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81.
We argue that given the experimental constraints on the Higgs boson mass the least fine-tuned parameter space of the minimal supersymmetric standard model is with negative top-squark masses squared at the grand unification scale. While the top-squark mass squared is typically driven to positive values at the weak scale, the contribution to the Higgs boson mass squared parameter from the running can be arbitrarily small, which reduces the fine-tuning of electroweak symmetry breaking. At the same time the top-squark mixing is necessarily enhanced and the maximal mixing scenario for the Higgs boson mass can be generated radiatively even when starting with negligible mixing at the unification scale. This highly alleviates constraints on possible models for supersymmetry breaking in which fine-tuning is absent. 相似文献
82.
The preparation of three new N‐Fmoc‐protected (Fmoc=[(9H‐fluoren‐9‐yl)methoxy]carbonyl) β2‐homoamino acids with proteinogenic side chains (from Ile, Tyr, and Met) is described, the key step being a diastereoselective amidomethylation of the corresponding Ti‐enolates of 3‐acyl‐4‐isopropyl‐5,5‐diphenyloxazolidin‐2‐ones with CbzNHCH2OMe/TiCl4 (Cbz=(benzyloxy)carbonyl) in yields of 60–70% and with diastereoselectivities of >90%. Removal of the chiral auxiliary with LiOH or NaOH gives the N‐Cbz‐protected β‐amino acids, which were subjected to an N‐Cbz/N‐Fmoc (Fmoc=[(9H‐fluoren‐9‐yl)methoxy]carbonyl) protective‐group exchange. The method is suitable for large‐scale preparation of Fmoc‐β2hXaa‐OH for solid‐phase syntheses of β‐peptides. The Fmoc‐amino acids and all compounds leading to them have been fully characterized by melting points, optical rotations, IR, 1H‐ and 13C‐NMR, and mass spectra, as well as by elemental analyses. 相似文献
83.
Radovan erný Guillaume Renaudin 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(11):i154-i155
The crystal structure of the intermetallic compound henicosamagnesium pentacosazinc, Mg21Zn25, has been determined by single‐crystal X‐ray diffraction. It is isomorphous with Zr21Re25 and deviates slightly from the rules that define the Frank–Kasper phases. 相似文献
84.
Dr. Tomáš Čarný Dr. Tibor Peňaška Samuel Andrejčák Prof. Dr. Radovan Šebesta 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(58):e202202040
Cross-coupling reactions are essential tools in target molecule synthesis. However, the use of highly reactive organometallic reagents limits their applicability. Here, we present a mechanochemical Pd-catalyzed cross-coupling reaction between aryl halides and organozinc pivalates that can be carried out under ambient temperature and atmosphere. This operationally simple procedure affords a wide range of biaryl and aryl-heteroaryl derivatives in high yields and short times. The reaction tolerates various functional groups and can be realized on a synthetically useful scale. Its practical value was demonstrated in the short synthesis of the pharmaceutical diflunisal. 相似文献
85.
The derivatisation of aldehydes in their α-position is an important facet of organic synthesis. Organocatalytic radical reactions afford α-functionalised aldehydes via a SOMO activation pathway. New organo-SOMO reactions of aldehydes with copper(I)-acetylide and alkylindium reagents are detailed. These reactions proceed well under the catalysis of chiral imidazolidinones. The corresponding functionalised aldehydes were obtained with acceptable yields, but with only low enantiomeric ratios. 相似文献
86.
Enantioselective Michael additions catalyzed by hydrogen-bonding catalysts produce many important compounds. Solvent-free reaction conditions in a ball mill can provide an improved enantioselectivity over the reaction in solution, due to lack of disruptive solvation of reagents. A range of 15 structurally different hydrogen-bonding organocatalysts were tested in two Michael additions to β-nitrostyrene under solvent-free conditions and compared with corresponding experiments in solution. With several thiourea catalysts, these Michael additions proceeded with higher enantioselectivities under solvent-free conditions than in solution. 相似文献
87.
88.
Teresa Küblbck Gaspard Ang Greta Bikelyt Jiina Pokorn Radovan Skcel Thomas M. Klaptke 《Angewandte Chemie (International ed. in English)》2020,59(30):12326-12330
A progressive halogen‐free multicolored smoke system to obtain white, red, violet, yellow, green, and blue smoke color is presented. The nitrogen‐rich salt guanidinium 5,5′‐azotetrazolate (GZT), which is usually applied as a gas generator or propellant ingredient, was combined with different smoke dyes (Solvent Red 1, Solvent Violet 47, Solvent Green 3, Solvent Yellow 33). These two‐component smoke mixtures offer a convenient and safe multicolor approach without the need for potassium chlorate or any other hazardous material. The common smoke characteristics with respect to burn time/burn rate, yield factor, transfer rate, as well as energetic properties were determined and compared with classic chlorate‐based formulations currently used. To the best of our knowledge, nothing comparable is known in the literature and a completely new research area in modern pyrotechnics is opened. 相似文献
89.
Nitrones serve as starting materials for the synthesis of many heterocycles. Oxidation of secondary amines using hydrogen peroxide and the catalytic amount of methyltrioxorhenium in ionic liquids is a useful method for the preparation of nitrones. Ultrasonic irradiation and ionic liquids have a positive influence on the reaction. The nitrones required were isolated in good yields. Corresponding hydroxylamines, which can be easily oxidised to nitrones, often accompanied the main products. Methyltrioxorhenium in ionic liquids was re-used in several reaction cycles without any deteriorating effect on the course of the reaction. 相似文献
90.
Guy Samson Jean Claude Morisette Radovan Popovic 《Photochemistry and photobiology》1988,48(3):329-332
Abstract— The copper quenching effect on fluorescence in Dunaliella tertiolecta was studied. 30% of variable fluorescence was quenched in the presence of 70 μ,M CuS04 . We confirmed that the copper inhibitory effect on photosystem II (PSII) activity is located on its oxidizing side. Further, we indicate that the complementary area is decreased by copper. Since the quantum yield of PSII photochemistry was lowered and the rate of PSII primary electron acceptor QA remained unaffected, we can conclude that some PSII reaction centers were inactivated by copper. 相似文献