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排序方式: 共有511条查询结果,搜索用时 31 毫秒
61.
Diego Cortes José Luis Rios Angel Villar Serafin Valverde 《Tetrahedron letters》1984,25(30):3199-3202
The structures of Cherimolin and Dihydrocherimolin, two new antimicrobial γ-lactone-bis-tetrahydrofuranes from Mill., Annonaceae, have been elucidated by spectroscopic analysis. 相似文献
62.
Polymer films containing dispersions of liquid crystal microdroplets have considerable potential for use in displays and other light control devices. These polymer-dispersed liquid crystal (PDLC) films operate by electric field control of light scattering, rather than by polarization control as in the case of twisted nematic systems. The scattering characteristics of the PDLC films are determined by the refractive indices of the polymer and liquid crystal and by the size of the microdroplets. We have found that it is possible to regulate the microdroplet size by controlling the droplet formation rate (i.e. the cure kinetics of the film). Using calorimetry and scanning electron microscopy, we determined the influence of cure kinetics on microdroplet size for epoxy-based PDLCs. We found that droplet size increased with increasing cure time constant. However, the relationship changed as cure temperature was varied, perhaps as a result of competing cure processes. We also determined the phase behaviour of the epoxy-based PDLCs. The liquid crystal acted as a plasticizer, depressing the glass transition temperature of the PDLC samples slightly below that of the pure epoxy. The temperature and enthalpy of the nematic to isotropic transition of the liquid crystal material in the microdroplets were both functions of cure temperature. From the transition enthalpy it was possible to estimate a, the fraction of liquid crystal contained in the droplets; we found that a decreased with increasing cure temperature, presumably as a result of greater liquid crystal solubility in the epoxy matrix at higher temperatures. 相似文献
63.
64.
Victor?de?FreitasEmail author Nuno?Mateus 《Analytical and bioanalytical chemistry》2011,401(5):1463-1464
Pyranoanthocyanins constitute one of the most important classes of anthocyanin-derived pigments occurring naturally in red wine. Nonetheless, correct assignment of their structures and pathways of formation in red wine has been relatively recent—less than two decades. Study of these newly discovered pigments is progressively unfolding the chemical pathways that drive the evolution of red wine colour during ageing. The objective of this paper is to review current knowledge regarding the pathway of formation in red wine of a great variety of pyranoanthocyanin structures, namely carboxypyranoanthocyanins, methylpyranoanthocyanins, pyranoanthocyanin-flavanols, pyranoanthocyanin-phenols, portisins, oxovitisins, and pyranoanthocyanin dimers. The chromatic features of some of the compounds, for example their colour expression and acid–base equilibria in aqueous media, are also discussed. 相似文献
65.
Yeast cell wall particles: a promising class of nature-inspired microcarriers for multimodal imaging
Figueiredo S Moreira JN Geraldes CF Rizzitelli S Aime S Terreno E 《Chemical communications (Cambridge, England)》2011,47(38):10635-10637
This communication demonstrates that yeast cell wall particles (YCWPs) are a promising class of nature-inspired biocompatible microcarriers for the delivery of amphipathic/lipophilic imaging reporters. When a paramagnetic MRI agent is loaded, the longitudinal relaxivity per particle at 0.5 T is the highest ever reported for Gd-based systems. 相似文献
66.
67.
A novel organocatalytic asymmetric [3+2] cycloaddition reaction between methyleneindolinones and allylic compounds yielding complex spirocyclopentaneoxindoles has been developed. It provides extraordinary levels of enantioselective control involving a chiral phosphine as a nucleophilic organocatalyst. Simple precursors were used under mild conditions to construct oxindole derivatives with high enantiopurity and structural diversity. This method should be useful in medicinal chemistry and diversity-oriented syntheses of these intriguing compounds. 相似文献
68.
Conreaux D Belot S Desbordes P Monteiro N Balme G 《The Journal of organic chemistry》2008,73(21):8619-8622
Various 3-iodo-4-methoxypyridin-2-ones and related pyrone and coumarin derivatives have been demonstrated as readily available precursors of 2-substituted furan-fused heterocycles by means of in situ sequential Sonogashira-acetylide coupling, dealkylation, and regioselective furan annulation reactions. A Et3N-induced S(N)2 process has been established that accounts for the dealkylation process. 相似文献
69.
The study of ecological systems has generated deep interest in exploring the complexity of chaotic food chains. The role of chaos in ecosystems is not entirely understood. One approach to have a better comprehension of ecological chaos is by analyzing it in mathematical models of basic food chains. In this article it is considered a classical chaotic food chain model from the literature. We use the theory of symbolic dynamics to study the topological entropy and the parameter space ordering of kneading sequences associated with one-dimensional maps that reproduce significant aspects of the model dynamics. The topological entropy allows us to distinguish different chaotic states in some realistic system parameter region. Another numerical invariant is introduced in order to characterize isentropic dynamics. Studying a set of maps with the same topological entropy, we exhibit numerical results about the relation between the second topological invariant and each of the control parameters in consideration. This work provides an illustration of how our understanding of ecological models can be enhanced by the theory of symbolic dynamics. 相似文献
70.
Vale N Matos J Moreira R Gomes P 《Journal of the American Society for Mass Spectrometry》2008,19(10):1476-1490
Electrospray ionization-ion trap mass spectrometry (ESI-MS) of imidazolidin-4-one peptidomimetic derivatives of the antimalarial
drug primaquine (PQ) is reported. These compounds contain the imidazolidin-4-one moiety either at the N- or the C-terminal of a dipeptide backbone, thus respectively mimicking PQ-Amino Acid-Proline (PQAAPro) and PQProAA derivatives of
PQ. Both the peptidomimetics and precursors previously developed by us are promising drug candidates, as they were found to
be active against rodent Plasmodium berghei malaria and Pneumocystis carinii pneumonia. Collision-induced dissociation (CID) and tandem-mass spectra (MS) of the title compounds, and fragmentation pathways
thereof, led to the following findings: (1) CID patterns present some parallelism with the reactivity towards hydrolysis previously
found for the same or related compounds; (2) a positional shift of the imidazolidin-4-one ring is reflected on both degree
and pathways of fragmentation, which makes tandem-MS a key tool for differentiation of imidazolidin-4-one isomers; (3) the
major MS/MS fragmentation of PQProAA mimetics involves release of a neutral diketopiperazine (DKP), in parallel to the “diketopiperazine
pathway” described in tandem-MS studies of oligopeptides; (4) the relative abundance of a major fragment in tandem-MS spectra
is inversely correlated with the size of the N-terminal AA in PQProAA mimetics. Overall, this work embodies an original and valuable contribution towards a deeper insight
into the molecular properties of novel antimalarials, which can be viewed as representative of both the 8-aminoquinoline and,
especially, the imidazolidin-4-one structural classes. 相似文献