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排序方式: 共有211条查询结果,搜索用时 15 毫秒
81.
Sievänen E Noponen V Král V Bríza T Kolehmainen E 《Magnetic resonance in chemistry : MRC》2008,46(4):392-397
Two geminal difluorosteroids, 3,3-difluoro-5beta-cholan-24-oic acid (1) and 3,3-difluoro-5alpha-androstan-17-one (2), have been prepared from corresponding ketosteroids with diethylaminosulphurtrifluoride (DAST) treatment in moderate yields. The structures of 1 and 2 have been characterized by (1)H, (13)C, (19)F NMR, and ESI mass spectral techniques. 相似文献
82.
The objective of this work is to investigate the influence of process parameters on the pressurized liquid extraction (PLE) of Ilex paraguariensis leaves. A factorial 26−2 experimental design was employed using responses as the extraction yield and the chromatographic profile of the extracts. The extraction time, polarity of solvent, amount of sample, numbers of PLE cycles, flushing volume and extraction temperature were selected as independent variables (factors). Results obtained indicated that the solvent polarity was the most significant variable in the study, while the amount of sample and extraction temperature also showed significant effect. The other variables did not present significant influence in the yield of extraction. GC/MS analysis of the extract enabled the identification of saturated hydrocarbons, fatty acids, fatty acid methyl esters, phytosterols and theobromine in the extracts. Quantitative analysis of four compounds presented in the extracts (caffeine, phytol, vitamin E and squalene) was performed by the GC/MS in the SIM mode. 相似文献
83.
84.
Eliane Lazzari Katia Arena Elina B. Caramão Paola Dugo Luigi Mondello Miguel Herrero 《Electrophoresis》2021,42(1-2):58-67
Pyrolysis processes are an alternative to minimize the environmental problem associated to agrifood industrial wastes. The main product resulting from these processes is a high-value liquid product, called bio-oil. Recently, the use of comprehensive two-dimensional liquid chromatography (LC × LC) has been demonstrated as a useful tool to improve the characterization of the water-soluble phases of bio-oils, considering their complexity and high water content. However, the precise composition of bio-oils from different agrifood byproducts is still unknown. In the present study, the qualitative and quantitative screening of eight aqueous phases from different biomasses, not yet reported in the literature, using LC × LC is presented. The two-dimensional approach was based on the use of two reverse phase separations. An amide column in the first dimension together with a C18 column in the second dimension were employed. Thanks to the use of diode array and mass spectrometry detection, 28 compounds were identified and quantified in the aqueous phase samples with good figures of merit. Samples showed a distinct quali-quantitative composition and a great predominance of compounds belonging to aldehydes, ketones and phenols, most of them with high polarity. 相似文献
85.
Outi Elina Maasalo 《Journal d'Analyse Mathématique》2008,106(1):191-207
We consider a metric space equipped with a doubling measure and a length metric. We prove that p-superharmonic functions are integrable with a small exponent on Hölder domains of the space. 相似文献
86.
Péres VF Saffi J Melecchi MI Abad FC Martinez MM Oliveira EC Jacques RA Caramão EB 《Journal of chromatography. A》2006,1105(1-2):148-153
Piperaceae family is original from tropical regions and it shows more than 700 species around the world. Piper gaudichaudianum Kunth is the specie more abundant in Brazil, occurring from Northeast to South Brazil. In this paper, it was investigated the influence of some experimental parameters on the pressurized liquid extraction (PLE) of P. gaudichaudianum Kunth leaves, using petroleum ether as extractor solvent. The optimization of the main variables involved in the PLE process (extraction temperature and time) has been done by response surface methodology (RSM) using, as responses, the extraction yield and the chromatographic profile (GC/MS) of the extracts. The optimized procedure employed 3 g of ground leaves, 10 min of extraction and one cycle of extraction at 85 degrees C. The major compounds present in the petroleum ether extracts were: palmitic acid, stearic acid and nerolidol. The results presented in this work show the possibility of using a fast and easy process to recover compounds from P. gaudichaudianum Kunth. 相似文献
87.
Vaz de Campos MC Oliveira EC Filho PJ Piatnicki CM Caramão EB 《Journal of chromatography. A》2006,1105(1-2):95-105
Naphthenic acids, C(n)H(2n+Z)O(2), are a complex mixture of alkyl-substituted acyclic and cycle-aliphatic carboxylic acids. The content of naphthenic acids and their derivatives in crude oils is very small, which hinders their extraction from matrixes of wide and varied composition. In this work, liquid-liquid extraction, followed by solid phase extraction with an ion exchange resin (Amberlyst A-27) and ultrasound desorption were used to isolate the acid fraction from heavy gas oil of Marlim petroleum (Campos, Rio de Janeiro, Brazil). The analysis was accomplished through gas chromatography coupled to mass spectrometry with electron impact ionization, after derivatization with N-methyl-N-(tert-butyldimethylsilyl)trifluoracetamide (MTBDMSTFA). The results indicate the presence of carboxylic acids belonging to families of alicyclic and naphthenic compounds which contain up to four rings in the molecule. 相似文献
88.
Rosana Cassia de Souza Schneider Vanessa Zanon Baldissarelli Márcia Martinelli Elina Bastos Caramão 《Analytica chimica acta》2004,505(2):223-226
Optimization of the analytical conditions for determination of the partially hydrogenated products of castor oil obtained by catalytic hydrogen transfer, using Pd/C, with limonene as hydrogen source, was carried out in this work. The study involved the use of three capillary columns: SPB-1, HP-5 and HP-20 M. The best resolution for the oleic acids isomers, C18:1, was obtained with HP-20 M column while, the best resolution, identification and quantification for the products containing OH and CO were obtained with a HP-5 column after the silylation of the methyl esters. 相似文献
89.
Zainab Al-Shuhaib Henning Böckemeier Lawrence Coghlan Elina Dörksen Iestyn V. Jones Patrick J. Murphy Robert Nash James M. Page 《Tetrahedron letters》2013
The Pd-mediated π-allyl cyclisation of bis-Cbz- and bis-Boc-protected guanidines 4 and 14b led to the formation of five- and six-membered heterocycles 5 and 17 in high yields. 相似文献
90.
Anna C. Closs Elina Fuks Maximilian Bechtel Prof. Dr. Oliver Trapp 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(47):10702-10706
Organocatalysis is a powerful approach to extend and (enantio-) selectively modify molecular structures. Adapting this concept to the Early Earth scenario offers a promising solution to explain their evolution into a complex homochiral world. Herein, we present a class of imidazolidine-4-thione organocatalysts, easily accessible from simple molecules available on an Early Earth under highly plausible prebiotic reaction conditions. These imidazolidine-4-thiones are readily formed from mixtures of aldehydes or ketones in presence of ammonia, cyanides and hydrogen sulfide in high selectivity and distinct preference for individual compounds of the resulting catalyst library. These organocatalysts enable the enantioselective α-alkylation of aldehydes under prebiotic conditions and show activities that correlate with the selectivity of their formation. Furthermore, the crystallization of single catalysts as conglomerates opens the pathway for symmetry breaking. 相似文献