The major challenge in the chemical synthesis of oligosaccharides is the stereoselective synthesis of the glycosidic bond. 1,2-trans glycosides can be synthesized with high selectivity by the use of neighboring group participation of a C-2 acyl group and is applicable to gluco- and manno-type sugars. In contrast, the stereoselective synthesis of 1,2-cis glycosides remains more challenging. By utilizing neighboring group participation using C-2 chiral auxiliaries, the stereoselective synthesis of 1,2-cis gluco- and manno-type sugars could be achieved. These reactions proceed via an intermediate sulfonium ion, which is depicted in scheme 1. This intermediate can be prepared by making three disconnections, of which two have already been explored[1,3].
Using a thioglycoside precursor, β-sulfonium ion intermediates could by prepared utilizing an unexplored disconnection. The synthetic route to this glycoside donors allows for a late stage introduction of the C-2 auxiliary, giving more control over the outcome of the stereoselectivity using a single glycosyl donor. Using low-temperature NMR studies, the activation of the glycosyl donor, as well as the formation of the β-sulfonium ion were investigated. Subsequently, the influence of the thioaryl moiety, as well as the benzyl protecting groups on the stereoselectivity was explored.
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