Regioselective modifications of natural polysaccharides

Another polysaccharide that was subjected to regioselective modifications is alginate, that consists of 1-->4-linked ?-D-mannuronic acid (M) and its C-5 epimer ?-L-guluronic acid (G) units. This natural copolymer is an important component of algae such kelp, and is also an exopolysaccharide of bacteria including Pseudomonas aeruginosa. Alginates are widely used in food, cosmetic and pharmaceutical industry. The sulfation of these polysaccharides exhibits compounds with carboxylic and sulfate groups close to each others as in heparin ones. Randomly sulfated alginates show anticoagulant activity, so regioselective modification of the polysaccharide backbone may help to understand the relationship between structure and properties in alginate sulfates. Indeed, a semi-synthetic sulfated alginate derivative (propylene glycol alginate sodium sulfate, PSS), has been employed as anti-cardiovascular disease drug in China, without control of degree of sulfation. Due to incomplete solubility and highly heterogeneous structure of natural alginic acids the strategy to obtain a regioselectively sulfated alginate polysaccharide was applied to ?-D-polymannuronic acid, that is the simplest polysaccharide possessing the most homogeneous structure of all alginic acids. It was protected at O-2,3 diol by either application of an orthoester or benzylidene ring and in the latter case, the polymannuronic acid was derivatized at carboxylic function too in order to enhance its solubility in aprotic solvent. At the end of the se