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991.
992.
过渡金属催化卡宾对O-H键的不对称插入反应是合成手性醇及其衍生物的直接方法.近年来,人们发展了多种手性催化剂实现了重氮酯衍生的金属卡宾对醇、酚、羧酸甚至水的O-H键的高对映选择性插入反应,但是重氮酮作为卡宾前体的不对称O-H键插入反应鲜有成功的例子.以非手性双铑络合物和手性螺环磷酸组成的协同催化体系,首次实现了α-重氮酮对醇的O-H键的不对称插入反应,获得了较高的收率和高达95%ee的对映选择性.反应为手性α-烷氧基酮这类重要手性化合物提供了高效的合成方法.还通过密度泛函理论计算,对反应机理进行了初步研究,发现水很可能参与了手性磷酸促进的烯醇中间体质子转移过程. 相似文献
993.
Mizuho Sugiuchi Dr. Yukatsu Shichibu Prof. Katsuaki Konishi 《Angewandte Chemie (International ed. in English)》2018,57(26):7855-7859
2,3‐bis(diphenylphosphino)butane enantiomers (chiraphos, L) used as chiral auxiliaries results in the preferential formation of an unprecedented Au24 framework with inherent chirality. The crystal structure of [Au24L6Cl4]2+ ( 1 ) has a square antiprism‐like octagold core twinned by two helicene‐like hexagold motifs, where the inherent chirality is associated with the helical arrangement. The clusters carrying (R,R)‐ and (S,S)‐ diphosphines had right‐ and left‐handed strands, respectively. Circular dichroism spectra showed peaks in the visible to near‐IR region, some of which did not coincide with absorption bands, suggesting the enantiomeric Au24 frameworks possess unique chiroptical properties. The Au24 frameworks were thermally robust, which could be attributed to the superatomic concept (18 e? system) and the steric constraint effects of the bridging ligand units. 相似文献
994.
Vladimír Nosek Dr. Jiří Míšek 《Angewandte Chemie (International ed. in English)》2018,57(31):9849-9852
The highly enantioselective enzyme methionine sulfoxide reductase A was combined with an oxaziridine‐type oxidant in a biphasic setup for the deracemization of chiral sulfoxides. Remarkably, high ee values were observed with a wide range of substrates, thus providing a practical route for the synthesis of enantiomerically pure sulfoxides. 相似文献
995.
Enantio‐ and Diastereoselective Synthesis of Chiral Allenes by Palladium‐Catalyzed Asymmetric [3+2] Cycloaddition Reactions
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Prof. Dr. Barry M. Trost Dr. Daniel Zell Dr. Christoph Hohn Dr. Guillaume Mata Autumn Maruniak 《Angewandte Chemie (International ed. in English)》2018,57(39):12916-12920
A protocol for the asymmetric synthesis of highly substituted chiral allenes with control of point and axial chirality has been developed. A palladium‐catalyzed [3+2] cycloaddition using readily available racemic allenes gives access to densely functionalized chiral allenes with excellent yields and functional group tolerance. The catalytic asymmetric protocol utilizes a broad range of allenyl TMM (trimethylenemethane) donors to form cyclopentanes, pyrrolidines, and spirocycles with very good control of regio‐, enantio‐, and diastereoselectivity. The chiral allene moiety is shown to be a valuable functional group for rapid elaboration towards complex targets. 相似文献
996.
《Journal of mass spectrometry : JMS》2018,53(8):693-699
In this work, the use of MALDI traveling wave ion mobility spectrometry‐mass spectrometry (MALDI‐TWIMS‐MS) for stereoselective structural analysis of direct cleavage and identification of 2‐substituted piperidines obtained through solid‐phase asymmetric synthesis by using heterogeneous 8‐phenylmenthyl‐based chiral auxiliary resins. A strategy for gas‐phase chiral and structural characterization of small molecular weight molecules by using MALDI‐IMS‐MS technique is discussed. Because both MALDI and IMS do not directly offer chiral resolution, an easy methodology by adding a chiral phase is described to carry out in situ online ion/molecule complexation with different chiral analytes inside the mass spectrometer. Piperidine enantiomers were resolved, and separation obtained shows dependence of surface areas. To corroborate this assumption and elucidate the separation mechanism to accomplish an analytical technique by which fast determination of the chirality of molecules may be determined for a wide range organic compound applications, it was performed DFT calculations to determine the cross‐sectional areas of proton‐bound dimer complexes. Drift times are affected by cross‐sectional areas, correlating bigger times with bigger molecular volumes during the ion mobility experiments of proton‐bound dimer complexes. 相似文献
997.
《Journal of separation science》2018,41(6):1247-1256
In the last few years, halogen bonds have been exploited in a variety of research areas both in the solid state and in solution. Nevertheless, several factors make formation and detection of halogen bonds in solution challenging. Moreover, to date, few chiral molecules containing electrophilic halogens as recognition sites have been reported. Recently, we described the first series of halogen‐bond‐driven enantioseparations performed on cellulose tris(3,5‐dimethylphenylcarbamate) by high‐performance liquid chromatography. Herein the performances of amylose tris(3,5‐dimethylphenylcarbamate) as halogen bond acceptor were also investigated and compared with respect to cellulose tris(3,5‐dimethylphenylcarbamate). With the aim to explore the effect of polysaccharide backbone on the enantioseparations, the thermodynamic parameters governing the halogen‐dependent enantioseparations on both cellulose and amylose polymers were determined by a study at variable temperature and compared. Molecular dynamics were performed to model the halogen bond in polysaccharide‐analyte complexes. Chiral halogenated 4,4′‐bipyridines were used as test compounds (halogen bond donors). On this basis, a practical method for detection of stereoselective halogen bonds in solution was developed, which is based on the unprecedented use of high‐performance liquid chromatography as technical tool with polysaccharide polymers as molecular probes (halogen bond acceptors). The analytical strategy showed higher sensitivity for the detection of weak halogen bonds. 相似文献
998.
《Journal of separation science》2018,41(6):1424-1432
A chiral capillary monolithic column for enantiomer separation in capillary electrochromatography was prepared by coating cellulose tris(3,5‐dimethylphenylcarbamate) on porous glycidyl methacrylate‐co‐ethylene dimethacrylate monolith in capillary format grafted with chains of [2(methacryloyloxy)ethyl] trimethylammonium chloride. The surface modification of the monolith by the photografting of [2(methacryloyloxy)ethyl] trimethylammonium chloride monomer as well as the coating conditions of cellulose tris(3,5‐dimethylphenylcarbamate) onto the grafted monolithic scaffold were optimized to obtain a stable and reproducible chiral stationary phase for capillary electrochromatography. The effect of organic modifier (acetonitrile) in aqueous mobile phase for the enantiomer separation by capillary electrochromatography was also investigated. Several pairs of enantiomers including acidic, neutral, and basic analytes were tested and most of them were partially or completely resolved under aqueous mobile phases. The prepared monolithic chiral stationary phases exhibited a good stability, repeatability, and column‐to‐column reproducibility, with relative standard deviations below 11% in the studied electrochromatographic parameters. 相似文献
999.
《Journal of separation science》2018,41(6):1298-1306
For the enantioselective and simultaneous analysis of lactate and 3‐hydroxybutyrate, a validated online two‐dimensional high‐performance liquid chromatography system using 4‐nitro‐7‐piperazino‐2,1,3‐benzoxadiazole as a fluorescent derivatization reagent has been developed. For the reversed‐phase separation in the first dimension, a Capcell Pak C18 ACR column (1.5 × 250 mm, particle size 3 μm) was used, and the target fractions were isolated by their hydrophobicity. In the second dimension, a polysaccharide‐coated enantioselective column, Chiralpak AD‐H (2.0 × 250 mm, 5 μm), was used. The system was validated by the calibration curve, intraday precision, interday precision, and accuracy using standards and real human samples, and satisfactory results were obtained. The present method was applied to human plasma and urine, and in the plasma, trace amounts of d‐ lactate (8.4 μM) and l‐ 3‐hydroxybutyrate (1.0 μM), besides high levels of l‐ lactate (860.9 μM) and d‐ 3‐hydroxybutyrate (59.4 μM), were successfully determined. In urine, trace levels of d‐ lactate (3.7 μM), d‐ 3‐hydroxybutyrate (2.3 μM), and l‐ 3‐hydroxybutyrate (3.3 μM) in addition to a relatively large amount of l‐ lactate (15.4 μM) were observed. The present online two‐dimensional high‐performance liquid chromatography system is useful for the simultaneous determination of all the lactate and 3‐hydroxybutyrate enantiomers in human physiological fluids, and further clinical applications are ongoing. 相似文献
1000.
《Journal of separation science》2018,41(6):1338-1345
A new platform technology for the preparation of stable chiral stationary phases was successfully optimized. The chiral selector tert‐butylcarbamoylquinine was firstly covalently connected to the polymer poly(3‐mercaptopropyl)methylsiloxane by thiol‐ene click reaction. Secondly, the quinine carbamate functionalized polysiloxane conjugate was coated onto the surface of vinyl modified silica particles and cross‐linked via thiol‐ene click reaction. The amount of polysiloxane, chiral selector, radical initiator, reaction solvent (chloroform and methanol), reaction time, and pore size of the supporting silica particles were varied and systematically optimized in terms of achievable plate numbers while maintaining simultaneously enantioselectivity. The optimization was based on elemental analysis data, chromatographic results, and H/u‐curves (Van Deemter) of the resultant chiral stationary phases. The results suggest that better chromatographic efficiency (higher plate numbers) at equal enantioselectivity can be achieved with methanol (a poor solvent for the polysiloxane that is dispersed rather than dissolved) and a lower film thickness of quinine carbamate functionalized polysiloxane. In this study, chiral stationary phases based on 100 Å silica slightly outperformed 200 Å silica particles (each 5 μm). The optimized two step material exhibited significantly reduced mass transfer resistance compared to the one step material and equal performance as a brush‐type chiral stationary phase. 相似文献