Synthesis and Application of Branched Type II Arabinogalactans.

The synthesis of linear and (1 → 6)-branched ß-(1 → 3)-d-galactans, structures found in plant arabinogalactan proteins (AGPs), is described. The synthetic strategy relies on iterative couplings of monosaccharide and disaccharide thioglycoside donors, followed by a late-stage glycosylation of heptagalactan backbone acceptors to introduce branching. A key finding from the synthetic study was the need to match protective groups in order to tune reactivity and ensure selectivity during the assembly. Carbohydrate microarrays were generated to enable the detailed epitope mapping of two monoclonal antibodies known to recognize AGPs: JIM16 and JIM133.

2,6-Disubstituted benzoates as neighboring groups for enhanced diastereoselectivity in beta-galactosylation reactions: synthesis of beta-1,3-linked oligogalactosides related to arabinogalactan proteins.

Arabinogalactan proteins (AGPs) are plant glycoproteins which contain a beta-1,3-linked galactan core. The synthesis of the beta-galactopyranose-1,3-beta-galactopyranose linkage using various 2-O-acyl-protected glycosyl donors has been plagued with poor stereoselectivity and side reactions including orthoester formation and transesterification of the 2-O-acyl group from the donor to the acceptor. We have investigated the use of 2,6-disubstituted benzoyl groups as bulky neighboring groups on the glycosyl donor. A 2,4,6-trimethylbenzoyl group was found to be optimal and enabled the formation of the beta-galactopyranose-1,3-beta-galactopyranose linkage to disarmed ester-protected acceptors, suppressing transesterification and reducing orthoester formation while enhancing the beta-selectivity of galactosylation reactions. A series of beta-1,3-linked oligogalactosides were prepared and elaborated to neoglycoconjugates for the study of AGP biosynthesis and AGP binding proteins.