Stereoselective Synthesis of S-Linked α(2→8) and α(2→8)/α(2→9) Hexasialic Acids

A new approach for the synthesis of S-linked alpha(2 -> 8) and alternating alpha(2 -> 8)/alpha(2 -> 9) oligosialic acids by S-alkylation has been developed, using chemo- and stereoselective alkylation of a C2-thiolated sialoside donor (nucleophile) with either a C8- or C9-iodide-activated sialoside acceptor (electrophile). An efficient intramolecular acetyl group migration from the C7 to C9-position of the sialoside under mild basic conditions was used to generate the C8-iodide, the key sialyl acceptor (electrophile). Using this strategy, the syntheses of S-linked alpha(2 -> 8) and alpha(2 -> 8)/alpha(2 -> 9) hexasialic acids were achieved.