We report here a new catalytic and exclusively -selective glycosylation strategy for multigram scale synthesis of biologically valuable Tn antigens. The underlying iron-catalyzed glycal 1,2--aminoglycosylation method is effective with a variety of galactosyl donors and amino acid acceptors with consistently high stereoselectivity. Rapid and scalable postglycosylation transformations readily afford single diastereomeric Tn antigens in high yields.