Intramolecular aglycon delivery on polymer support: Gatekeeper monitored glycosylation

A novel use of polymer supported glycosyl donor for stereoselective synthesis of beta-manno glycoside is described. This system features the use of polymer support in an unprecedented manner, in which the polymer sector serves as a ''gatekeeper''. Polymer supported thiomannoside 10 that carries a p-alkoxybenzyl group as a linker at C-2 position was synthesized. This compound was subjected to the conditions of beta-mannosylation according to the procedure of p-methoxybenzyl assisted intramolecular aglycon delivery (IAD) to react with glycosyl acceptor 11. Treatment with DDQ afforded the mixed acetal 12 which was followed by the activation of the anomeric C-S linkage to initiate the IAD step. This process allows the product derived from IAD to be specifically released from the polymer to give the mixture which is highly enriched with the beta-manno glycoside 13. Compatibility with the orthogonal glycosylation strategy was confirmed by the reaction with the glycosyl fluoride 11c, and resultant disaccharide 14 was further transformed into 16 which corresponds to the core trisaccharide structure of Asn-linked oligosaccharide.