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.
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