The study explains how minimal changes in donor structure (i.e. the stereochemistry of a single stereocenter or changing a single remote benzyl group for a benzoate) impact the formation of different reactive intermediates. While it has previously been argued that 1,3-dioxanium ions cannot form from benzylidene mannosyl donors, spectroscopic evidence -both in the gas phase and in solution-and computational chemistry have shown these species to be favorable intermediates formed upon activation of the mannosyl donors. These species very well explain the "blackand-white" difference in stereochemical outcome of the 3-Obenzoyl vs the 3-O-benzyl benzylidene mannose donors, with the former providing solely α-mannosyl product and the latter predominantly the β-isomers. The computational studies in the glucose series strongly support the intermediacy of a productforming β-glucosyl triflate, for which spectroscopic proof has accumulated.