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91.
The reaction of 4-chloro-2-phenylquinazoline with K15NN2 has been studied by 15N-NMR. spectroscopy. 15N-chemical shifts in 5-phenyl-1 (3)-[15N]-tetrazolo[1,5-c]quinazoline and -Nα(Nγ)-[15N]-4-azido-2-phenylquinazoline are reported. The characteristic IR. absorption frequencies of the tetrazole group have been determined in a series of annelated 15N-labelled compounds. From these studies and the chemistry of the labelled tetrazoles, it is concluded that all haloazines examined react with KN3 by the direct nucleophilic substitution mechanism. An addition of nucleophile-ring opening-ring closure (ANRORC) mechanism was not observed. The synthesis of several 15N-labelled tetrazoloazines is described. 相似文献
92.
A novel synthetic approach to 2-oxo-Δ3-A-nor-steroids is described. The photochemical ring contraction of epoxyketones, cf. 4 → 5 , serves as the key step of this procedure, which gives notably higher yields than the previously published syntheses (e.g., overall yield 6 → 10 40%). 相似文献
93.
PF Hendrix SL Lachnicht MA Callaham X Zou 《Rapid communications in mass spectrometry : RCM》1999,13(13):1295-1299
Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
94.
Cervical spine MR imaging using multislice gradient echo imaging: comparison with cardiac gated spin echo 总被引:2,自引:0,他引:2
M V Kulkarni P A Narayana C B McArdle J W Yeakley N F Campagna F W Wehrli 《Magnetic resonance imaging》1988,6(5):517-525
Forty-one patients with suspected cervical spine disorders were studied using multislice gradient echo imaging (GE) technique, with a 1.5-T system. The images were compared to cardiac-gated spin echo (CGSE) images in the diagnosis of suspected cord and spinal disorders. Images were graded for ability to detect cord lesion, cord-CSF contrast, CSF-bone contrast and contrast between CSF and extradural abnormality. The signal-to-noise ratio and contrast-to-noise ratio were used to compare images. There was 44% decrease in contrast between cord lesion and normal cord on GE when compared to CGSE, except for spinal cord hemorrhage. There was a 40% improvement between bone and CSF contrast on GE compared to CGSE. GE images were significantly better qualitatively as well as quantitatively in the detection of extradural lesions. This effect was more marked in axial plane where CGSE images are extremely suboptimal. CGSE images are better than GE for spinal cord lesions, while GE are superior in the diagnosis of degenerative disease in the cervical spine. 相似文献
95.
A 18-step transformation of dehydroabietic acid ( 1 ) into ketone 40 , convenient starting material for the prparation of 14α-methylsteroids, is described. 相似文献
96.
97.
98.
Summary Nanocapsule preparative procedures are able to change the availability of drugs from delivery devices. In particular, the
influence of the organic solvent, used in the interfacial polymerisation process, on the shape, mean particle size, size distribution
and type of colloidal suspension was investigated. The organic solvents employed in the preparation of PECA nanocapsules were
ethanol, acetone or acetonitrile. A non-ionic surfactant (Pluronic F68) was used during the preparation process. The amount
of monomer represented another variable, capable of influencing the final carrier properties of PECA nanocapsules. The presence
of ethanol achieved the formation of both nanoparticles and nanocapsules. However, the presence of acetone or acetonitrile
achieved not only a highly homogeneous size colloidal system, but also the formation of only one type of PECA nanosphere (nanocapsule).
The different amount of monomer led to the formation of nanocapsules with a wall thickness proportional to the initial concentration
of the polymerising agent.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献
99.
100.
The stereochemistry of the isomeric 6, 7-methylene steroids 2 and 3 was established by reduction to the known 7-methyl compounds 6 and 8 , respectively. These results revise previous steric assignments based on n.m.r. data [4]. 相似文献