Semi-synthesis of a 3-O-sulfated red seaweed galactan-derived disaccharide alditol

β-d-Galp3-SO3-(1→4)-3,6-anhydro-l-GalOH (agarobiitol 32-sulfate, 4) was semi-synthetically prepared as follows: production of agarobiitol (1) from agarose by partial reductive hydrolysis, protection of the primary hydroxyl groups of 1 with trityl groups to produce the 11,62-di-O-tritylated derivative (2), regioselective dibutylstannylene-mediated sulfation of 2 to give the 32-O-sulfated-11,62-di-O-tritylated compound (3), and detritylation of compound 3 to give the final product (4). This semi-synthetic route allowed the preparation of a red seaweed galactan-derived disaccharide alditol with sulfate group located at C-3 of the galactopyranosidic ring. Because red seaweed galactans are glycosidically linked at C-3 of the β-d-Galp unit, a sulfated derivative with this structure could not be obtained by partial reductive hydrolysis of sulfated red seaweed galactans.