9-O-Sulfation on alpha-NeuAc-(2-->8)-NeuAc and inter-residue lactonization

Treatment of alpha-NeuAc-(2-->8)-NeuAc (1) with SO3-pyridine (4 equiv) in DMF resulted in selective 9-O-sulfation on the nonreducing end residue and the formation of an inter-residual delta-lactone. The lactonization could result from the C-2 carboxylic acid of the nonreducing residue condensing with the hydroxyl group or/and sulfated group at C-9 of the reducing residue to form a six-membered ring between two adjacent sialic acid residues. When alpha-NeuAc-(2-->9)-NeuAc (5) was used as a sulfation substrate, only 9-O-sulfation on the nonreducing end residue was observed. According to capillary electrophoresis (CE) analysis, 9-O-sulfation on the disialic acid is a fast reaction, while sulfation on other hydroxyl groups is insignificant under the conditions used.