Thioglycosides glycosylation with -selectivity

Another popular promoter for glycosylation with thioglycosides is the reagent dimethyl(methylthio)sulfonium triflate (DMTST).60 A comparison between DMTST and NIS/TfOH for the formation of sialoglycoconjugates demonstrated that the iodine system provided superior yields and anomeric selectivity in nearly all cases.61 The yields and stereoselectivity also increased with an increase in solvent polarity, suggesting that enhanced stabilization of the oxacarbenium intermediate, the rate-limiting step in glycosylation, was responsible for the increased... 

The synthesis of the hexasaccharide demonstrates that our new technology provides a convenient, efficient method for the synthesis of oligosaccharides. The most noteworthy feature of our procedure that makes it particularly suited for these operations is the ability to selectively and efficiently activate the anomeric position under mild conditions additionally, by coupling the glycosyl fluoride with a thioglycoside glycosyl acceptor, the stage is set for the process to be effectively repeated. 

The new building block 18 was prepared following the route outlined in Scheme 3. Thioglycoside 20 was glycosylated with the lactose building block 19 and the chloroacetate was selectively removed with thiourea to give building block 18. 

A variety of l,2-m 5-glycopyranosyl esters have been prepared in high yields and with excellent selectivity by reaction of alkyl thioglycoside derivatives with carboxylic acids in the presence of -iodosuccinimide. An example is given in Scheme aP 1,2-rra/w-Glycosyl acetates are available from methyl a- or -thioglycopyranosides by treatment with acetic... 

Glycosylation of thioglycosides.1 1-Thioglycosides when activated with 1.5 equiv. of C,H,SeOTf, prepared in situ from QH SeCl and AgOTf, react at 0° to -40° with alcohols to form glycosides in 60-95% yield. The reaction is generally p-selective. 

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]. 

Such a synthesis is shown in Scheme 4.18a, in which a glycosyl bromide is coupled with the thioglycosyl acceptor to afford a thioglycosyl disaccharide. The glycosyl bromide was obtained from the corresponding 1-thioglycoside by treatment with Br2. It was shown [222-224] that phenyl selenoglycosides can be selectively activated in the... 

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