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Radical addition to 2-cyclohexyl-5-methylidene-6-methyl-1,3-dioxan-4-one ( 2 ) affords stereoselectively 5,6-trans-products trans- 3 . The size of the radical has no influence on the selectivity. These trans-acetals are converted into threo-3-hydroxy-butanoates 4 . Methyl 2-methylidene-3-[(tert-butyl)diphenylsilyloxy]butanoate ( 5 ), treated under the same conditions, leads mainly to the erythro-isomer of 4 after deprotection. An influence of the steric bulkyness of the radical is observed. The stereochemical course of the reactions is discussed. 相似文献
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Towards a goal of detecting scaled-up DNA adducts as altered deoxynucleotides by mass spectrometry, we have set up a practical and general method for isolating DNA-derived deoxyribonucleoside-5'-monophosphates devoid of ribonucleotides starting with a 1 g sample of mammalian tissue. The method is practical because costs have been minimized, and it is general because it can be applied to a more difficult sample such as mouse skin or non-fresh calf liver. The procedure, consisting of a series of steps that were largely gleaned and tuned from prior literature, proceeds as follows: (1) homogenize the tissue in sodium dodecyl sulfate; (2) digest with ribonuclease A, ribonuclease TI, alpha-amylase and proteinase K; (3) partition between water and phenol; (4) precipitate the DNA with ethanol followed by redissolving and dialysis; and (5) digest with nuclease P1 and phosphodiesterase I followed by ultrafiltration and boric acid gel chromatography. The yellow to brown color of DNA from difficult tissues only persisted up to the ultrafiltration step. Apparently this DNA was contaminated with iron-containing proteins. Residual ribonucleotides were not observable (<0.1%) by HPLC in the final sample. Without boric acid gel chromatography, residual contamination by ribonucleotides was about 1% even when the DNA was purified before digestion by phenol partitioning followed by use of a Genomic Tip kit from Qiagen. 相似文献
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L. Criegee G. Franke A. Giese Th. Kahl G. Poelz U. Timm H. Werner W. Zimmermann 《Nuclear Physics B》1977,121(1):38-44
Photons of 3 GeV and 5 GeV were scattered on 7 different elements, ranging from Be to Au, and detected with a pair spectrometer. The angular distributions show diffractive patterns consistent with known nuclear sizes. Forward cross sections are 20–30% lower than expected from an A2 dependence. This shadowing effect is qualitatively explained by photon interactions via intermediate hadronic states. 相似文献