A practical, useful glycosylation method employing only allyl glycoside building blocks has been developed. The donor's glycosylation reactivity is turned on via isomerization of its anomeric allyl protecting group into the corresponding prop-1-enyl moiety. Subsequent chemoselective activation with NIS/TfOH achieves high yields in glycosidic bond construction at room temperature. The efficacy and efficiency of this approach in carbohydrate synthesis is demonstrated in the concise synthesis of the fully protected Shigella flexneri serotype Y O-antigen. © 2010 The Royal Society of Chemistry.