A convenient synthetic strategy of the common acidic pentasaccharide repeating unit corresponding to the O-antigen of enterotoxigenic E. coli O168 and Shigella dysenteriae type 4 has been successfully developed. A stereoselective [2 + 3] block glycosylation method has been exploited to get the target pentasaccharide derivative. Most of the synthetic intermediates were solid and prepared in high yields from commercially available reducing sugars following a series of protection-deprotection reactions. A α-D-mannose moiety has been used as the source of α-D-glucosamine moiety. A late-stage TEMPO mediated selective oxidation reaction finally resulted in the pentasaccharide containing a glucuronic acid unit.