Total Synthesis of the Escherichia coli O111 O ‐Specific Polysaccharide Repeating Unit

The first total synthesis of the O ‐antigen pentasaccharide repeating unit from Gram‐negative bacteria Escherichia coli O111 was achieved starting from four monosaccharide building blocks. Key to the synthetic approach was a bis‐glycosylation reaction to combine trisaccharide 10 and colitose 5 . The colitose building block ( 5 ) was obtained de novo from non‐carbohydrate precursors. The pentasaccharide was equipped at the reducing end with an amino spacer to provide a handle for subsequent conjugation to a carrier protein in anticipation of immunological studies.