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1.
The phenyldimethylsilyl-substituted monoketene PhMe(2)SiCH=C=O (1) and bisketene (PhMe(2)SiC=C=O)(2) (3) have been prepared and compared to the corresponding Me(3)Si- and t-BuMe(2)Si-substituted species. The (13)C, (17)O, and (29)Si NMR spectra fit the pattern shown by other silylketenes and provide no evidence for transmission of a substituent effect of the Ph group through the silicon to the ketenyl group, as has been proposed for PhMe(2)Si-substituted radicals. The UV spectrum of 1 does show a longer lambda and greater epsilon than for t-BuMe(2)SiCH=C=O, and this may indicate some interaction of the phenyl group with the ketene chromophore, while the greater reactivity of 1 in hydration compared to t-BuMe(2)SiCH=C=O is ascribed to the inductive effect of the phenyl. The very similar ring-opening reactivity of the bis(phenyldimethylsilyl)cyclobutenedione (6) to form 3 compared to the bis(Me(3)Si) analogues also provides no evidence of a significant interaction of the phenyl with the ketene. A new type of stabilized 1,8-bisketene based on the arylbis(dimethylsilyl) grouping, namely, 1,4-bis(ketenyldimethylsilyl)benzene (12), has been prepared for the first time. 相似文献
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Thomas T. Tidwell 《Tetrahedron letters》1979,20(48):4615-4618
Diethylketene and organometallic reagents form enolates capturable by methyl iodide or trimethylsilyl chloride with greater than 95% positional selectivity. 相似文献
5.
calculations predict that cyano and trifluoromethyl groups both have large destabilizing effects on α-heteroatom stabilized cations, whereas CN is a much weaker destabilizer than CF3 of less stabilized cations. 相似文献
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Juliana S Luz Celso RR Ramos Márcia CT Santos Patricia P Coltri Fernando L Palhano Debora Foguel Nilson IT Zanchin Carla C Oliveira 《BMC biochemistry》2010,11(1):22
Background
The archaeal exosome is formed by a hexameric RNase PH ring and three RNA binding subunits and has been shown to bind and degrade RNA in vitro. Despite extensive studies on the eukaryotic exosome and on the proteins interacting with this complex, little information is yet available on the identification and function of archaeal exosome regulatory factors. 相似文献8.
Fu N Allen AD Kobayashi S Tidwell TT Vukovic S Matsuoka T Mishima M 《The Journal of organic chemistry》2008,73(5):1768-1773
Cyclobutenediones 5 disubstituted with HO (a), MeO (b), EtO (c), i-PrO (d), t-BuO (e), PhO (f), 4-MeOC6H4O (g), 4-O2NC6H4O (h), and 3,4-bridging OCH2CH2O (i) substituents upon laser flash photolysis gave the corresponding bisketenes 6a-i, as detected by their distinctive doublet IR absorptions between 2075 and 2106 and 2116 and 2140 cm-1. The reactivities in ring closure back to the cyclobutenediones were greatest for the group 6b-e, with the highest rate constant of 2.95 x 10(7) s-1 at 25 degrees C for 6e (RO = t-BuO) in isooctane, were less for 6a (RO = OH, k = 2.57 x 10(6) s-1 in CH3CN), while 6f-i were the least reactive, with the lowest rate constant of 3.8 x 10(4) s-1 in CH3CN for 6h (RO = 4-O2NC6H4O). The significantly reduced rate constants for 6f-i are attributed to diminution of the electron-donating ability of oxygen to the cyclobutenediones 5f-h by the ArO substituents compared to alkoxy groups and to angle strain in the bridged product cyclobutenedione 5i. The reactivities of the ArO-substituted bisketenes 6f-h in CH3CN varied by a factor of 50 and gave an excellent correlation of the observed rate constants log k with the sigma p constants of the aryl substituents. Computational studies at the B3LYP/6-31G(d) level of ring-closure barriers are consistent with the measured reactivities. Photolysis of squaric acid (5a) in solution provides a convenient preparation of deltic acid (7). 相似文献
9.
Renata Monteiro-Maia Maria B Ortigão-de-Sampaio Rosa T Pinho Luiz RR Castello-Branco 《Journal of immune based therapies and vaccines》2006,4(1):1-6
We hypothesize that the energy strategy of a cell is a key factor for determining how, or if, the immune system interacts
with that cell. Cells have a limited number of metabolic states, in part, depending on the type of fuels the cell consumes.
Cellular fuels include glucose (carbohydrates), lipids (fats), and proteins. We propose that the cell's ability to switch
to, and efficiently use, fat for fuel confers immune privilege. Additionally, because uncoupling proteins are involved in
the fat burning process and reportedly in protection from free radicals, we hypothesize that uncoupling proteins play an important
role in immune privilege. Thus, changes in metabolism (caused by oxidative stresses, fuel availability, age, hormones, radiation,
or drugs) will dictate and initiate changes in immune recognition and in the nature of the immune response. This has profound
implications for controlling the symptoms of autoimmune diseases, for preventing graft rejection, and for targeting tumor
cells for destruction. 相似文献
10.