Stereoselective tris-glycosylation to introduce beta-(1-->3)-branches into gentiotetraose for the concise synthesis of phytoalexin-elicitor heptaglucoside.

Dodecyl thioglycosides (3, 4, 5) were prepared by conventional transformation of d-glucose and used as new glycosyl donors for a short-step synthesis of phytoalexin elicitor heptaglucoside. A gentio-tetraoside derivative (6) having three hydroxyl groups was synthesized by NIS-TfOH promoted glycosylate in more than 90% yield followed by selective removal of temporary protective groups. Undesired formation of alpha-glycosides at the introduction of beta-(1-->3)-branches into gentio-oligosaccharides was found to be suppressed by use of a thiophilic reagent system, BSP (1-benzenesulfinyl piperidine)-Tf2O, giving the heptaglucoside in only four glycosylation steps.