Glycosylations of Simple Acceptors with 2‐O‐Acyl l‐Idose or l‐Iduronic Acid Donors Reveal Only a Minor Role for Neighbouring‐Group Participation

Several l-idose and l-iduronic acid glycosyl donors (mostly thioglycosides but also halides and trichloroacetimidates) with acyl protecting groups at the C-2 position were prepared and evaluated in glycosylation reactions with simple acceptors. In glycosaminoglycan oligosaccharide syntheses in the literature, the presence of C-2 acyl protecting groups in l-ido-configured glycosyl donors generally results in exclusive formation of 1,2-trans glycosidic linkages, a finding that has typically been attributed to neighbouring-group participation. However, glycosylations of simple alcohols with l-ido-configured donors (particularly thioglycosides), reported here, generally displayed incomplete stereocontrol and gave mixtures of the 1,2-trans and 1,2-cis products, suggesting that neighbouring-group participation has lesser importance in these reactions. Glycosyl donors and reaction conditions were identified that gave improved, but not exclusive, selectivity for the desired α-l-anomer (1,2-trans) as the major product. Interestingly, glycosylations under the same reaction conditions with more complex monosaccharide acceptors gave exclusively the expected 1,2-trans products. The role of neighbouring-group participation in these glycosylations was explored with density functional theory (DFT) calculations, which revealed that the non-stereoselective addition of the acceptor alcohol to the intermediate oxocarbenium ion is competitive with the stereospecific addition of the acceptor to the acyloxonium ion intermediate.