- A novel protocol for the regioselective bromination of primary alcohols in unprotected carbohydrates or glycosides
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The regioselective and efficient bromination of primary hydroxyl groups in unprotected carbohydrates or glycosides is successfully achieved by using (chloro-phenylthio-methylene)dimethylammoniumchloride (CPMA) in the presence of tetrabutylammonium bromide
- Xue, Weihua,Zhang, Lifen
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p. 1429 - 1432
(2012/09/07)
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- Multichromophoric Cyclodextrins. 1. Synthesis of O-Naphthoyl-β-cyclodextrins and Investigation of Excimer Formation and Energy Hopping
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The synthesis and photophysical properties of β-cyclodextrins bearing seven 2-naphthoyloxy chromophores in specific positions, either on the primary face or the secondary face, or 14 2-naphthoyloxy chromophores, seven on each face, are reported. These multichromophoric cyclodextrins are good models for the study of excitation energy migration among chromophores in well-defined positions. The investigation was performed in dichloromethane and in a mixture of ethanol and methanol that can form a glass at low temperature. The absorption spectra show that the interactions between chromophores in the ground state are weak, whereas the fluorescence spectra reveal the existence of excimers at room temperature but not at low temperature in a rigid glass. Further evidence of excimer formation is provided by the fluorescence decays. Since excimers act as energy traps, the energy hopping process was studied in a rigid glass at low temperature by steady-state and time-resolved fluorescence depolarization techniques. The steady-state anisotropy is found to be one seventh of the theoretical limiting anisotropy 0.4, which means that excitation energy hops between chromophores with essentially randomly oriented transition moments at a rate much higher than the chromophore intrinsic decay rate. Energy hopping is indeed very fast as shown by the fluorescence anisotropy decay which is at least as fast as the apparatus time resolution (a few tens of picoseconds).
- Berberan-Santos, Mário N.,Canceill, Josette,Brochon, Jean-Claude,Jullien, Ludovic,Lehn, Jean-Marie,Pouget, Jacques,Tauc, Patrick,Valeur, Bernard
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p. 6427 - 6436
(2007/10/02)
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- Dioxolanylium Ions Derived from Carbohydrates. IX. Rearrangement between Dioxolanylium and Dioxanylium Ions
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The formation of 4,6-acetoxonium and benzoxonium ion derivatives of methyl gluco-, manno-, galacto- and idopyranosides from the 6-azido-6-deoxy compounds on treatment with nitrosonium ion is described and the formation and rearrangement of the 3,5-benzoxonium ion derived from methyl α-d-xylofuranoside discussed.
- Jacobsen, Steffen
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p. 157 - 164
(2007/10/02)
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