Visible Light Photoredox-Catalyzed O-Sialylation Using Thiosialoside Donors.

An efficient protocol for the O-sialylation using thiosialoside donors under visible light photocatalysis was developed. Thiosialosides were activated under the irradiation with blue light in the presence of Ru(bpy)3(PF6)2 as photocatalyst, Umemoto's reagent as CF3 radical source and Cu(OTf)2 as an additive in acetonitrile/dichloromethane at -30 °C, and the subsequent reaction with glycosyl acceptors generally produced the desired sialosides in good to excellent yields with the satisfactory α-selectivity.