Defining the SN1 side of glycosylation reactions: stereoselectivity of glycopyranosyl cations

S6.3 Glycosylation mechanisms and strategies
Location (hall): 
Start/end time: 
Tuesday, July 2, 2019 - 17:15 to 17:30

Thomas Hansen1, Ludivine Lebedel2, Wouter A. Remmerswaal1, Stefan van der Vorm1, Dennis P.A. Wander1, Mark Somers1, Herman S. Overkleeft1, Dmitri V.  Filippov1, Jerome Désiré2, Agnès Martin-Mingot2, Yves Blériot2, Gijs A. van der Marel1, Sébastien Thibaudeau2, Jeroen D. C. Codée1

1Leiden University, Leiden Institute of Chemistry, Einsteinweg 55, 2333 CC Leiden, The Netherlands, 2Poitiers University, IC2MP, 4 rue Michel Brunet, 86073 Poitiers, France

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.