Carbosilane dendrimers bearing globotriaoses: Syntheses of globotrioasyl derivative and introduction into carbosilane dendrimers

As an application of a one-pot reaction involving Birch reduction and subsequent S(N)2 reaction in liquid ammonia, synthetic assembly of trisaccharidic moieties of globotriaosyl ceramide onto carbosilane dendrimers was accomplished using tris(3-bromopropyl) phenylsilane and tris[tris(3-bromopropyl)silylpropyl] phenylsilane as the core scaffolds. The common globotriaosyl derivative having benzylsulfide functionality at the terminal of the aglycon was efficiently prepared from D-galactose and D-lactose as starting materials. The glycosyl donor derived from galactose and the glycosyl acceptor derived from lactose were condensed in the presence of silver triflate as the best promoter to provide corresponding trisaccharide with newly formed alpha-1-4 linkages in 90% yield. Fully benzylated protection of the trisaccharide was deprotected under the Birch reduction condition followed by acetylation to give an acetate in which alkene was converted into benzyl sulfide by radical addition of alpha-toluenethiol in high yields. On the other hand, carbosilane dendrimers were prepared from appropriate chlorosilanes as starting materials by a combination of hydrosylation followed by alkenylation. The terminal C=C double bonds of the carbosilanes were converted into corresponding alcohols by means of the usual hydroboration reaction, and the alcohols underwent further chemical manipulation to give carbosilane dendrimers with peripheral bromine atoms.