PURPOSE: To obtain the subject compound in high yield by condensing a sialic acid thioglycoside with a saccharide derivative in the presence of N- iodosuccinimide and trifluoromethanesulfonic acid.

CONSTITUTION: A sialic acid thioglycoside [e.g. methyl (methyl 5- acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto-2- nonulopyranosido)nate] is used as a saccharide donor and a galactose and a lactose derivatives which are minimally protected are used as a saccharide receptor. Dehydrating treatment is carried out in a solvent such as acetonitrile and the resultant product is cooled. N-Iodosuccinimide and trifluoromethanesulfonic acid are then added thereto and reacted. Thereby, the objective compound is obtained.