Stereoselective Synthesis of Di- and Trisaccharide Fucoidan Fragments Bearing alpha-D-Glucuronic Acid Residue

The first stereoselective synthesis of disaccharide alpha-D-GlcA-(1 -> 2)-alpha-L-Fuc-OPr ( 1), trisaccharide (1 -> 3)-alpha-L-Fuc-[alpha-D-GlcA-(1 -> 2)]-alpha-L-Fuc-OPr(3), and their selectively O-sulfated derivatives 2 and 4 bearing sulfo-groups at O(4) of the fucose units has been performed. Compounds 1-4 represent the fragments of the chain of the fucoidan from Cladosiphon okamuranus brown seaweed. Glucuronylation by a series of selectively O-acetylated glucuronyl bromides was studied to obtain the target products. It has been found that 3-O-acetylated donor 6 is the most efficient agent for alpha-glycoside bond formation that can be connected with intramolecular remote participation of 3-O-acetyl group favoring alpha-stereoselectivity.