Brpnsted acids glycosylation

Glycals can be transformed into 1,6-anhydro sugar derivatives by intramolecular cyclization in the presence of Lewis and Brpnsted acids, a reaction that has been termed the intramolecular Ferrier glycosylation.168 Sharma el al.169 showed that a montmorillonite clay-supported silver reagent can be an efficient catalyst for this transformation. The 1,6-anhydro-2,3-dehydro sugars obtained were then selectively dihydroxylated to furnish 1,6-anhydro saccharides. 

In the case of Brpnsted acid-mediated glycosyl fluoride donor glycosylations, the effect of counter-ion was also found to be as important as that of solvent [21]. For instance, trifluoromethanesulfonic acid favoured the formation of the a-glycoside, whereas bis-trifluoromethanesulfonamide favoured p. Interestingly, this phenomenon was emphasised by the demonstration of matched and mismatched selectivities... 

Activation of 0-glycosyl trichloroacetimidates as glycosyl donors typically requires moderately strong acids, such that a simple A/,A/ -diarylthiourea, ArNHC(=S)NHAr [e.g., Ar = 3,5-bis(trifluoromethyl), = 8-5], would not be expected to catalyse the process. However, it can act as a co-catalyst with simple Brpnsted acids such as benzoic (p. = 4). The system gives significant rate and yield enhancements, and good selectivity for the / -anomer. A multiply hydrogen-bonded complex of reactants and catalysts is proposed. 

---