Probing the Mechanism of Sulfoxide-Catalyzed Hemiacetal Activation in Dehydrative Glycosylation

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Abstract
The concept of sulfoxide-covalent catalysis has been established in the context of a versatile hemiacetal hydroxyl activation/substitution reaction for the formation of anomeric linkages. Mechanistic studies focused on the hemiacetal activation process show that this transformation proceeds in the presence of a sulfonic anhydride and an acid scavenger through the intermediacy of a glycosyl sulfonate species (10), which serves as a resting state prior to the addition of an external nucleophile and subsequent glycosidic bond formation. Successful determination of the proportion of 18O incorporation in 10 as a function of its formation, via the technique of dynamic monitoring of 13C−16/18O isotopic chemical shift perturbations, provides strong evidence that hemiacetal activation proceeds through initial nucleophilic addition of the hemiacetal hydroxyl to the S(IV)-center of putative sulfonium sulfonate 6. Further confirmation was obtained through the independent synthesis, structure verification, and 1H NMR detection of glycosyl oxosulfonium 11 during the sulfoxide-catalyzed conversion of hemiacetal 3 to glycosyl sulfonate 10.