Syntheses of p-trifluoroacetamidophenyl 3-O-(2-acetamido-2-deoxy-3-O-( alpha -L-fucopyranosyl)- beta -D-glucopyranosyl)- beta -D-galactopyranoside and of p-trifluoro-acetamidophenyl 3-O-(2-acetamido-2-deoxy-3-O-( alpha -L-fucopyranosyl)-4-O-( beta -D-galactopyranosyl- beta -D-glucopyranosyl) beta -D-galactopyranoside, a trisaccharide and a tetrasaccharide fragment of the Le super(x) tumour-associated antigen

p-Trifluoroacetamidophenyl 3-O-(2-acetamido-2-deoxy-3-O-( alpha -L-fucopyranosyl)- beta -D-glucopyranosyl)- beta -D-galactopyranoside and p-trifluoro-acetamidophenyl 3-O-(2-acetamido-2-deoxy-3-O-( alpha -L-fucopyranosyl)-4-O-( beta -D-galactopyranosyl)- beta -D-glucopyranosyl)- beta -D-galactopyranoside, which are trisaccharide and tetrasaccharide fragments respectively of the Le super(x) tumour-associated antigen, were synthesized, using mainly thioglycosides as glycosyl donors and dimethyl(methylthio)sulfonium triflate ("DMTST") as promoter.