Methods are described for the chemo-and regioselective monofunctionalization of the secondary hydroxyl face of cyclodextrins. Monofunctionalization takes place either by nucleophilic epoxide opening of mono(2A,3A-anhydro)heptakis(6-0-teri-butyldimethylsilyl)-(2AS)-/?-cyclodextrin by ethylenediamine, lithium azide, or ammonia or by direct monoalkylation of one of the C(2)-hydroxyl groups of heptakis(6-0-tert-butyldimethylsilyl)cyclodextrins with primary alkyl bromides, with cyano-, ethynyl-, or ester-containing functional groups. The latter route enables the synthesis of mono(2A-0-(a-(4-(aminomethyl)tolyl))hexakis(2B,2c,2D,2E,2F,2G-0-methyl)heptakis(6-0-ter^butyldimeth-ylsilyl)-/3-cyclodextrin and its 2-aminomethyl isomer. These are lipophilic precursors for cyclodextrin derivatives having one reactive functional group and an enlarged molecular cavity formed by partial methylation of the secondary hydroxyl face. The direct monoalkylation route of the secondary face leaves the structure of the cavity intact, while this is distorted in the route using nucleophilic epoxide opening. Two amino-functionalized cyclodextrins were used for coupling reactions with a monofunctionalized calix [4]arene. In this way water-soluble cyclodextrin derivatives could be obtained of which the secondary faces were flexibly capped with a calix[4]arene moiety.
The synthesis of homo-and heterocyclodextrin (CD) dimers, containing two CD moieties that are linked through their secondary sides by aliphatic or 2,2'-bipyridyl spacers is described. In these dimers, the glucose units to which the spacers are linked have been transformed into altrose units. The dimers with an octamethylene spacer show self-complexation of the spacer in one of the CD moieties in aqueous solution, as revealed by 1 H and 13 C NMR spectroscopy. Using high-resolution (600 and 800 MHz) NMR spectroscopy and a variety of 2D NMR techniques, an assignment of nearly all of the 1 H NMR signals of two of the CD dimers was made, affording detailed information about the structure of these compounds in water. The self-inclusion of the spacers leads to lower binding affinities for ditopic guest molecules like p-toluidino-6-naphthalene sulfonate (TNS) derivatives and porphyrins. When a rigid 2,2'-bipyridyl group is used to connect the two CD moieties, self-inclusion of the spacer is not possible. This results in the formation of different complexes with ditopic guest molecules, for example, a 2:2 complex with a porphyrin. The CD heterodimers described in this paper contain an a-CD and a b-CD moiety. These dimers display site-specific binding of guest molecules.
The synthesis o f three dansyl appended cyclodextrin derivatives, differing in the spacer length between cyclodextrin and the dansyl moiety, is described. In compound 4 the fluorophore is directly attached to the cyclodextrin. Com pound 5 contains an ethyl spacer and compound 6 a triethylene glycol spacer. These com pounds are designed to detect neutral organic guest molecules like cyclohexanol and adamantanecarboxylic acid in water by fluorescence spectroscopy. At neutral pH none of the compounds is sensitive towards guest molecules. For compound 4 this is due to the fact that the dansyl group is located outside the cyclodextrin cavity. For compunds 5 and 6 the low sensitivity is the result o f a strong self inclusion o f the dansyl group. Lowering the pH results in protonation o f the dimethylamino group o f the dansyl moiety, which makes the self-inclusion less favourable leading to a strongly increased response towards guests. This phenomenon allows the sensors to be switched on and off by lowering or increasing the pH o f the solution. Compound 6 is able to detect adamantanecarboxylic acid at 5 x 10' 7 m ol" 1 dm 3 concentration at pH 1.
A luminescent ruthenium(II) complex with six cyclodextrin binding sites is shown to switch off its emission upon binding of N,NA-dinonyl-4,4A-bipyridinium bromide and to recover luminescence upon displacement of the bipyridinium ion by a steroid.
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