Studies have been conducted to elucidate the mechanism of glycosylation reactions using a prop-1-enyl donor isomerized directly from an allyl glycoside. The reactions promoted by NIS/TfOH can take place in high yields in acetonitrile at room temperature. Activation of the anomeric prop-1-enyl group often leads to both the desired glycoside (e.g., 9) and the addition-reaction product (e.g., the anomeric mixed acetal 10). TfOH perhaps has a dual role in the reaction: i.e., (a) producing IOTf in situ to activate the prop-1-enyl group and (b) catalyzing the transformation from the addition-reaction product to the desired glycoside (e.g., from 10 to 9). The latter process involves multiple competing pathways. © 2012 American Chemical Society.