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a, QS-21A is a heavily glycosylated saponin isolated from Quillaja saponaria Molina in both apiose (QS21Aapi) and xylose (QS21Axyl) forms. b, Synthesis of QS-21Aapi. Glycosidic linkages were constructed by the use of Ph2SO·Tf2O-promoted dehydrative glycosylation and glycosyl trichloroacetimidate coupling in the construction of the trisaccharide–triterpene substructure. Ara, arabinose; PMB, 4-methoxybenzyl; Rha, rhamnose; TBS, tert-butyldimethylsilyl.
