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abstracta convergent [ 3 + 2 ] block synthetic strategy was developed for the synthesis of the pentasaccharide repeating unit of the cell wall oantigen of escherichia coli o11 strain in excellent yield in a minimum number of steps . 
 several suitably functionalized thioglycoside derivatives were used as glycosyl donors during the synthesis of the target compound . 
 a thioglycoside was the glycosyl donor used to couple with another thioglycoside derivative in a highly stereoselective manner exploiting the difference of their reactivity profile . 
 a combination of niodosuccinimide ( nis ) and perchloric acid supported over silica gel ( hclo4sio2 ) was used as a thiophilic glycosylation activator system in all stereoselective glycosylation reactions . 
 hclo4sio2 acted as a userfriendly solid acid catalyst . 
 yields were very good in all glycosylation steps with a high stereoselective outcome . 
 the synthetic pentasaccharide could be coupled to an appropriate protein to furnish a glycoconjugate derivative for its use in immunochemical studies .