Stereoselective Synthesis of L-Guluronic Acid Alginates

The glycosylation properties of gulopyranosides have been mapped out, and it is shown that gulose has an intrinsic preference for the formation of 1,2-cis-glycosidic bonds. It is postulated that this glycosylation behaviour originates from nucleophilic attack at the oxacarbenium ion, which adopts the most favourable H-3(4) conformation. Building on the stereoselectivity of gulose, a guluronic acid alginate trisaccharide was assembled for the first time by using gulopyranosyl building blocks.