A practical synthesis of α-D-Manp-(1 → 3)-α-D-Manp-(1 → 2)-[α-D-Glcp-(1 → 3)]-α-D-Manp-(1 → 2)-α-D-Manp-(1 → 2)-α-D-Manp, an O-specific heterohexasaccharide fragment of Citrobacter braakii O7a, 3b, 1c