Highly Efficient Sialylation towards α(2-3)- and α(2-6)-Neu5Ac-Gal Synthesis: Significant ‘Fixed Dipole Effect’ ofN-Phthalyl Group on α-Selectivity

Highly alpha-selective sialylation of sialic acid N-phenyltrifluoroacetimidate with galactose acceptors, up to 100:0 and 92% for alpha(2-6)-sialoglycoside, while 97:3 and 77% for alpha(2-3)-sialoglycoside linkage formations, has been realized by introducing the C-5 N-phthalyl group on the donor. The 'fixed dipole effect' of N-phthalyl group was proposed in order to explain the high reactivity and alpha-selectivity. The N-phthalyl group was removed by treatment with methylhydrazine acetate, of which Neu alpha-Gal units can be readily applicable to the synthesis of a variety of N-linked oligosaccha-rides.