Stereodirecting Effect of C5-Carboxylate Substituents on the Glycosylation Stereochemistry of 3-Deoxy-D-manno-oct-2-ulosonic Acid (Kdo) Thioglycoside Donors: Stereoselective Synthesis of α- and β-Kdo Glycosides

The stereodirecting effect of C5-ester functions on the glycosylation stereoselectivity of 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) ethyl thioglycoside donors is presented. The coupling of 5-O-arylcarbonyl or acetyl protected Kdo thioglycosides with acceptors proceeds in an alpha-selective and high-yielding manner, leading to formation of alpha-linked Kdo glycosides products. On the other hand, the glycosylation stereoselectivity of the 5-O-2-quinolinecarbonyl (Quin) or 4-nitropicoloyl substituted Kdo thioglycoside donors is switchable: (1) The glycosylation of the 5-O-Quin carrying Kdo donors with primary glycosyl acceptors shows complete beta-stereoselectivity, furnishing the corresponding beta-glycosides in good-to-excellent yield. (2) The stereochemical outcome of the secondary acceptors with these Kdo donors is determined mainly by the stereoelectronic nature of the acceptor. Only or predominant alpha anomeric products are obtained when the Kdo donors couple with the disarmed or highly crowded secondary carbohydrate acceptors, while the selectivity may switch to predominant beta in the glycosylation of the 5-O-4-nitropicoloyl carrying donor with more reactive secondary alcohols. The synthetic use of the newly developed Kdo donors 1c and 7b has been demonstrated by facile preparation of a structurally unique trisaccharide motif 19 which possesses both alpha- and beta-Kdo glycosidic bonds.