For the preparation of l-iduronic acid, trehalose was converted into a derivative of a novel disaccharide, β-l-idopyranosyl β-l-idopyranoside, through diastereoselective hydroboration of the 5, 5'-di-eno intermediate. The 6- and 6'-hydroxy groups were then oxidized in two steps to give a disaccharide composed of 2 units of l-iduronate moieties, which underwent acidic cleavage of the glycosidic bond to give the target compound.