A highly stereoselective construction of 1,2- trans-β-glycosidic linkages capitalizing on 2- azido-2-deoxy-D-glycosyl diphenyl phosphates as glycosyl donors
نویسندگان
چکیده
The scope of TMSOTf-promoted glycosidation of 2-azido-2-deoxyglycopyranosyl diphenyl phosphates is investigated. The 3,4,6-tri-O-benzyl-protected glucosyl and galactosyl donors and the 4,6-O-benzylidene-protected galactosyl donor each react with a range of acceptor alcohols in the presence of a stoichiometric amount of TMSOTf in propionitrile at –78 °C to afford 1,2-trans-β-linked disaccharides in high yields with α:β ratios ranging from 9:91 to 1:>99, regardless of the anomeric composition of the donor used. The use of propionitrile as a solvent at –78 °C has proven to be among the best choice for the highest levels of β-selectivity reported to date for this type of glycosidation. A plausible reaction mechanism, which features a large equilibrium preference for α-glycosyl-nitrilium ions over βnitrilium ions, is proposed based on byproducts formed through their intermediacy and accounts for the observed excellent β-selectivities.
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