a, GM2, GD3 and GD2 have a similar trisaccharide core. b, Synthesis of GM2 tetrasaccharide through α-selective sialyl phosphite donor glycosylation. c, Synthesis of GD3 tetrasaccharide by thiosialoside glycosylation. Incorporation of the C5-trifluoroacetamide group in both donor and acceptor enhances efficiency of the coupling. Piv, pivaloyl (2,2,2-trimethylacetyl).