An oligosaccharide active against Helicobacter pylori was synthesized in a highly efficient manner for the first time. The anti-H. pylori oligosaccharide structure is a core-2 branched-type oligosaccharide with a characteristic alpha-N-acetylglucosamine at the nonreducing end. The oligosaccharide was synthesized from the nonreducing end to the reducing end, with an N-benzyl-2,3-oxazolidinone-carrying glycosyl donor used to introduce an alpha-N-acetylglucosamine at the nonreducing end. Complete chemoselective activation of a bromo sugar in the presence of a thioglycoside acceptor was achieved, and the use of 2,6-dimethylphenyl thioglycoside prevented the aglycon transfer observed when the corresponding phenyl thioglycoside is used as an acceptor.