Summary of main observation and conclusion An effective approach relying on a Lewis acid-or Brφnsted acid-assisted gold(Ⅰ)-catalyzed glycosylation has been reported in the synthesis of a panel of the representative natural spirostanol saponins,namely polyphyllin D (1),polyphyllin V (2),dioscin (3),formosanin C (4),and a derivative of polyphyllin D bearing a terminal azide group (5).This approach highlights the engagement of low Ioadings of Ph3AuPOTf (≤ 0.5mol％) in the presence of Ga(OTf)3,In(OTf)3,or HOTf (～10 mol％) as a co-catalyst,at practical reaction rates as an alternative to the conventional ～10 mol％Ioadings of the gold(Ⅰ) catalyst in the glycosylation.Polyphyllin D (1) was obtained in 41％ overall yield over six steps compared to the maximum 30％ yield in previous syntheses,where conventional donors and promoters were used.By exploiting a regioselective rhamnosylation,a "one-pot" approach was adopted to assemble 1 and 5,thus further strengthening the efficiency of the gold(Ⅰ)-catalyzed glycosylation.
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