The synthesis of a protected pentasaccharide27bcorresponding to the antithrombin III binding region of heparin is presented. This pentasaccharide was prepared from two disaccharides (12cand23) and a monosaccharide (1). The glucuronic acid containing disaccharide12cwas prepared from easily available monomers 6 and 7. Oxidation to the uronic acid was performed in the disaccharide stage. L-Idose derivative16, prepared via a new route, was coupled with 1,6-anhydro derivative17, oxidized and transformed into disaccharide23. Coupling of12cand23to tetrasaccharide24ahas been investigated. Better yields were obtained without collidine, the reason for which is explained. Coupling of24band1afforded the pentasaccharide27b, protected with acetyl at the positions to be sulphated, benzyl at the other hydroxyl functions and azide at the 2-position of the glucosamine residues. Conversion of27binto the sulphated pentasaccharide Ib can be performed according to published procedures.