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91.
Metalation of mesitylene with butyllithium in hexane in the presence of tetramethylethylenediamine yields mono-, di- and trilithiated benzylic compounds (1–5), characterized as their silylated derivatives. Dilithiation occurred partly at the same but preferentially at different methyls. The symmetrical tris-1,3,5-(lithiomethylene)benzene was the main product. Similar metalation occurred with m-xylene. o-Xylene gave also dilithiated products, but at a slower rate, and p-xylene gave slowly and exclusively the gem-dilithio-derivative.The results were explained by greater stabilization of polylithio derivatives with charges all on the starred carbons of the conjugated system, rather than those with charges uniformly dispersed on all carbons. CNDO/2 calculations of the energy of the anions account only partially for the order of stability of most of the studied compounds, but a good correlation between this experimental order and the energies of the lithium compounds was found. 相似文献
92.
A NEW FLUORESCENT PHOTOPRODUCT OF TRYPTOPHAN EVIDENCED BY LONG WAVELENGTH EXCITATION OF FLUORESCENCE
Abstract— Besides the normal tryptophan (Trp) fluorescence in aqueous solution (emission maximum at 350 nm), a new emission, peaking around 380 nm, appears by long wavelength excitation. Its fluorescence yield (φs 0.24) is higher than that of tryptophan (φTrp = 0.13). The growth of this emission is observed under different experimental conditions, mainly under UV anaerobic irradiation. To explain this observation, the formation of a C3 -hydroxylated derivative is tentatively suggested. 相似文献
93.
Uwe K. A. Klein Gertrud Hönes Friedrich W. Hafner 《Photochemistry and photobiology》1984,40(5):569-574
Abstract —The time-dependent rotational relaxation of Rhodamine 6G in fresh and dried human blood serum was investigated using picosecond phasefluorometry. Measurements were also carried out in aqueous solutions of amino acids, glucose, urea, and bovine serum albumin to evaluate our model for the interactions. It is shown that the distribution of Rhodamine 6G between the aqueous and the protein phase of the blood serum strongly depends on protein concentration and temperature. 相似文献
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New theorems are established for cages (or polyhedra) with trivalent vertices. One theorem says that all such cages have at least three Kekulé structures (or perfect matchings). Thence, resonance generally appears as a possibility. Another theorem says that for every even vertex count >70 there is at least one cage of a preferable subclass, while for vertex count <70 the sole preferable cage is that of the truncated icosahedron. Thence, the unique role of the buckminsterfullerene structure for C60 is mathematically indicated.[/p]Work supported by the Welch Foundation of Houston, Texas. 相似文献
96.
K. P. Klein H. K. Reimschuessel 《Journal of polymer science. Part A, Polymer chemistry》1971,9(9):2717-2725
Syntheses and reactions of 4-carboxy-2-pyrrolidone, 6,6-dimethyl-4-carboxy-2-piperidone, and 5,5-dimethyl-4-carboxy-2-pyrrolidone are described. Whereas 4-carboxy-2-pyrrolidone polymerizes upon heating to form a polyimide, the two lactams containing geminal dimethyl groups undergo isomerization. The 6,6-dimethyl-4-carboxy-2-piperidone isomerizes to 5,5-dimethyl-3-carboxymethyl-2-pyrrolidone, and 5,5-dimethyl-4-carboxy-2-pyrrolidone affords isopropylidene succinimide. Possible mechanisms for these reactions are discussed. 相似文献
97.
Joseph T. Klein Larry Davis Richard C. Effland 《Journal of heterocyclic chemistry》1987,24(3):725-731
The synthesis of a novel 2′,3′-dihydrospiro(benzofuran-2(3H),4′(1′H)isoquinoline] ring system ( IV ) by a nucleophilic aromatic fluoride displacement-cyclization is described. Preparation of various derivatives of IV as well as the precursor 4-(2-fluorobenzyl)-1,2,3,4-tetrahydro-4-isoquinolinols is also described. 相似文献
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