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41.
[structure: see text] A new strategy for the synthesis of peptidyl and peptidomimetic P1-aldehydes 3 on HCAM solid support is described. The appropriate C-terminal aldehyde precursors were prepared and anchored to a resin support via a semicarbazone linkage (HCAM resin). After synthetic elaboration, acidic hydrolysis efficiently delivered C-terminal target aldehydes 3a-h in good overall yields and in excellent purity. 相似文献
42.
T. N. Ul'yanova G. G. Dvoryantseva L. M. Alekseeva Yu. N. Sheinker A. S. Elina 《Chemistry of Heterocyclic Compounds》1971,7(6):790-795
The dependence of the chemical shifts of the protons of quinoxaline and quinoxaline and pyrazine N-oxides and N,N-dioxides on the acid concentration was studied. The changes in the PMR spectra on passing from the neutral bases to the monocations were examined. The first protonation of pyrazine and quinoxaline N-oxides occurs at the N4 nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 846–852, June, 1971. 相似文献
43.
44.
45.
Péres VF Saffi J Melecchi MI Abad FC de Assis Jacques R Martinez MM Oliveira EC Caramão EB 《Journal of chromatography. A》2006,1105(1-2):115-118
This paper describes a comparative study of extraction methods of terpenes (terpenic alcohols and phytosterols), fatty acids and Vitamin E from leaves of Piper gaudichaudianum Kunth. The analysis of extracts was done by gas chromatography with mass spectrometric detection. The identification and quantification was made by co-injections of the extract with certified standards. The use of pressurized liquid extraction (PLE; Dionex trade name: ASE, for accelerated solvent extraction) decrease significantly the total time of extraction, the amount of solvent and the manipulation of sample and solvents in comparison with soxhlet (SE) and ultrasound-assisted (USE). In addition, PLE was more effective for the extractions of terpenes (terpenic alcohols and phytosterols), fatty acids and Vitamin E. 相似文献
46.
Assis Jacques R dos Santos Freitas L Flores Peres V Dariva C de Oliveira JV Bastos Caramão E 《Journal of separation science》2006,29(18):2780-2784
The objective of this work was to investigate the extraction of Ilex paraguariensis leaves by means of three extraction techniques: pressurized liquid extraction (PLE, also called accelerated solvent extraction – ASE), maceration, and sonication. Samples of mate tea leaves were collected from an experiment conducted under agronomic control at Indústria e Comércio de Erva‐Mate Bar?o LTDA, Brazil. Six solvents with increasing polarities (n‐hexane, toluene, dichloromethane, ethyl acetate, acetone, and methanol) were used in this investigation. Chemical analysis of the extracts was performed by GC coupled with a mass spectrometer detector. The identification and quantification were accomplished by coinjections of certified standards. The results showed that no significant differences in the qualities of the extracts were noticed regarding the extraction methods. On the other hand, the PLE technique was found to be more effective for the extractions of caffeine, phytol, palmitic, and stearic acid. The use of PLE led to a significant decrease in the total extraction time, amount of solvent consumption, and manipulation of samples compared to maceration and ultrasound‐assisted extraction methods. 相似文献
47.
Hydrogen bonding in phosphonate cavitands: Investigation of host-guest complexes with ammonium salts
Elina Kalenius Raisa Neitola Michele Suman Enrico Dalcanale Pirjo Vainiotalo 《Journal of the American Society for Mass Spectrometry》2010,21(3):440-450
The H-bonding in alkylammonium complexes of phosphonate cavitands were studied by mass spectrometric methods and theoretical
calculations. The alkylammonium ions included primary, secondary, and tertiary methyl- and ethylammonium ions. Their complexation
with mono-, tetra-, and two di-phosphonate cavitands, which differ according to the number and position of H-bond acceptor
P = O groups, was evaluated by using different competition experiments, energy-resolved CID, gas-phase H/D-exchange, and ligand-exchange
reactions, together with ab initio theoretical optimization of the complexes. The phosphonate cavitands with two or more adjacent
P = O groups were found to be selective towards secondary alkylammonium ions, due to simultaneous formation of two stable
hydrogen bonds. In the ion-molecule reactions (both H/D- and ligand-exchange), the formation of two stable hydrogen bonds
was observed either to slow down the reaction or to completely prevent it. This was, however, limited to situations where
two hydrogen bonds are formed between the H-bond donor sites of the alkyl ammonium ion and the vicinal H-bond acceptor sites
of the cavitand. 相似文献
48.
Persianova I. V. Sheinker Yu. N. Titkova R. M. Elina A. S. 《Chemistry of Heterocyclic Compounds》1977,13(7):781-784
The direction of protonation (1-N or 5-N) or 2-, 2,3-, and 2,6-substituted derivatives of 1,5-naphthyridine was determined on the basis of the basicity constants.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 965–968, July, 1977. 相似文献
49.
R. M. Titkova A. S. Elina N. P. Kostyuchenko 《Chemistry of Heterocyclic Compounds》1972,8(9):1118-1121
The N-oxidation reactions of 2-hydroxy- and 2-amino-1,5-naphthyridines were investigated. 2-Hydroxy-1,5-naphthyridine 5-oxide and 2-hydroxy-, 2-amino-, and 2-acetamido-1,5-naphthyridine 1,5-dioxides were obtained. The structures of the compounds obtained were proved by means of IR and PMR spectroscopy.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1237–1240, September, 1972. 相似文献
50.
M. M. Kaganskii G. G. Dvoryantseva A. S. Elina 《Chemistry of Heterocyclic Compounds》1973,9(3):369-373
The UV spectra of the bases and cations of a number of 2-substituted quinoxaline derivatives were investigated, and their basicity constants were determined. The investigated compounds were divided into two series, characterized, respectively, by ortho and meta orientations of the substituent and cationoid center, on the basis of correlations of the pKa values with the Hammett—Taft constants. The effect of the position of the protonation center relative to the substituent on the energy of the * transitions was examined within the framework of the simple MO LCAO method. The results of the calculations are compared with the experimentally observed shift in the1Lb band in the UV spectra of some 2-substituted quinoxalines on passing from the bases to the monocations.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 398–403, March, 1973. 相似文献