Acceptor reactivity in the total synthesis of alginate fragments containing α-L-guluronic acid and β-D-mannuronic acid.

The total synthesis of mixed-sequence alginate oligosaccharides, featuring both β-D-mannuronic acid (M) and α-L-guluronic acid (G), is reported for the first time. A set of GM, GMG, GMGM, GMGMG, GMGMGM, GMGMGMG, and GMGGMG alginates was assembled using GM building blocks, having a guluronic acid acceptor part and a mannuronic acid donor side to allow the fully stereoselective construction of the cis-glycosidic linkages. It was found that the nature of the reducing-end anomeric center, which is ten atoms away from the reacting alcohol group in the key disaccharide acceptor, had a tremendous effect on the efficiency with which the building blocks were united. This chiral center determines the overall shape of the acceptor and it is revealed that the conformational flexibility of the acceptor is an all-important factor in determining the outcome of a glycosylation reaction.