The synthesis of a cellobiouronic thioglycoside donor I (R1 = benzoyl; R2 = benzyl) protected with a selectively removable 3'-O-benzyl group is described. The donor I is suitable as a monomer building block in the construction of oligomer structures corresponding to the capsular polysaccharide of Streptococcus pneumoniae type 3. The carboxyl function was introduced through regioselective 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-oxidn. of a 4',6'-diol cellobiose deriv. If the oxidn. was performed on a 2,3,2',3',4',6'-hexa-ol deriv., oxidn. also of the secondary 2- and 3-hydroxyl groups was obsd. to give a tricarboxyl deriv. as one of the major products. The thioglycoside was formed by acidic mercaptolysis of a 1,6-anhydro bridge. The donor I was transformed into a suitable starting monomer acceptor through glycosylation with a spacer alc. and subsequent debenzylation. [on SciFinder (R)]