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排序方式: 共有220条查询结果,搜索用时 15 毫秒
1.
Steven D. Bull Simon W. Epstein Nadeam Mujtaba Edward D. Savory Juan A. Tamayo 《Tetrahedron》2006,62(33):7911-7925
Enolates of (S)-N,N′-bis-(p-methoxybenzyl)-3-iso-propylpiperazine-2,5-dione exhibit high levels of enantiodiscrimination in alkylations with (RS)-1-aryl-1-bromoethanes and (RS)-2-bromoesters, affording substituted diketopiperazines containing two new stereogenic centres in high de. Deprotection and hydrolysis of the resultant substituted diketopiperazines provides a route to the asymmetric synthesis of homochiral methyl 2-amino-3-aryl-butanoates and 3-methyl-aspartates in high de and ee. 相似文献
2.
P. J. Bream J. J. Lim S. Bull A. V. Andrianov S. Sujecki E. C. Larkins 《Optical and Quantum Electronics》2006,38(12-14):1019-1027
We describe the inclusion of nonequilibrium gain into a self-consistent 2.5D CW spectral laser diode model and report on the use of this model to investigate the origin of gain compression in a 975 nm high-brightness tapered QW laser diode. Nonequilibrium gain is calculated using a dynamic gain model, which simulates the dynamic relaxation of the quantum well carrier energy distributions under the influence of steady-state electrical and optical excitation. Calculated gain and spontaneous emission spectra are included in the laser model via parameterised look up tables. Both simulated and experimentally measured intracavity spontaneous emission spectra show an increased carrier density and a blue-shift of the gain maximum with increasing bias caused by carrier heating and spectral hole burning. The accurate incorporation of nonequilibrium gain compression is therefore vital for the accurate prediction of the operating characteristics of these devices and for the experimental determination of the active region temperature. 相似文献
3.
Freedberg DI Venable RM Rossi A Bull TE Pastor RW 《Journal of the American Chemical Society》2004,126(33):10478-10484
The HNCO NMR pulse sequence was applied to three selectively labeled (15)N and (13)C isotopic homologues of the peptide Ac-WAAAH(AAARA)(3)A-NH(2) to probe directly for hydrogen bonds between residues 8 and 11 (characteristic of a 3(10)-helix), 8 and 12 (alpha-helix), and 8 and 13 (pi-helix). The experiments demonstrate conclusively, and in agreement with circular dichroism studies, that the center of the peptide is alpha-helical; there is no discernible 3(10)- or pi-helix at these specific positions. Molecular dynamics simulations of the preceding peptide and Ac-(AAAAK)(3)A-NH(2) in water using the potential energy parameter set CHARMM22/CMAP correctly yield an alpha-helix, in contrast to simulations with the set CHARMM22, which result in a pi-helix. 相似文献
4.
Yuichiro Hori Miyako Nishiura Tomomi Tao Reisuke Baba Steven D. Bull Kazuya Kikuchi 《Chemical science》2021,12(7):2498
Reversible enzymatic post-translational modification of the ε-amino groups of lysine residues (e.g. N-acylation reactions) plays an important role in regulating the cellular activities of numerous proteins. This study describes how enzyme catalyzed N-deprotection of lysine residues of non-fluorescent peptide-coumarin probes can be used to generate N-deprotected peptides that undergo spontaneous O- to N-ester transfer reactions (uncatalyzed) to generate a highly fluorescent N-carbamoyl peptide. This enables detection of enzyme catalyzed N-deacetylation, N-demalonylation, N-desuccinylation and N-demethylation reactions activities towards the N-modified lysine residues of these probes using simple ‘turn on’ fluorescent assays.We developed “turn-on” fluorescent probes that detect enzymatic lysine deacylation and demethylation critical for epigenetic and other cellular phenomena, using intramolecular O- to N-ester transfer reactions. 相似文献
5.
Burton A Elomari S Chen CY Medrud RC Chan IY Bull LM Kibby C Harris TV Zones SI Vittoratos ES 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(23):5737-5748
The syntheses, structure solutions, and physicochemical and catalytic characterizations of the novel zeolites SSZ-53 and SSZ-59 are described. SSZ-53 and SSZ-59 were synthesized under hydrothermal conditions with the [1-(4-fluorophenyl)cyclopentylmethyl]trimethyl ammonium cation and 1-[1-(4-chlorophenyl)cyclopentylmethyl]-1-methyl azocanium cation, respectively, as structure-directing agents. The framework topology of SSZ-53 was solved with the FOCUS method, and the structure of SSZ-59 was determined by model building. Rietveld refinement of synchrotron X-ray powder diffraction data confirms each proposed model. SSZ-53 and SSZ-59 each possess a one-dimensional channel system delimited by 14-membered rings. Results from transmission electron microscopy, electron diffraction, catalytic experiments (spaciousness index and constraint index tests), and argon and hydrocarbon adsorption experiments are consistent with the proposed structures. 相似文献
6.
Cheeseman M Feuillet FJ Johnson AL Bull SD 《Chemical communications (Cambridge, England)》2005,(18):2372-2374
A new way of combining chiral auxiliaries and substrate-directable reactions for asymmetric synthesis is described that employs a three-step sequence of aldol-cyclopropanation-retro-aldol reactions for the stereoselective synthesis of enantiopure cyclopropane carboxaldehydes. 相似文献
7.
A study of certain reaction parameters of the recently described procedure for converting ketones to the corresponding carbonitriles using tosylmethyl isocyanide, has led to a modified method suitable for the efficient synthesis of 17-cyanosteroids. These products readily undergo methylation to give the corresponding 17-acetyl compounds. This two-step conversion of steroidal 17- to 20-ketones proceeds in excellent overall yields, and constitutes a particularly mild and simple alternative method to those hitherto used for this purpose. 相似文献
8.
John D. Watkins James E. Taylor Steven D. Bull Frank Marken 《Tetrahedron letters》2012,53(26):3357-3360
A simple and electrolyte-free ion-transfer electrosynthesis micro-reactor system (volume 100 μL, up to 10 mg batches) for processes at liquid–liquid interfaces is developed and demonstrated for the reduction of aldehydes and imines. These cathodic reactions occur at an amphiphilic carbon nanofiber membrane accompanied by proton cation transfer from an aqueous phase into an organic phase. 相似文献
9.
Robert M. Archer Dr. Sylvain F. Royer William Mahy Dr. Caroline L. Winn Prof. Michael J. Danson Dr. Steven D. Bull 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(8):2895-2902
Practical syntheses of 2‐keto‐3‐deoxy‐D ‐xylonate (D ‐KDX) and 2‐keto‐3‐deoxy‐L ‐arabinonate (L ‐KDA) that rely on reaction of the anion of ethyl 2‐[(tert‐butyldimethylsilyl)oxy]‐2‐(dimethoxy phosphoryl) acetate with enantiopure glyceraldehyde acetonide, followed by global deprotection of the resultant O‐silyl‐enol esters, have been developed. This has enabled us to confirm that a 2‐keto‐3‐deoxy‐D ‐gluconate aldolase from the archaeon Sulfolobus solfataricus demonstrates good activity for catalysis of the retro‐aldol cleavage of both these enantiomers to afford pyruvate and glycolaldehyde. The stereochemical promiscuity of this aldolase towards these enantiomeric aldol substrates confirms that this organism employs a metabolically promiscuous pathway to catabolise the C5‐sugars D ‐xylose and L ‐arabinose. 相似文献
10.
Dr. Michael Andresini Dr. Arianna Tota Prof. Leonardo Degennaro Dr. James A. Bull Prof. Renzo Luisi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(69):17293-17321
Recent years have seen a marked increase in the occurrence of sulfoximines in the chemical sciences, often presented as valuable motifs for medicinal chemistry. This has been prompted by both pioneering works taking sulfoximine containing compounds into clinical trials and the concurrent development of powerful synthetic methods. This review covers recent developments in the synthesis of sulfoximines concentrating on developments since 2015. This includes extensive developments in both S−N and S−C bond formations. Flow chemistry processes for sulfoximine synthesis are also covered. Finally, subsequent transformations of sulfoximines, particularly in N-functionalization are reviewed, including N−S, N−P, N−C bond forming processes and cyclization reactions. 相似文献