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101.
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2H-labelled precursors and high-field 2H- NMR spectroscopy have been used to demonstrate that the first tetramic acid derived intermediate in the biosynthesis of malonomicin is 5-aminomethyl-4-hydroxy-3-succinoyl-3-pyrrolin-2-one, formed by condensation of L-2,3-diaminopropanoic acid and 3-oxoadipic acid. 相似文献
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N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns 下载免费PDF全文
Dr. Lars H. Finger Jannick Guschlbauer Dr. Klaus Harms Prof. Dr. Jörg Sundermeyer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(45):16292-16303
Depending on the amount of methanol present in solution, CO2 adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO2Me] and [EMMIm][OCO2Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO2 adduct 4 has been investigated and compared with the corresponding NHC–CO2 zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO2H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO2 zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]2 cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO2. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO2Me][X]. The first NHO‐SO2 adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol. 相似文献
105.
Qian Zhang Corinna Harms Jens Mitzel Pawel Gazdzicki K. Andreas Friedrich 《Current Opinion in Electrochemistry》2022
The durability of polymer electrolyte membrane fuel cells (PEMFCs) needs to be further improved to cope with application requirements and economic competitiveness. This article highlights the challenges in the reliable determination of degradation rates and lifetime. The reliable evaluation of performance degradation rates is fundamental to quantify and benchmark durability and to allow comparisons between PEMFC durability tests performed using different materials or in different laboratories. The use of efficient recovery procedures enables the discrimination of reversible and irreversible voltage losses and facilitates the understanding of recovery mechanisms. In the end, recent contributions about lifetime diagnoses and prediction are presented, which are promising to be implemented in PEMFC applications. 相似文献
106.
Experimental studies were undertaken to determine the torsional stress concentration factors (Kt) associated with circular holes in bone. Reflective photoelasticity was used to determine the stress field around a circular
hole through one wall of the bone. A single adult sheep femur was used as the torsional model, in which six circular holes
were concentrically machined through the posterior cortex. These holes ranged from 10.4 percent to 66.4 percent of the mediolateral
bone diameter. From the photoelastic data, a stress concentration curve was developed for bone. The maximum stress location
on the boundary of the hole was found to shift from the previously expected 45-deg location.
Studies on tubes made of steel and plastic, both coated with photoelastic coating, were also performed. Three different pieces
of steel tubing with similar inner to outer diameters were coated with different thicknesses of photoelastic coating. The
variation in coating thickness did not appear to influence the stress-concentration factors in steel. TheKt in steel for 10 percent and 20 percent defects agreed with theKt associated with similar defects in bone. A single piece of plastic tubing was used in which six holes from 10 percent to
60 percent of the tube's outer diameter were concentrically machined through one wall. The location of the maximum stress
around the boundary of the hole was found to shift, and this agreed with the maximum stress shift found in the bone.
Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11. 相似文献
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Alves GA Amato S Anjos JC Appel JA Astorga J Bernard T Bracker SB Cremaldi LM Darling CL Dixon RL Errede D Gay C Green DR Jedicke R Karchin PE Kwan S Lueking LJ de Mello Neto JR Metheny J Milburn RH de Miranda JM da Motta Filho H Napier A Passmore D Rafatian A dos Reis AC Ross WR Santoro AF Sheaff M Souza MH Spalding WJ Stoughton C Streetman ME Summers DJ Takach SF Wallace A Wu Z 《Physical review letters》1994,72(6):812-815