Stereoselective Synthesis of a Tetrasaccharide Fragment from RhamnogalacturonanⅡSide Chain A

Comprehensive Summary An efficient synthesis of a complex tetrasaccharide fragment 1 structurally related to rhamnogalacturonan ll 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-assistedα-D-xylosylation was developed to synthesize theα-D-xylopyranosidic linkage and a 3,4-O-benzoyl-controlledα-L-galactosylation reaction was usedforthe stereoselective synthesis of the correspondingα-L-galactopyranosidic linkage.