Oral Presentation HUPO 2019 - 18th Human Proteome Organization World Congress

The chemical synthesis of sialoglycans remains a challenge in carbohydrate chemistry due to the difficulty of stereoselective glycosidation of sialic acid. Although there are several efficient methods for highly stereoselective sialidation, the stereoselectivity frequently varies depending on the structures of glycosyl partners and reaction conditions. Very recently, we have developed a novel stereoselective glycosidation method using macrobicyclic α-sialyl donors as synthetic equivalent of the bridgehead oxonium cation, in which the C1 and C5 positions were tethered with an alkyl chain. In this study, we have examined the substrate scope and carried out the synthesis of various sialic acid-containing compounds in a fully stereoselecitive manner.The glycosidations of macrobicyclic sialyl donors with primary to tertiary alcohols provided corresponding α-sialosides as a single stereochemical outcome in high yields (84%~). In addition, we have found that a macrobicyclic donor bearing dibenzylphosphate group as a leaving group could be used for C-glycoside formation. Next, we applied the method to the synthesis of α(2,8)-linked sialic acid dimer, which is challenging due to the extremely poor reactivity of the 8-OH of sialic acid. We envisioned conformational change by tethering the C1 and C5 position would boost the reactivity of the 8-OH. As expected, the 8-OH in a macrobicyclic sialic acid was successfully glycosylated with the macrobicyclic sialyl donor to afford a dimeric sialic acid in high yield. Furthermore, by repeating the 8-OH sialylation, we have produced an pentamer, which then underwent the global deprotection to deliver a α(2,8)-linked pentasialoside^[1].In conclusion, we have developed a reliable and powerful method for the stereoselective synthesis of diverse sialic acid–containing compounds. Our method would be applicable to extending the scope of automated glycan synthesis.