No abstract

An efficient synthesis of a complex tetrasaccharide fragment 1 structurally related to rhamnogalacturonan II side chain A has been accomplished through a stepwise glycosylation strategy. Challenges involved in the synthesis include the facile construction of the sterically crowded L-fucopyranose core and the stereoselective formation of two 1,2-cis-glycosidic linkages. The 3,4-disubstituted L-fucopyranoside structure was successfully constructed through a 'counterclockwise' glycosylation sequence, namely, the less reactive axial 4-OH group of the central fucose unit was glycosylated first, then the 3-OH. Besides, a 2-pyridinecarbonyl-assisted α-D-xylosylation was developed to synthesize the α-D-xylopyranosidic linkage and a 3,4-O-benzoyl-controlled α-L-galactosylation reaction was used for the stereoselective synthesis of the corresponding α-L-galactopyranosidic linkage.

An expedient synthesis of the nonreducing hexasaccharide fragment of axinelloside A has been completed via a linear stepwise glycosylation approach. Challenges involved in the synthesis include the highly stereoselective construction of five consecutive 1,2-cis-glycosidic linkages and the formation of a sterically crowded 2,3-disubstituted L-fucoside subunit. Protecting group-directing glycosylation strategies such as the remote participation effect of the benzoyl substituent and the stereocontrolling effect of the 4,6-O-benzylidene group were employed for the synthesis of the desired 1,2-cis-glycosidic linkages. Moreover, the 2,3branched L-fucoside framework was established through a 3-O and then 2-O glycosylation sequence in which the 3-hydroxyl group of the core L-fucose unit was glycosylated first and then the 2-hydroxyl. The synthetic hexasaccharide is properly protected, so it can be employed as a precursor to synthesize its natural form.