α‐Selective Organocatalytic Synthesis of 2‐Deoxygalactosides

Abstract

Alpha rules: A thiourea acts as an efficient organocatalyst for the glycosylation of protected galactals to form oligosaccharides containing a 2-deoxymonosaccharide moiety. The reaction is highly stereoselective for α-linkages and proceeds by way of a syn-addition mechanism.

A propos of glycosyl cations and the mechanism of chemical glycosylation; the current state of the art

“…90 The same thiourea has also been employed as a catalyst for the selective addition of alcohols to glycals resulting in the formation of 2-deoxy-α-glycosides, although in this instance it is considered to function merely as a mild protic acid. 91–92 In related work chiral binol-derived phosphoric acid catalysts have been employed to promote cyclization of 1-(γ-hydroxypropyl)glycals and their homologs to spiroketals. It was demonstrated that the anomeric selectivity of the reaction is dependent on the chirality of the catalyst, and cyclic hydrogen bonded transition state was invoked to explain the phenomenon.…”

Exploring the glycosylation properties of a sialyl thioimidate donor

“…To further explore the glycosylation properties of 6, we proceeded to examine thioglycosides 12 19 and 13 20 as acceptors. Reaction of 12 with donor 6 was conducted under the optimized conditions, and expectedly, the desired sialoside 20 was isolated in very high yield and moderate a/b 2:1 selectivity.…”

Organocatalyzed Glycosylation Reactions of Carbohydrates

“…By deploying 1 mol% of thiourea-derived catalyst 9 the authors succeeded in glycosylations of protected glycal 15. 2-Deoxyglycoside 17 was obtained with high yields (Scheme 4.6) [67,68].…”