Trichloroacetimidates, glycosylations, silver

Despite these restrictions, the acceptor-bound succinoyl MPEG28 was found to permit glycosylations with glycosyl trichloroacetimidates promoted by boron trifluoride, triflic anhydride, and trimethylsilyl or triethylsilyl triflates, with glycosyl halogenates promoted by silver triflate, and with thioglycosides promoted by the iodonium ion. It is compatible with long-term ester protection, with allylic,... 

Douglas, S P, Whitfield, D M, Krepinsky, J J, Silver trifluoromethanesulfonate(triflate) activation of trichloroacetimidates in glycosylation reactions, J. Carbohydr. Chem., 12, 131-136, 1993. 

Wei, G, Gu, G, Du, Y, Silver triflate. A mild alternative catalyst for glycosylation conditions using trichloroacetimidates as glycosyl donors, J. Carbohydr. Chem., 22, 385-393, 2003. 

Glycosylations of IV.57 and IV.58 using both the trichloroacetimidate IV.59 and the chloride IV.60 as L-iduronic acid donors demonstrated that the trichloro-acetimidates are superior candidates for the anomeric activation of L-iduronic acid moieties to form the desired a-linked disaccharide (Scheme 48). Condensation of IV.57 with IV.59 in the presence of trimethylsilyl triflate afforded the a-linked disaccharide IV.61 in 63% yield and the j8-linked anomer IV.62 in 27% yield. Interestingly, glycosylation of IV.57 or IV.58 with chloride IV.60 in the presence of silver triflate afforded very slowly, but exclusively, the corresponding )8-linked disaccharides in 70 and 60% yield, respectively. 

In addition the selective activation of both glycosyl trichloroacetimidates (using catalytic triethylsilyl trifluoromethanesulfonate at —78 °C) and glycosyl bromides (using silver trifluoromethanesulfonate and collidine) over selenoglycosides can be achieved. 

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