Synthesis of the starfish ganglioside AG2 pentasaccharide

This Letter reports the first synthesis of the AG2 pentasaccharide, using silylene-oxazolidinone double-locked sialic acid building blocks. The di-DTBS-protected sialic acid building block was easily prepared and readily activated with NIS and TfOH to provide the sialylated lactose unit in good yield with moderate selectivity. After obtaining the trisaccharide unit, the oxazolidinone-protected C4–OH on the sialic acid residue was readily deprotected by treatment with NaOMe. Coupling with the galactofuranosylβ(1-3)galactopyranosyl fluoride building block produced the desired AG2 pentasaccharide in a highly stereoselective manner. Finally, the desired AG2 pentasaccharide was obtained in good yield following global deprotection.