Controlling the stereochemistry in glycosylation reactions is crucial for the assembly of oligosaccharides and glycoconjugates, which are required to fuel glycobiological, glycomedical and glycomaterial studies. However, full control over the stereochemical outcome of glycosylation reactions continues to be a major challenge. The wide variety of structurally different glycans has spurred the development of many different strategies to steer the stereochemical outcome of a glycosylation reaction. Neighboring group participation by acyl groups at C-2 is commonly used in the construction of 1,2-trans linkages. It is the most general means to control glycosylation stereoselectivity and (relatively) independent on the decoration pattern of the glycosyl donor. For the construction of 1,2-cis-glycosyl linkages no general strategy is available and translating methodology from one donor to another is often very challenging, if not impossible. The Crich β-mannosylation methodology, in which a 4,6-Obenzylidene protecting group is used in conjunction with nonparticipating protecting groups at the C-2 and C-3 functionalities, has become one of the most powerful means to construct 1,2-cismannosyl linkages. Strong evidence for the intermediacy of mannosyl α-triflates as product forming intermediates in glycosylations following an SN2-type mechanism has accumulated. However, the method is sensitive to changes in the protecting group pattern, and when a C-3-acyl group is present, the donors give rise to highly stereoselective αmannosylations reactions. In contrast, benzylidene protected glucosyl donors provide mixtures of α/β-products with the anomeric product ratio critically hinging on the nucleophilicity of the acceptor. The stereoselectivity of these glucosylations appears to be relative insensitive to changes in the protecting group at C-3, although systematic studies have not been reported to dissect the influence of C-3 ester groups. In the preceding study we have forwarded evidence for the generation of 1,3-dioxanium ions from 3-acyl-4,6-benzylidene mannosyl α-triflates, that can account for the formation of the observed αproducts. We have not been able to detect the corresponding glucosyl 1,3-dioxanium ions. There has been significant debate as to the origin of the extremely high α-selectivity in the 3-acyl benzylidene mannose case. While Crich and co-workers have initially postulated long-range participation as a reasonable explanation for this stereoselectivity, they have more recently argued strongly against the possible formation of the 1,3dioxanium ions as this would require the formation of a highly strained dioxanium ion from a C-3-acyl rotamer that is hardly populated requiring an intramolecular substitution reaction on a high-energy ring conformer. Rather they argued that the C-3ester destabilizes the anomeric triflate, promoting reactions through an oxocarbenium ion intermediate. No comparison to the glucose case has been made and explanations for the mannose/glucose dichotomy are lacking. To unravel the origin of the contrasting behavior of the 3-acyl benzylidene mannosyl and glucosyl donors we here report on a combination of systematic glycosylation reactions, using a series of alcohol nucleophiles of gradually changing nucleophilicity, with gas-phase IR spectroscopy to detect the 1,3-dioxanium ions and computational experiments investigating different reaction paths of the anomeric triflates. In combination with the preceding report, describing the formation of mannosyl 1,3-dioxanium ions and the absence of these species in glucosylation reaction, a clear picture emerges on the diverging reaction pathways followed in the mannose and glucose systems. The computational studies pin-point the structural features responsible for the contrasting behavior of the mannosyl and glucosyl donors and illuminate the delicate balancing act between the various reactive intermediates. The study explains well why long-range participation can take place in one donor (i.e. the mannose system), but fails to control stereoselectivity in closely related systems (i.e. the glucose donors) and as such provides a blue print for the design of future stereoselective glycosylation methodology.