Modulating glycosylation with exogenous nucleophiles: an overview

Abstract

The major challenge in carbohydrate synthesis is stereochemical control of glycosidic bond formation. Different glycosylation methods have been developed that are based on the modulation effect of external nucleophiles. This review highlights the development, synthetic application, challenges and outlook of the modulated glycosylation methods.

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

“…64 Recent examples on the study of additives include the addition of sulfides and sulfoxides, which afford stable glycosyl sulfonium and oxysulfonium salts, 30,65–66 and of dimethylformamide and other secondary amides, and even catalytic oxindoles, 67 leading to the formation of glycosyl imidates. 68–69 The additives area has been reviewed recently and as such we will not comment further on it here, 70 but rather we will limit ourselves to counter ions.…”

Acceptor Reactivity in the Total Synthesis of Alginate Fragments Containing α‐L‐Guluronic Acid and β‐D‐Mannuronic Acid

“…[15] To probe whether the anomeric thio function was at the basis of the improved reactivity of 4 we added thiophene [15b] to the condensation of 7 and 10,t of ind that this external sulphide had no notable effect on the reaction (Table 1, entry 7). This glycosylation proceeded strikingly better than the analogous coupling of 7 and 10,and the tetrasaccharide 20 was obtained in 91 %y ield as as ingle anomer.R emarkably,t he large difference in yield for the glycosylations between 7 and the acceptors 4 and 10,iscaused by the difference in the anomeric functionality,athiocresol and azidopropanol group,respectively,atthe reducing end of the disaccharide acceptor,d istal from the reacting C4'ÀOH.…”

Rapid assembly of the doubly-branched pentasaccharide domain of the immunoadjuvant jujuboside A via convergent B(C6F5)3-catalyzed glycosylation of sterically-hindered precursors

“…25 The nitrile solvent is proposed to occupy the α-face of the oxocarbenium intermediate due to the anomeric effect, resulting in addition of nucleophiles from β-face. Accordingly, after some optimization, we discovered that the combination of B(C 6 F 5 ) 3 catalyst and t -BuCN/CF 3 Ph solvent system afforded pentasaccharide 16 in 81% combined yield and 1.2:1 α/β selectivity, overcoming the inherent complete stereopreference for the undesired α-anomer (entry 6).…”