Phosphite-based sialic acid donors in the synthesis of α(2→9) oligosialic acids

The combination of a phosphite donor and an anomeric thiocresol-protected acceptor, both with a TFA protecting group at C-5 of the sialic acid, provides good α-selectivity and yield in sialylation. Although the convergent synthetic strategy of using a phosphite disialo-donor and a disialo-acceptor assembles tetra-sialic acid efficiently, overcoming the low α-selectivity of α-anomer and purifying it remain to be achieved. Furthermore, mono- and di-sialic acids were, respectively, conjugated on carrier protein, keyhole limpet hemocyanin. The enzymatic hydrolysis method is recommended for estimating the amount of sialic acid on a protein conjugate.