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

Abstract

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.