Iodine-mediated glycosylation en route to mucin-related glyco-aminoacids and glycopeptides

The use of iodine-DDQ as a promoter for glycosylation of Fmoc-Ser-OBn and Fmoc-Thr-OBn with phenylseleno 3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranoside in toluene-dioxane gave 49% and 73% yields, respectively, of the corresponding alpha-glycosides as the sole glycoside products. Reductive acetylation of the azide groups and cleavage of the benzyl esters by hydrogenolysis gave building blocks that were used in solid-phase synthesis to prepare triglycosylated tetrapeptides (Ac(3)GalNAc-alpha-Ser)(3)-Gly and (Ac(3)GalNAc-alpha-Thr)(3)-Gly in 27% and 49% overall yield, respectively.