No abstract

Tetrabutylammonium triphenyldifluorosilicate (TBAT) can be employed as a fluoride source to cleave silicon-carbon bonds thus generating in situ carbanions that coupled with a variety of electrophiles, including aldehydes and ketones, in moderate to high yields. Among the examples reported is the first instance of fluoride-induced intermolecular coupling between allyltrimethylsilane and imine derivatives. Also, of particular note is the TBAT-initiated coupling of primary alkyl halides with allyltrimethylsilane. TBAT is an easily handled crystalline solid that has several advantages over tetrabutylammonium fluoride (TBAF) as a fluoride source; it is anhydrous, nonhygroscopic, soluble in most commonly used organic solvents, and less basic than TBAF.

Hypervalent azido- and cyanosilicate derivatives, prepared in situ by the reaction of trimethylsilyl azide or trimethylsilyl cyanide, respectively, with tetrabutylammonium fluoride, are effective sources of nucleophilic azide or cyanide. Primary and secondary alkyl halides and sulfonates undergo rapid and efficient azide or cyanide displacement in the absence of phase transfer catalysts with the silicate derivatives. Application of these reagents to the stereoselective synthesis of glycosyl azide derivatives is reported.

Palladium-catalyzed cross-coupling of phenyl, vinyl, and allyl siloxane derivatives proceeded in good to excellent yield with aryl iodides, electron-deficient aryl bromides, and allylic benzoates. Methyl and 2,2,2-trifluoroethyl siloxane derivatives can be employed in the coupling reaction. Electron-donating and -withdrawing groups are tolerated on the aryl halide without affecting the coupling. The scope and limitations of this alternative to Stille and Suzuki couplings is outlined.

The Mitsunobu reaction occurs typically with inversion of configuration in secondary alcohol derivatives. In this paper, a mechanistic explanation for lactonizations of hindered alcohols under Mitsunobu conditions with retention is proposed. This involves the intermediacy of an acyloxyphosphonium salt followed by acyl transfer to the alcohol.

Treatment of 2-acetoxy glycopyranosyl azides with Ph3P gave isoxazolines by ring closure of the phosphorimine. Coupling of in situ generated isoxazolines with acylating reagents gave mixtures of alpha- or beta-glycopyranosyl amides. The alpha/beta ratio depended upon the acylating reagent and metal salts employed. For example, coupling of isoxazoline 3 with Z-Asp-(SPy)-OBn in the presence of CuCl2 gave exclusively alpha-N-glucopyranosylasparagine derivative 8. This general procedure has been applied to mono-, di-, and trisaccharide systems.

This article reports on the synthesis, characterization, and binding studies of surface-functionalized, negatively charged catanionic vesicles. These studies demonstrate that the distribution of glycoconjugates in the membrane leaflet can be controlled by small alterations of the chemical structure of the conjugate. The ability to control the glycoconjugate concentration in the membrane provides a method to explore the relationship between ligand separation distance and multivalent lectin binding at the bilayer interface. The binding results using the O-linked glucosyl conjugate were consistent with a simple model in which binding kinetics are governed by the density of noninteracting glucose ligands, whereas the N-linked glycoconjugate exhibited binding kinetics consistent with interacting or clustering conjugates. From the noninteracting ligand model, an effective binding site separation of the sugar sites for concanavalin A of 3.6-4.3 nm was determined and a critical ligand density above which binding kinetics are zeroth order with respect to the amount of glycoconjugate present at the bilayer was observed. We also report cryo-transmission electron microscopy (cryo-TEM) images of conjugated vesicles showing morphological changes (multilayering) upon aggregation of unilamellar vesicles with concanavalin A.

Pentavalent aryl and heteroaryl bis(catechol) silicates undergo palladium-catalyzed cross-coupling with aryl and heteroaryl triflates in the presence of a fluoride source in excellent yields. These solid, air-stable bis(catechol) silicates are prepared from a high-yielding displacement reaction between catechol and an aryl siloxane in the presence of an amine base. The cross-coupling reaction is tolerant of a wide range of electron-donating and electron-withdrawing groups. Several examples of di-ortho-substituted triflates are successfully coupled with these reagents.