐(4‐methylphenyl)‐4,5‐dihydropyrazole‐3‐carboxylate

The title diastereoisomers, methyl 5‐(S)‐[2‐(S)‐methoxy­carbonyl)‐2,3,4,5‐tetra­hydro­pyrrol‐1‐yl­carbonyl]‐1‐(4‐methyl­phenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxyl­ate and methyl 5‐(S)‐[2‐(R)‐methoxycarbonyl)‐2,3,4,5‐tetrahydropyrrol‐1‐ylcarbonyl]‐1‐(4‐methyl­phenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxylate, both C₁₉H₂₃N₃O₅, have been studied in two crystalline forms. The first form, methyl 5‐(S)‐[2‐(S)‐methoxy­carbonyl)‐2,3,4,5‐tetrahydropyrrol‐1‐ylcarbonyl]‐1‐(4‐methylphenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxyl­ate–methyl 5‐(S)‐[2‐(R)‐methoxy­carbonyl)‐2,3,4,5‐tetra­hydro­pyrrol‐1‐yl­carbonyl]‐1‐(4‐methylphenyl)‐4,5‐dihydropyrazole‐3‐carboxylate (1/1), 2(S),5(S)‐C₁₉H₂₃N₃O₅·2(R),5(S)‐C₁₉H₂₃N₃O₅, contains both S,S and S,R isomers, while the second, methyl 5‐(S)‐[2‐(S)‐methoxycarbonyl)‐2,3,4,5‐tetrahydro­pyrrol‐1‐ylcarbonyl]‐1‐(4‐methyl­phenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxyl­ate, 2(S),5(S)‐C₁₉H₂₃N₃O₅, is the pure S,S isomer. The S,S isomers in the two structures show very similar geometries, the maximum difference being about 15° on one torsion angle. The differences between the S,S and S,R isomers, apart from those due to the inversion of one chiral centre, are more remarkable, and are partially due to a possible rotational disorder of the 2‐­(methoxycarbonyl)tetrahydropyrrole group.     

Synthesis,Bioactivities and Structure Activity Relationship of N‐4‐Methyl‐1,2,3‐thiadiazole‐5‐carbonyl‐N′‐phenyl Ureas

Twenty nine novel N‐4‐methyl‐1,2,3‐thiadiazole‐5‐carbonyl‐N′‐phenyl ureas were designed and synthesized, and their structures were confirmed by proton nuclear magnetic resonance (¹H NMR), infra red spectroscopy (IR) and high‐resolution mass spectroscopy (HRMS). Compounds V‐9 , V‐11 , V‐12 , V‐15 , V‐19 , V‐21 , V‐22 and V‐24 exhibit excellent activity against Culex pipiens pallens. Compounds V‐12 and V‐22 present good insecticidal activity against Plutella xylostella L. Their median lethal concentrations (LC₅₀) are 164.15 and 89.69 mg·L^?1, respectively. Compound V‐11 also has potential wide spectrum of fungicide activity. Its median effective concentrations (EC₅₀) detected from 3.82 µg·mL^?1 against Physalospora piricola to 31.60 µg·mL^?1 against Cercospora arachidicola. Compounds V‐15 and V‐24 show outstanding induction activities as same as positive controls TDL and ningnanmycin, furthermore V‐24 has the highest induction activity of 41.85%±4.43%. To elucidate the structure activity relationship in these compounds, a 3D‐QSAR model has been built. The established model showed a reliable predicting ability with q² values of 0.643 and r² values of 0.982.     

新型5-(对氯苯基)-3-(邻，对-二氯苯基)-4，5-二氢吡唑肟酯衍生物的合成，结构表征及抗菌活性

合成了10个未见文献报道的1-(5-(2-氯苯基)-3-(2,4-二氯苯基)-4,5-二氢-N-吡唑肟酯类衍生物,并经过元素分析、HRMS、核磁共振氢谱对其结构进行了表征。对新合成的化合物进行了初步抗Bacillus subtilis, Staphylococcus aureus, Escherichia coli 和 Pseudomonas aeruginosa生物活性测试,结果表明:化合物7c 和7f对供试病菌具有较好的体外杀灭活性,其MIC值达到1.562 μg/mL;化合物7c ,7d和7f 具有中等的抑制DNA回旋酶活性(IC₅₀ = 1.6~2.5 µg/mL)。在生物活性结果的基础上对系列化合物的构效关系进行了初步的探讨。     

Guest‐Induced Photophysical Property Switching of Artificial Light‐Harvesting Dendrimers

An artificial light‐harvesting multiporphyrin dendrimer ( 8P_ZnP_FB ) composed of a focal freebase porphyrin ( P_FB ) with eight zinc(II) porphyrin ( P_Zn ) wings exhibited unique photophysical property switching in response to specific guest molecule binding. UV/Vis titration studies indicated stable 1:2 host–guest complex formation between 8P_ZnP_FB and meso‐tetrakis(4‐pyridyl)‐porphyrin ( TPyP ) for which the first and second association constants were estimated to be >10⁸ M ^?1 and 3.0×10⁷ M ^?1, respectively. 8P_ZnP_FB originally shows 94 % energy transfer efficiency from P_Zn to the focal P_FB . By the formation of the host–guest complex ( 8P_ZnP_FB? 2 TPyP ) the emission intensity of 8P_ZnP_FB is significantly decreased, and an ultrafast charge separation state is generated. The energy transfer process from P_Zn wings to the P_FB core in 8P_ZnP_FB is almost entirely switched to an electron transfer process by the formation of 8P_ZnP_FB? 2 TPyP .     

Fumonisins determination in urine by LC-MS-MS

The present work shows the optimization and validation results of an analytical methodology based on imunoaffinity clean-up (IAC) followed by liquid chromatography with tandem mass detection (LC-MS-MS) for the analysis of the mycotoxins fumonisin B₁ (FB₁) and B₂ (FB₂) in human urine samples, in order to evaluate fumonisins exposure in two Portuguese populations. As far as we know, imunoaffinity clean-up procedure was used, for the first time, in the analysis of fumonisins (FBs) in urine. Using this analytical methodology, the limit of quantification achieved was 10 ng mL⁻¹ for FB₁ and for FB₂. Recoveries were higher than 73.4% for fortification levels between 10 and 100 ng mL⁻¹ and intra-day and inter-day repeatibility were lower than 8.6%. The natural occurrence of FB₁ and FB₂ in 68 human urine samples obtained from the central zone of Portugal was studied. None of the studied samples presented detectable levels of FB₁ and FB₂.     

Hydrolysis of C,N‐chelated diorganotin(IV) chlorides and catalysis of transesterification reactions

Diorganotin(IV) dichlorides of formula L^CNRSnCl₂ (where R is nBu or Ph) containing one L^CN chelating ligand were hydrolyzed with aqueous sodium hydroxide in benzene. The composition of the products is strongly dependent on the amount of hydroxide. The partially hydrolyzed compounds of composition (L^CNRSnCl)₂(µ‐O) were isolated as crystalline products. A hydrolysis where more than one molar equivalent of NaOH is employed gave only a mixture of unidentifiable products. The structure of (L^CNPhSnCl)₂(µ‐O) was determined by X‐ray diffraction techniques in the solid state. In solution there was a mixture of diastereoisomers found, where the tin atoms serve as a stereogenic centers. The catalytic activity of starting dichlorides as well as (L^CNPhSnCl)₂(µ‐O) in various transesterification processes was investigated. The activity is very low in the case of starting dichlorides. When two molar equivalents of NaH are added or (L^CNPhSnCl)₂(µ‐O) is employed in the catalytic experiments, the activity is comparable to the literature data. Copyright © 2009 John Wiley & Sons, Ltd.     

Preparation and structural analysis of (±)‐cis‐ethyl 2‐sulfanylidenedecahydro‐1,6‐naphthyridine‐6‐carboxylate and (±)‐trans‐ethyl 2‐oxooctahydro‐1H‐pyrrolo[3,2‐c]pyridine‐5‐carboxylate

Bicycle ring closure on a mixture of (4aS,8aR)‐ and (4aR,8aS)‐ethyl 2‐oxodecahydro‐1,6‐naphthyridine‐6‐carboxylate, followed by conversion of the separated cis and trans isomers to the corresponding thioamide derivatives, gave (4aSR,8aRS)‐ethyl 2‐sulfanylidenedecahydro‐1,6‐naphthyridine‐6‐carboxylate, C₁₁H₁₈N₂O₂S. Structural analysis of this thioamide revealed a structure with two crystallographically independent conformers per asymmetric unit (Z′ = 2). The reciprocal bicycle ring closure on (3aRS,7aRS)‐ethyl 2‐oxooctahydro‐1H‐pyrrolo[3,2‐c]pyridine‐5‐carboxylate, C₁₀H₁₆N₂O₃, was also accomplished in good overall yield. Here the five‐membered ring is disordered over two positions, so that both enantiomers are represented in the asymmetric unit. The compounds act as key intermediates towards the synthesis of potential new polycyclic medicinal chemical structures.     

The psuedo‐michael reaction of 2‐aminoimidazolines 2. Part 1. Synthesis and structure assignment of isomeric 5(1H)‐Oxo and 7(1H)‐Oxo‐2,3‐dihydroimidazo[1,2‐a]pyrimidine‐6‐carboxylates

The pseudo‐Michael reaction of 1‐aryl‐2‐aminoimidazolines‐2 with diethyl ethoxymethylenemalonate (DEEM) was investigated. Extensive structural studies were performed to confirm the reaction course. For derivatives with N1 aromatic substituents, it was found that the reaction course was temperature dependent. When the reaction temperature was held at ?10 °C only the formation of 1‐aryl‐7(1H)‐oxo‐2,3‐dihydroimi‐dazo[1,2‐a]pyrimidine‐6‐carboxylates ( 4 ) was observed in contrast to earlier suggestions. Under the room temperature conditions, the same reaction yielded mixtures, with varying ratio, of isomeric 1‐aryl‐7(1H)‐oxo‐ ( 4a‐4f ) and 1‐aryl‐5(1H)‐oxo‐2,3‐dihydroimidazo[1,2‐a]pyrimidine‐6‐carboxylates ( 5a‐5f ). The molecular structure of selected isomers, 4b and 5c , was confirmed by X‐ray crystallography. Frontal chro‐matography with delivery from the edge was applied for the separation of the isomeric esters. The isomer ratio of the reaction products depended on the character of the substituents on the phenyl ring. The 1‐aryl‐7(1H)‐oxo‐carboxylates ( 4a‐4f ) were preferably when the phenyl ring contained H, 4‐CH₃, 4‐OCH₃ and 3,4‐Cl₂ substituents. Chloro substitution at either position 3 or 4 in the phenyl ring favored the formation of isomers 5a‐5f . The isomer ratios were confirmed both by ¹H NMR and chromatography. The reaction of the respective hydrobromides of 1‐aryl‐2‐aminoimidazoline‐2 with DEEM, in the presence of triethylamine, gave selectively 5(1H)‐oxo‐esters ( 5a‐5f ).     

Design,Synthesis and Herbicidal Activities of Tetrahydroisoindoline‐1,3‐dione Derivatives Containing Alkoxycarbonyl Substituted 2‐Benzoxazolinone

Several different alkoxycarbonyl‐substituted 2‐benzoxazolinone moieties have been incorporated into a tetrahydroisoindoline‐1,3‐dione scaffold to provide 25 compounds ( 1a – 1u and 2a – 2d ). The structures of these compounds were confirmed by ¹H and ¹³C NMR, HRMS and X‐ray single‐crystal diffraction. Some of these compounds ( 1g , 1h , 1j , 1k ) exhibited excellent herbicidal activities against Abutilon theophrasti, Amaranthus retroflexus and Echinochloa crus‐galli at a rate of 375 g AI·ha^?1. Among them, compounds 1h and 1j displayed the best post‐emergence herbicidal effect against Abutilon theophrasti with ED₅₀ values of 1.8 and 5.3 g AI·ha^?1, respectively, which are superior to that of the commercial acifluorfen (44.3 g AI·ha^?1). Field trials demonstrated that compound 1h exhibited similar herbicidal activity to a high concentration atrazine, and found to be safer for maize than atrazine. The results of this study therefore show that compound 1h could potentially be used as a post‐emergence herbicide for maize fields.     

Quantitative Determination of Fumonisins B1 and B2 by High-Performance Liquid Chromatography with Fluorescence Detection

Abstract

A high-performance liquid chromatographic method has been developed for the quantitative determination of the recently described mycotoxins, fumonisins B₁ (FB₁) and B₂ (FB₂) utilizing pre-column derivatization with o-phthaldialdehyde, isocratic elution, and fluorescence detection. Prior to analysis, sample extracts were purified on strong anion exchange cartridges. The method has been applied to the analysis of naturally contaminated corn and mixed horse feed samples as well as fungal culture material, for the presence of the mycotoxins. Detection limits are approximately 50 ng g^?1 for FB₁ and 100 ng g^?1 for FB₂. The method proved to be highly reproducible and recoveries of the toxins from the purification steps were found to be 99.5% and 85.9% for FB₁ and FB₂, respectively.     

Synthesis and antiproliferative activity of (Z + E)‐1‐[4‐(2‐(cyclopentadienyltricarbonylmanganese)‐2‐oxo‐ethoxy)phenyl]‐1,2‐di(p‐hydroxyphenyl)‐but‐1‐ene against breast cancer cells

This paper describes the synthesis of (Z + E)‐1‐[4‐(2‐(cyclopentadienyltricarbonylmanganese)‐2‐oxo‐ethoxy)phenyl]‐1,2‐di(p‐hydroxyphenyl)‐but‐1‐ene. Two synthetic pathways were explored. The best pathway consisted of the alkylation of 1,2‐bis‐[4‐(tert‐butyl‐dimethylsilyloxy)phenyl]‐1‐(4‐hydroxyphenyl)but‐1‐ene with BrCH₂COOEt. The ester obtained was transformed into the Weinreb amide by reaction with HN(OMe)Me–HCl. The reaction of lithium manganese tricarbonylcyclopentadienide with the Weinreb amide produced 1‐[4‐(2‐(cyclopentadienyltricarbonylmanganese)‐2‐oxo‐ethoxy)phenyl]‐1,2‐di(p‐tert‐butyldimethylsiloxyphenyl)‐but‐1‐ene. The deprotection of phenolic functions of the latter compound led to the formation of the final compound. The Z and E isomers could be separated but the isomerization of these isomers from one to another is an easy process. The Z + E compound 2 was tested against the hormone‐dependent MCF‐7 and hormone‐independent MDA‐MB‐231 breast cancer cell lines. The IC₅₀ values of compound 2 were 4.80 ± 2.00 µm and 4.79 ± 0.70 µm for MCF‐7 cells and MDA‐MB‐231 cells, respectively, which was three times better than the ferrocenyl analogue. Copyright © 2012 John Wiley & Sons, Ltd.     

Aza‐nido‐dodecaboranes and ‐borates from Aza‐closo‐dodecaboranes by the Addition of Neutral or Anionic Bases: Mechanism

The addition of neutral (L = py, NEt₃, NHEt₂, NH₂tBu) and anionic Lewis bases (X^‐ = OH^‐, Br^‐, N₃^‐, Me^‐, NHBu ^‐, NHtBu^‐, [FeCp(CO)₂]^‐) to aza‐closo‐dodecaboranes RNB₁₁H₁₁ ( 1 ) or to derivatives thereof with boron bound non‐hydrogen ligands yields nido‐clusters RNB₁₁H₁₁L or [RNB₁₁H₁₁X]^‐ or derivatives thereof, respectively, the non‐planar pentagonal aperture N—B4—B9—B8—B5 of which hosts a B8—B9 hydrogen bridge. The base is either bound to B8 ( 3 )or B4 ( 5 )or B2( 7 ). The structures of these adducts are concluded from ¹H and ¹¹B NMR including 2D‐NMR spectra, and in the case of MeNB₁₁H₁₁(NHEt₂) (type 3 ) also by a crystal structure analysis. With two of the adducts MeNB₁₁H₁₁L (L = py, NHEt₂), isomers of the type 3 , 5 , and 7 , and with two of the adducts, MeNB₁₁H₁₁(NH₂tBu) and {MeNB₁₁H₁₁[FeCp(CO)₂]}^‐, isomers of the type 3 and 7 could be identified. The position of boron‐bound ligands during the addition of bases to 1 shows, that only vertices of the ortho‐belt of 1 are involved in the opening process. A mechanism is made plausible that starts by the attack of the base at B2 of 1 and opening of the N‐B2 bond, denoted as a [3c, 1c]‐collocation, to give 2 with an endo‐H atom, whose migration into an adjacent bridge position and opening of a second B—N bond, denoted as a [3c, 2c]‐translocation, gives 3 ; this isomer can be transformed into 7 by a sequence of [3c, 2c]‐translocations via the isomers 4 , 5 , and 6 . The chiral type 3 species MeNB₁₁H₁₁L with L = NHEt₂, NH₂tBu undergo a rapid enantiomerization, for whose mechanism the exchange of the bridging and the amine‐H atom has been made plausible.     

Kinetics and products of propargyl (C3H3) radical self‐reactions and propargyl‐methyl cross‐combination reactions

Propargyl (HCC CH₂) and methyl radicals were produced through the 193‐nm excimer laser photolysis of mixtures of C₃H₃Cl/He and CH₃N₂CH₃/He, respectively. Gas chromatographic and mass spectrometric (GC/MS) product analyses were employed to characterize and quantify the major reaction products. The rate constants for propargyl radical self‐reactions and propargyl‐methyl cross‐combination reactions were determined through kinetic modeling and comparative rate determination methods. The major products of the propargyl radical combination reaction, at room temperature and total pressure of about 6.7 kPa (50 Torr) consisted of three C₆H₆ isomers with 1,5‐hexadiyne(CHC CH₂ CH₂ CCH, about 60%); 1,2‐hexadiene‐5yne (CH₂CC CH₂ CCH, about 25%); and a third isomer of C₆H₆ (∼15%), which has not yet been, with certainty, identified as being the major products. The rate constant determination in the propargyl‐methyl mixed radical system yielded a value of (4.0 ± 0.4) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for propargyl radical combination reactions and a rate constant of (1.5 ± 0.3) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ for propargyl‐methyl cross‐combination reactions. The products of the methyl‐propargyl cross‐combination reactions were two isomers of C₄H₆, 1‐butyne (about 60%) and 1,2‐butadiene (about 40%). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 118–124, 2000     

The E and Z isomers of 3‐(benzoxazol‐2‐yl)­prop‐2‐enoic acid

The carboxyl­ic acid group and the double bond are coplanar in (E)‐3‐(benzoxazol‐2‐yl)­prop‐2‐enoic acid, C₁₀H₇NO₃, whereas in isomeric (Z)‐3‐(benzoxazol‐2‐yl)­prop‐2‐enoic acid, also C₁₀H₇NO₃, they are almost orthogonal. In both isomers, a strong O—H⋯N hydrogen bond, with the carboxyl­ic acid group as a donor and the pyridine‐like N atom as an acceptor, and weak C—H⋯O interactions contribute to the observed supramolecular structures, which are completed by π–π stacking interactions between oxazole and benzenoid rings.