Glycosyl cations – also known as glycosyl oxocarbenium ions – are key reactive intermediates in the glycosylation reaction but given their high reactivity and fleeting nature these species remain poorly understood and their role in shaping the stereochemical outcome of a glycosylation reaction enigmatic. We here present on a computational method that relates the stereochemical outcome of reactions involving these species to the full ensemble of conformations these species can adopt. This computational method maps the complete conformational energy landscape (CEL) and predicts the stereoselectivity of SN1-type glycosylation reactions in a quantitative manner (Figure 1). The vast majority of glycosyl oxocarbenium ions, including those derived from L-fucose, L-rhamnose, D-glucose, D-mannose and D-galactose, are stereoselectively attacked to provide 1,2-cis glycoside products. Experimental evidence for the computed privileged conformers is obtained by the generation and direct spectroscopic characterization of selected glycosyl oxocarbenium ions under superacid conditions. The fundamental insight offered by the calculations into the structure and reactivity of glycosyl oxocarbenium ions, and the intrinsic cis-selectivity of these reactive species will pave the way for the development of new and improved glycosylation chemistry, enabling the more effective generation of oligosaccharides to fuel glycobiology and glycobiotechnology research.
Figure 1. Mapping the conformational preference and reactivity of the glucosyl (A) and mannosyl (B) oxocarbenium ions to understand the role of these reactive intermediates in the assembly of complex oligosaccharides.
