Stereoselective Synthesis of α(2,9) Di- to Tetrasialic Acids, Using a 5,4-N,O-Carbonyl Protected Thiosialoside

An efficient stereoselective synthesis of alpha(2,9) tetra- to disialic acids 1-3, using the 5,4-N,O-carbonyl protected thiosialoside 4, is described. The cyclic protecting group was effective for alpha-sialylation without the need for acetonitrile as the solvent. The donor 4 enabled the formation of a tetramer in excellent yield and selectivity. Deprotection of the cyclic protecting groups of the protected di- to tetrasialica acids proceeded smoothly to give the fully deprotected alpha(2,9) tetra- to disialic acids 1-3.