Abstract
Controlling the chemical glycosylation reaction remains the major challenge in the synthesis of oligosaccharides. Though 1,2-trans glycosidic linkages can be installed using neighboring group participation, the construction of 1,2-cis linkages is difficult and has no general solution. Long-range participation (LRP) by distal acyl groups may steer the stereoselectivity, but contradictory results have been reported on the role and strength of this stereoelectronic effect. It has been exceedingly difficult to study the bridging dioxolenium ion intermediates because of their high reactivity and fleeting nature. Here we report an integrated approach, using infrared ion spectroscopy, DFT computations, and a systematic series of glycosylation reactions to probe these ions in detail. Our study reveals how distal acyl groups can play a decisive role in shaping the stereochemical outcome of a glycosylation reaction, and opens new avenues to exploit these species in the assembly of oligosaccharides and glycoconjugates to fuel biological research.
Highlights
Controlling the chemical glycosylation reaction remains the major challenge in the synthesis of oligosaccharides
Long-range participation (LRP) potentially allows for the utilization of the relative stereochemistry of C-3, C-4, or C-6 groups to control the facial selectivity in glycosylation reactions thereby enabling the stereoselective synthesis of C-2-deoxy and 1,2-cis glycosides
Glycosyl cations have been characterized in superacid solution by nuclear magnetic resonance (NMR), protonation of the acetyl groups under these conditions prevents the assessment of their ability to engage in neighboring group participation (NGP) and LRP27,28
Summary
Controlling the chemical glycosylation reaction remains the major challenge in the synthesis of oligosaccharides. We report the use of infrared ion spectroscopy (IRIS) to characterize glucosyl, mannosyl, and galactosyl dioxolenium ions formed via LRP and show how the stability and reactivity of these species depend on the position and configuration of the acyl group.
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