Synthesis of D-fructofuranosides using thioglycosides as glycosyl donors

Benzylated and benzoylated ethyl thioglycosides of D-fructofuranose have been synthesized and tested as glycosyl donors in couplings to various primary and secondary carbohydrate accepters. Treatment of 2-O-acetyl-1,3,4,6-tetra-O-benzoyl-D-fructofuranose with ethyl mercaptan in a BF3 .-etherate-promoted reaction gave the benzoylated ethyl 2-thio-alpha,beta-D-fructofuranosides, which after deacylation and benzylation afforded the benzylated derivatives. These thiofructofuranosides, using dimethyl(methylthio)sulfonium triflate (DMTST) or N-iodosuccinimide as promoter, were found to be excellent donors, which gave disaccharide coupling products in quantitative or almost quantitative yields with all tested accepters, yields rarely found in oligosaccharide synthesis. The benzoylated donors gave only alpha-linked fructofuranosides, due to participation of the 3-O-benzoyl group, whereas the benzylated donors gave alpha/beta-mixtures.