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21.
Jean-Franois Nierengarten Tilo Habicher Roland Kessinger Francesca Cardullo Franois Diederich Volker Gramlich Jean-Paul Gisselbrecht Corinne Boudon Maurice Gross 《Helvetica chimica acta》1997,80(7):2238-2276
The macrocyclization between buckminsterfullerene, C60, and bis-malonate derivatives in double Bingel reaction provides a versatile and simple method for the preparation of covalent bis-adducts of C60 with high regio- and diastereoselectivity. A combination of spectral analysis, stereochemical considerations, and X-ray crystallography (Fig. 2) revealed that out of the possible in-in, in-out, and out-out stereoisomers, the reaction of bis-malonates linked by o-, m-, or p-xylylene tethers afforded only the out-out ones (Scheme 1). In contrast, the use of larger tethers derived from 1,10-phenanthroline also provided a first example, (±)- 19 (Scheme 2), of an in-out product. Starting from optically pure bis-malonate derivatives, the new bis-functionalization method permitted the diastereoselective preparation of optically active fullerene derivatives (Schemes 4 and 5) and, ultimately, the enantioselective preparation (enantiomeric excess ee > 97%) of optically active cis-3 bis-adducts whose chirality results exclusively from the addition pattern (Fig. 6). The macrocyclic fixation of a bis-malonate with an optically active, 9,9′-spirobi[9H-fluorene]-derived tether to C60 under generation of 24 and ent- 24 with an achiral addition pattern (Scheme 4) was found to induce dramatic changes in the chiroptical properties of the tether chromophore such as strong enhancement and reversal of sign of the Cotton effects in the circular dichroism (CD) spectra (Figs. 4 and 5). By the same method, the functionafized bis-adducts 50 and 51 (Schemes 10 and 11) were prepared as initiator cores for the synthesis of the fullerene dendrimers 62 , 63 , and 66 (Schemes 12 and 13) by convergent growth. Finally, the new methodology was extended, to the regio- and diastereoselective construction of higher cyclopropanated adducts. Starting from mono-adduct 71 , a clipping reaction provided exclusively the all-cis-2 tris-adduct (±)- 72 (Scheme 14), whereas the similar reaction of bis-adduct 76 afforded the all-cis-2 tetrakis-adduct 77 (Scheme 15). Electrochemical investigations by steady-state voltammetry (Table 2) in CH2Cl2 (+0.1M Bu4NPF6) showed that all macroeyciic bis(methano)fullerenes underwent multiple reduction steps, and that regioisomerism was not much influencing the redox potentials, All cis-2 bis-adducts gave an instable dianion which decomposed during the electrochemical reduction. In CH2Cl2, the redox potential of the fullerene core in dendrimers 62, 63 , and 66 is not affected by differences in size and density of the surrounding poly(ether-amide) dendrons. The all-cis-2 tris- and tetrakis(meihano)fullercnes (±) -72 and 77 , respectively, are reduced at more negative potential than previously reported all-e tris- and tetrakis-adducts with methane bridges that are also located along an equatorial belt. This indicates a larger perturbation of the original fullerene π-chromophore and a larger raise in LUMO energy in the former derivatives. 相似文献
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24.
M. G. O. Santana-Marques A. J. V. Ferrer-Correia K. A. Caldwell M. L. Gross 《Journal of the American Society for Mass Spectrometry》1993,4(10):819-827
Aromatic β-hydroxyoximes undergo unusual fragmentation reactions as protonated or cationized species, as radical cations, or as (M - H)? ions, As protonated species, they expel OH ’ from the oxime functionality in violation of the even electron rule. Parallel eliminations of alkyl radicals follow OH’ loss when the aromatic ring is substituted with an alkyl chain. Alkyl radical losses appear to be characteristic of radical cations that can isomerize to ions in which the alkyl chain bears a radical site and the charged site is the conjugate acid of a basic functionality (e.g., oxime or imine). Evidence for the mechanisms was found in the ion chemistry of oxime and imine radical cations. The imine reference compounds were conveniently generated by fast atom bombardment-induced reduction of oximes, removing the requirement for using conventional chemical synthesis. Protonated imines and the (M - H)? ions of oximes fragment extensively via charge-remote processes to eliminate the elements of alkanes. This chemistry is not shared by the protonated oximes. 相似文献
25.
O-GlcNAcylation of serine and threonine residues is a dynamic and essential post-translational modification involved in signaling pathways in eukaryotes. Studies of O-GlcNAcylation would be aided by small-molecule inhibitors of O-GlcNAc transferase (OGT), the sole enzyme know to mediate this modification, but discovery of such molecules has been hampered by poor expression of cloned OGT and lack of suitable high-throughput screens. This Communication describes the development an expression system to access large amounts of the catalytic domain of OGT and the implementation of a fluorescence-based substrate analogue displacement assay that has led to the discovery of a set of OGT inhibitors. This work lays the foundation for both structural and functional analysis of the catalytic domain of OGT. 相似文献
26.
Zhou S Xu C Wang J Gao W Akhverdiyeva R Shah V Gross R 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):7926-7932
Acidic sophorolipid (SL) molecules derived from yeasts represent a novel type of asymmetrical bolaamphiphiles due to their unique structural features that include an asymmetrical polar head size (disaccharide vs COOH), a kinked hydrophobic core (cis-9-octadecenoic chain), and a non-amide polar-nonpolar linkage. Light microscopy, small- and wide-angle X-ray scattering, FT-IR spectroscopy, and dynamic laser light scattering were used to investigate the supramolecular structures of the self-assembled aggregates of SL molecules at different pH values. In acidic conditions (pH < 5.5), giant twisted and helical ribbons of 5-11 microm width and several hundreds of micrometers length were observed for the first time. Increase in solution pH values slowed ribbon formation, decreased ribbon yield, and increased the helicity and entanglements of the giant ribbons. An interdigitated lamellar packing model of acidic SL-COOH molecules with a long period of 2.78 nm, stabilized by both the strong hydrophobic association between the cis-9-octadecenoic chains and strong disaccharide-disaccharide hydrogen bonding, is proposed. The neutralization of SL-COOH in water to SL-COONa produced clear solutions with the formation of short-range ordered aggregates. At concentrations below 1.0 mg/mL, the size of self-assembled aggregates increased as the concentration increased. At concentrations above 1.0 mg/mL, narrowly distributed micellar aggregates with a constant hydrodynamic radius (R(h)) of about 100 nm are formed. The large micelles show strong angular dependence with the fast mode appearing at scattering angle theta >/= 60 degrees. 相似文献
27.
James J. Gallagher Edward K. Chess Saleh M. Arghestani Michael L. Gross 《Journal of mass spectrometry : JMS》1983,18(3):118-126
Mechanisms for decomposition of 1- and 2-phenyltetralins were investigated using low resolution mass spectrometry and metastable ion techniques. Four primary decompositions were observed for 1-phenyltetralin radical cations: (1) the loss of C6H6 via a 1,4-elimination; (2) the elimination of ethene via competing losses from carbons 3 + 4 and carbons 2 + 3; (3) the loss of C8H8, probably through a stepwise Diels-Alder cycloreversion to expel styrene; and (4) the loss of methyl radical involving carbon 2 and possibly carbon 4. Three major decompositions were observed for 2-phenyltetralin radical cations: (1) the loss of C8H8, possibly through a Diels-Alder cycloreversion to expel styrene; (2) the loss of C6H6 via a 1,3 elimination; and (3) the loss of methyl radical from carbon 1. Various exchange reactions occur prior to these losses, but they proved to be incomplete even for metastable ions. 相似文献
28.
Silvia Gross Andrea Zattin Vito Di Noto Sandra Lavina 《Monatshefte für Chemie / Chemical Monthly》2006,60(3):583-593
Silica-based inorganic–organic hybrid thin films embedding the organically modified oxohafnium clusters (Hf4O2(OMc)12, OMc=OC(O)–C(CH3)=CH2) were obtained by photo-activated free radical copolymerisation of the methacrylate groups of the cluster with those of the
pre-hydrolysed (methacryloxypropyl)trimethoxysilane (MAPTMS, (CH2=C(CH3)C(O)O)(CH2)3Si(OCH3)3). By this route, a covalent anchoring of the cluster to the forming silica network was achieved. Samples characterized by
two different Si/Hf compositions (18:1, 5:1) were prepared. The surface and in-depth composition of the thin films were investigated
through Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). XPS depth profiles performed on the thin
layers evidenced a homogenous in depth distribution of the hafnium guest species within the whole silica films and sharp film-substrate
interfaces.
Broad band dielectric spectroscopy (BDS) measurements permitted to investigate the electric response of the obtained films
in the frequency and temperature range of 40 Hz – 1 MHz and 0–160°C. 相似文献
29.
Urška L. Štangar Urh Černigoj Polonca Trebše Ksenija Maver Silvia Gross 《Monatshefte für Chemie / Chemical Monthly》2006,137(5):647-655
Summary. Transparent TiO2 films with a high photodegradation activity towards an azo dye in aqueous solution were prepared by sol–gel processing. Films
on soda–lime glass supports protected with a thin silica barrier layer exhibited better crystallization and monodisperse nanoparticles,
higher absorption of light below 370 nm, and higher photocatalytic activity than those films deposited on bare glass supports
proving the detrimental effect of interdiffused sodium ions on the development of the anatase nanostructure. The effect of
substrate was more pronounced in thinner films (300 nm) than in thicker ones (1200 nm), which were achieved by adding a template
(i.e. Pluronic F127) to the sol. 相似文献
30.
Viroids: a class of subviral pathogens 总被引:13,自引:0,他引:13