Glycosylations with Thioglycoside Donors

Gold(III)-catalyzed glycosylations with thioglycoside donors... 

Boons and coworkers have used the polystyrylboronic acid as a temporary polymeric support for oligosaccharide synthesis (Scheme 16.3). Polystyrylboronic acid 5, easily prepared from the corresponding bromide by treatment with trimethyl borate and n-BuLi in high loading capacity, was introduced on the 4,6-0 positions of the acceptor 6 by heating in pyridine. The immobilized boronate acceptor 7 was glycosylated with thioglycoside donor... 

Syntheses of thioglycosides are sometimes possible using Koenigs-Knorr glyco-sylation conditions (Scheme 4) [31], although mercury(II) salts have been rather frequently used to promote glycosylation with thioglycoside donors (Section 4.3.2). 

Four other oligosaccharides (18-21, Fig. 6) corresponding to fragments of either the linear or the branched hexasaccharide moieties of the arabinan have been synthesized as methyl glycosides [55,74]. These were obtained in a maimer similar to the disaccharide and trisaccharide via glycosylation of methyl glycoside acceptors with thioglycoside donors. 

B. Mukhopadhyay, B. Collet, and R. A. Field, Glycosylation reactions with disarmed thioglycoside donors promoted by /V-iodosuccinimide and HC104-silica, Tetrahedron Lett., 46 (2005) 5923-5925. 

Wong and coworkers showed that 5-azido sialyl donors protected with O-acetyl ester are useful for a-selective glycosylations of primary hydroxyls (Scheme 4.6e) [169]. It was proposed that the linear and electron-withdrawing nature of the C-5 azido moiety stabilizes the reactive axial acetonitrile adduct to allow the incoming nucleophile to approach the a-face in an SN2-like fashion. In addition, a chemose-lective glycosylation method has been developed for the synthesis of NeuAca-(2 —> 9) NeuAc as thioglycoside donor for use in the subsequent glycosylations [169]. 

Chemical entities discussed in this chapter as glycosyl donors share the principal structural feature C(anomeric)—sulfur atom bond with thioglycosides, discussed earlier. However, the electron density on the sulfur atom is diminished, and consequently its chemical reactivity differs considerably, because of substitution with electron-withdrawing groups such as carboxylic or phosphoric acid residues. This... 

Photochemical oxidation has been employed for the activation of O-glycosides [552], thioglycosides [553] and telluroglycosides [554], It is also effective for O-glycosylation with the phenyl selenoglycoside donors [555], although this approach has not been widely taken up for preparative use. 

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