Promoters stereoselectivity glycosylations

Wong and co-workers753 and Hashimoto and co-workers754 have demonstrated that triflic acid is an ideal promoter in chemo- and stereoselective glycosylation with phosphites [Eq. (5.285)]. 

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... 

Takeuchi, K, Tamura, T, Mukaiyama, T, Stereoselective glycosylation of thioglycosides promoted by respective combinations of A-iodo- or A-bromosuccinimide and trityl tetrakisfpentafluorophe-nyl)borate. Application to one-pot sequential synthesis of trisaccharide. Tetrahedron Lett., 124-125, 2000. 

Uchiro, H, Mukaiyama, T, An efficient method for catalytic and stereoselective glycosylation with thioglycosides promoted by trityl tetrakis(pentafluorophenyl)borate and sodium periodate, Chem. 

Asymmetric synthesis of nucleosides via molybdenum-catalyzed alkynol cycloisomerization coupled with stereoselective glycosylations of deoxyfuranose glycals and 3-amidofuranose glycals. Journal of the American Chemical Society, 118, 6648-6659 (b) McDonald, F.E. and Chatterjee, A.K. (1997) Group VI metal-promoted endo-azacyclizations via alkyne-derived metal vinylidene carbenes. Tetrahedron Letters, 38, 7687-7690 ... 

Scheme 5.35 Effect of 6-O-acyl groups on the stereoselectivity of heterogeneously promoted p-glycosylation.

---