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91.
Andrus MB Meredith EL Hicken EJ Simmons BL Glancey RR Ma W 《The Journal of organic chemistry》2003,68(21):8162-8169
The total synthesis of (+)-geldanamycin (GA), following a linear route, has been completed using a demethylative quinone-forming reaction as the last step. Key steps include the use of two new asymmetric boron glycolate aldol reactions. To set the anti-C11,12 hydroxymethoxy functionality, (S,S)-5,6-bis-4-methoxyphenyldioxanone 8 was used. Methylglycolate derived from norephedrine 5 set the C6,7 methoxyurethane stereochemistry. The quinone formation step using nitric acid gave the non-natural o-quino-GA product 55 10:1 over geldanamycin. Other known oxidants gave an unusual azaquinone product 49. o-Quino-GA 55 binds Hsp90 with good affinity but is less cytotoxic compared to GA. 相似文献
92.
Deprotonation of the 3-trialkylsilyloxy-1,4-diene 3a and subsequent electrophilic substitution of the non-isolated 3-trialkylsilyoxypentadienyllithium 4 gives the α- and γ-products 8 and/or 6 in good yields. Whereas alkylation of 4 proceeds with variable regioselectivity (Table 1) aldehydes and ketones attack preferentially the γ-position of 4 (Table 2). The desired γ-products 6 may be directly subjected to inter- and intramolecular [4 + 2]-additions as demonstrated by the reactions 5a (? 6d ) → 7 and 6h → 19 (Schemes 4 and 12). Alternatively, smooth fluoride-promoted silylether-cleavage 6 → 11 (Scheme 8) provides a convenient approach to substituted vinyl ketones such as to the natural products 11f (Table 3). The stereoselective conversion 6k → 23 (Scheme 13) implies an endo-selective intramolecular Diels-Alder addition ( 26 → 23 ) and exemplifies the use of 4 as an equivalent of the hypothetical anion IV . Furthermore, some electrophilic substitutions of the hexadienyllithium 15 have been studied (Scheme 10). 相似文献
93.
A method is proposed for the colorimetric determination of carbon in titanium. In the method the sample is dissolved in a mixture of sulfuric and fluoboric acids, and nitric acid is added to dissolve the titanium carbide. The solution is boiled and filtered, and the yellow coloration from the nitrated organic complex is read with a spectrophotometer. A spectrophotometric study of the color was made. A comparison was made between the colorimetric determination of carbon in titanium and the colorimetric determination of carbon in steels. The proposed method is rapid and is readily adaptable to the determination of carbon in a large number of samples at one time. The procedure is designed for titanium containing up to 0.7 per cent. carbon. None of the elements encountered in “commercially pure” titanium metal interferes. 相似文献
94.
M. emougin J. M. esrijelew L. S. Ssolowcitschik D. B. Judd D. L. Me Adam M. S. Kantrowitz R. H. Simmons F. R. Filz H. Fink R. Stahn A. Matthes L. Lilienfeld J. W. Me Bain W. L. Me Clatchie D. M. Greenberg M. M. Greenberg R. H. Miles Y. C. Tang H. L. Wang Erich Schmidt E. Graumann R. Schnegg W. Jandebeur Margarete Hecker W. Simson B. Schulze E. Rieger P. Guillot R. M. Cobb D. V. Lowe F. T. Carson L. Vidal P. Goldsmid H. Ahlquist S. Köhler C. V. Palmrose W. S. Hendrixson R. N. Miller W. H. wansoin H. John F. W. Poppet 《Analytical and bioanalytical chemistry》1937,109(7-8):281-299
95.
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98.
G. S. Hall N. Roach U. Simmons H. Cong M-l. Lee E. Cummings 《Journal of Radioanalytical and Nuclear Chemistry》1984,82(2):329-339
We have constructed an external beam proton-induced X-ray emission (PIXE) system for trace element analysis of biological and environmental samples. Optimization of sample preparation and experimental PIXE set up has produced a rapid, accurate, sensitive, and reliable PIXE analysis procedure. The analytical procedure was evaluated using maternal scalp hair from pregnant women 16–19 weeks' gestation. The samples were irradiated with 3.5 MeV protons from the Rutgers Nuclear Physics Department's 8 MV Tandem FN Van de Graaff accelerator. The concentrations of the elements calcium (Ca) to cadmium (Cd) in hair are reported for 50 subjects. The analytical procedure is suitable for non-invasive clinical analysis for evaluation of nutritional states and for environmental exposure to toxic metals. 相似文献
99.
A practical and cost-effective synthesis of 3,3-difluoropyrrolidine is reported. The synthesis involves the isolation of two intermediates, which are prepared via two efficient through processes: (1) a Claisen rearrangement followed by a Ru(VIII)-catalyzed oxidation to prepare the 2,2-difluorosuccinic acid and (2) an efficient cyclization to form N-benzyl-3,3-difluoropyrrolidinone followed by BH(3).Me(2)S reduction. 相似文献
100.
The possible relevance of the Soret effect in geoscientific studies is discussed. In particular, the question is addressed of whether the observation that salinity generally increases with depth in geologic basins can be explained by this effect. We proceed from the fact that salinity increases with depth, in many cases almost precisely linearly at larger depths. This suggests that salinity may be interdependent with the geothermal gradient that behaves in precisely the same way. Our work on a transient analysis suggests that the timescales involved in establishing corresponding steady state salinity distributions in the presence of a temperature gradient could be on the order of 100 million years. 相似文献