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排序方式: 共有8135条查询结果,搜索用时 9 毫秒
901.
Jim J. Huang Aris Ragouzeos Janet L. Rideout 《Journal of heterocyclic chemistry》1994,31(6):1685-1688
Benzylation of 6-benzylthiopurine was examined. Structural assignments of the products were determined by 1-D and 2-D nmr spectroscopy (HMQC, HMBC, and nOe). In the presence of base, the isomeric N3-, N7-, and N9-benzylated products 4, 3 , and 2 were isolated; however, only 9-benzyl-6-benzylthio-purine ( 2 ) was obtained in the absence of base. In the latter case, the initially formed N3- and N7-isomers were, in the presence of acid, converted to 9-benzyl-6-benzylthiopurine ( 2 ) via a 6-benzylthiopurine intermediate as evidenced by analysis of the reaction over time using reversed-phase hplc. 相似文献
902.
Morton E. Gurtin William O. Williams 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1967,18(1):132-135
Zusammenfassung Es wird eine Methode beschrieben für die Charakterisierung von Stoff-Phasen durch Angabe der Symmetrie-Gruppe als Funktion der Deformation und Temperatur. Wenn in einem elastischen Stoff die Phase durch Dichte und Temperatur bestimmt wird, dann gelten die meisten der üblichen Resultate für die Gesamt-Symmetrie-Gruppe auch für die Gruppe, die einer gegebenen Phase entspricht. 相似文献
903.
The concise, enantioselective total synthesis of the potent antitumor antibiotics (+)-FR900482 and (+)-FR66979 are described. Sharpless asymmetric epoxidation technology has been deployed to construct the optically active aziridine-containing fragment that is joined to the aromatic moiety in a highly convergent manner. Dimethyldioxirane effects the remarkable one-step deprotection/oxidative cyclization of an eight-membered ring amino-ketone to the unique hydroxylamine hemiketal ring system that is a distinctive structural motif of FR900482. This reaction has been exploited in a concise 33-step enantioselective total synthesis of FR900482. 相似文献
904.
Black PJ Cami-Kobeci G Edwards MG Slatford PA Whittlesey MK Williams JM 《Organic & biomolecular chemistry》2006,4(1):116-125
Alcohols have been employed as substrates for C-C bond-forming reactions which involve initial activation by the temporary removal of hydrogen to form an aldehyde. The intermediate aldehyde is converted into an alkene via a Horner-Wadsworth-Emmons reaction, nitroaldol and aldol reactions. The 'borrowed hydrogen' is then returned to the alkene to form a C-C bond. 相似文献
905.
A practical synthesis of acylated glycosyl isothiocyanates from sugar oxazolines, by reaction with thiophosgene, is reported. In the absence of any additive, the reaction is governed by the reverse anomeric effect, leading to the equatorially oriented isothiocyanate. However, in the presence of copper(II) chloride, the reaction proceeds preferentially with retention of the configuration at the anomeric center, providing the axial anomer as the major product. Noteworthy, this strategy allows accessing per-O-acetylated glycopyranosyl isothiocyanates with 1,2-cis relative configuration (e.g., the alpha-anomer in the D-gluco and D-galacto series), a problem that was outside the scope of previous methodologies. 相似文献
906.
Bacterial monooxygenase enzymes catalyze a regiospecific single-step hydroxylation of diphenylacetylene to yield meta- and para-hydroxydiphenylacetylene. 相似文献
907.
P. A. Lane C. R. Whitehouse T. Martin M. Houlton G. M. Williams A. G. Cullis S. S. Gill J. R. Dawsey G. Ball B. T. Hughes M. A. Crouch M. B. Allenson 《Journal of Crystal Growth》1992,120(1-4):245-251
This paper describes the first reported use of diethylaluminium hydride-trimethylamine adduct (DEAlH-NMe3) for the growth of GaAs/GaAlAs power heterojunction bipolar transistors (HBTs) by chemical beam epitaxy (CBE). This precursor possesses a significantly higher vapour pressure than the more conventionally used triethylaluminium (TEA), and leads to much less stringent requirements for bubbler and gas-line heating, and also much-improved GaAs/GaAlAs heterojunction definition when no carrier gas is employed. The use of all-gaseous n- and p-type dopants offers significant technological advantages in CBE, and the current paper also provides the first report of the use of hydrogen sulphide for n-type doping of CBE-grown GaAlAs HBT emitter regions. In conclusion, DC and RF data obtained from the heterojunction bipolar transistors fabricated to date are described. A DC gain of 40 has already been measured and encouraging early data obtained from RF-probed devices are also presented. 相似文献
908.
909.
A potentially biomimetic synthesis of the guanidine-containing marine natural product crambescidin 359 via a double Michael addition of guanidine to a suitably functionalised bis-enone is reported. 相似文献
910.