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排序方式: 共有224条查询结果,搜索用时 250 毫秒
1.
Wang D Schwinden MD Radesca L Patel B Kronenthal D Huang MH Nugent WA 《The Journal of organic chemistry》2004,69(5):1629-1633
The reaction of a variety of methyl esters with dimethylsulfoxonium methylide at 0-25 degrees C affords the chain-extended beta-keto dimethylsulfoxonium ylides. Subsequent treatment with hydrogen chloride in THF proceeds with loss of DMSO to afford the corresponding alpha-chloroketones. This sequence has been utilized to convert the methyl esters of CBZ-protected alanine and valine to the anti N-protected alpha-amino epoxides, which are important pharmaceutical intermediates. When the same protocol is applied to BOC-protected phenylalanine methyl ester, epimerization occurs so that the use of a more reactive aryl ester is required. This chemistry provides a practical route to alpha-chloroketones that avoids the use of toxic and explosive diazomethane. 相似文献
2.
The McLafferty-type rearrangements, which are the most facile fragmentation for the styryl- and the alkyl propenyl sulfoxides, have been proven to involve at least a β-hydrogen by deuterium labelling studies. γ-Hydrogen also rearranges, yet a cyclopropane instead of an alkene is eliminated. Furthermore, alkyl propenyl sulfoxides undergo hydrogen migration only to the surfinyl oxygen. 相似文献
3.
In-beam and matrix-isolation techniques have been used in the mass spectral studies of several categories of biologically significant compounds. These include amino acids, quaternary ammonium salts, vitamins and nucleosides. Molecular ions and/or (M+H)+ions are obtained, together with useful fragmentations, all of which are valuable in structural elucidations. Spectra obtained by this version of the in-beam technique are similar but not identical with those obtained by field desorption and secondary ion mass spectrometry. Ammonium and sodium chlorides, ammonium sulfate, p-toluenesulfonic and hydrochloric acids can all be used as a room temperature matrix. The detection limits for vitamin E and 2′-deoxyguanosine have been determined as 1 ngand 5 μg, respectively. 相似文献
4.
Abstract— Enzyme-generated triplet species transfer energy very efficiently to thylakoid membrane fractions enriched in either photosystem-I or photosystem-II. Independent of the nature of the triplet donor, the emission is always more intense with photosystem-I. Since the fluorescence quantum yield of chlorophyll in PS-I is lower and the rate of energy transfer usually smaller than to chlorophyll in PS-II, the yield of 1 S chlorophylls in PS-I is substantially higher. This is tentatively attributed to more favorable reverse intersystem crossing from an upper triplet state in PS-I. 相似文献
5.
Lilian F. J. Noël Ian A. Maxwell Willy J. M. Van Well Anton L. German 《Journal of polymer science. Part A, Polymer chemistry》1994,32(11):2161-2168
Recently a model has been developed capable of predicting absolute monomer concentrations and their ratios in the polymer, aqueous, and monomer droplet phases as a function of conversion in batch emulsion copolymerizations without using any adjustable parameters. In this article the sensitivity of model predictions of composition drift toward deviations of 10% in all model parameters (maximum swellabilities of monomer in the polymer phase, water solubilities, reactivity ratios, and monomer and polymer densities) was estimated using the monomer combination methyl methacrylate-styrene as an example. From the sensitivity analysis it can be concluded that the reactivity ratios are the most important parameters affecting composition drift. The effects of deviations in maximum swellabilities and monomer and polymer densities on composition drift can be neglected, while the water solubility is important only in those cases where the amount of monomer in the aqueous phase cannot be neglected as compared with the total monomer amount. © 1994 John Wiley & Sons, Inc. 相似文献
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格雷码辅助相移技术可以实现具有较强鲁棒性与抗噪能力的三维(3D)形貌测量。为解决由待测物体不均匀的表面反射率、噪声和物体运动等因素造成的级次边沿误码问题,提出了一种基于错位格雷码的动态3D形貌测量方法。将传统格雷码图案在投影前预先移动半个条纹周期得到错位格雷码图案,再采用传统格雷码解码方法对二值化后的错位格雷码图案解码,可得到与截断相位完全错开的解码结果。对该解码结果进行修正后即可利用得到的正确的相位级次辅助截断相位成功展开。同时,为了提高测量精度,引入了一个虚拟相位平面以进一步拓展投影条纹周期数。实验结果表明,所提方法在使用N帧格雷码图案的情况下,可以编码周期数为2N+1的投影条纹进行3D测量,其无需任何附加图案即可避免级次边沿误码问题,并有效提升了测量精度。复杂动态场景的3D重建结果证明,所提方法能够以2381 frame/s的速率实现高精度、高效率和高速的3D形貌测量。 相似文献
9.
Marcos A. P. Martins Adilson P. Sinhorin Clarissa P. Frizzo Lilian Buriol Elisandra Scapin Nilo Zanatta Helio G. Bonacorso 《Journal of heterocyclic chemistry》2013,50(1):71-77
5‐Bromo[5,5‐dibromo]‐1,1,1‐trihalo‐4‐methoxy‐3‐penten[hexen]‐2‐ones are explored as precursors to the synthesis of 3‐ethoxymethyl‐5‐trifluoromethyl‐1H‐pyrazoles from a cyclocondensation reaction with hydrazine monohydrate in ethanol. 3‐Ethoxymethyl‐carboxyethyl ester pyrazoles were formed as a result of a substitution reaction of bromine and chlorine by ethanol. The dibrominated precursor furnished 3‐acetal‐pyrazole that was easily hydrolyzed to formyl group. In addition, brominated precursors were used in a nucleophilic substitution reaction with sodium azide to synthesize the 3‐azidomethyl‐5‐ethoxycarbonyl‐1H‐pyrazole from the reaction with hydrazine monohydrate. These products were submitted to a cycloaddition reaction with phenyl acetylene furnishing the 3‐[4(5)‐phenyl‐1,2,3‐triazolyl]5‐ ethoxycarbonyl‐1H‐pyrazoles and to reduction conditions resulting in 3‐aminomethyl‐1H‐pyrazole‐5‐carboxyethyl ester. The products were obtained by a simple methodology and in moderate to good yields. 相似文献
10.
Lilian Alnsour Reem Issa Shady Awwad Dima Albals Idrees Al-Momani 《Molecules (Basel, Switzerland)》2022,27(5)
Phenolic and antioxidant compounds have received considerable attention due to their beneficial effects on human health. The aim of this study is to determine the content of total phenols and antioxidants in fifty-two coffee samples of different origins, purchased from the Jordanian local market, and investigate the effect of the degree of roasting on the levels of these compounds. The coffee samples were extracted using the hot water extraction method, while Folin–Ciocalteu (FC) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay methods were used to analyze these compounds. The results showed that the highest content of total phenol (16.55 mg/g equivalent to GAE) was found in the medium roasted coffee, and the highest content of antioxidants (1.07 mg/g equivalent to TEAC) content was found in the green coffee. Only light and medium roasted coffee showed a significant correlation (p < 0.05, R2 > 0.95) between the average of total phenolic and antioxidant content. A negative correlation between the antioxidant content and the degree of roasting (p < 0.05, R2 > 0.95) were shown, while it did not correlate with phenolic contents. Previously, a positive correlation between antioxidant and chlorogenic acids content was observed, with no correlation between the origin of coffee samples nor heavy metal content, which was previously determined for the same coffee samples. These findings suggest that the antioxidant content for coffee extracts is largely determined by its chlorogenic acid content, rather than the coffee origin or total phenolic and heavy metals content. 相似文献