Interrupted Pummerer Reaction in Latent-Active Glycosylation: Glycosyl Donors with a Recyclable and Regenerative Leaving Group

Latent O-glycosides, 2-(2propylthiol) benzyl (PTB) glycosides, were converted into the corresponding active glycosyl donors, 2-(2-propylsulfinyl) benzyl (PSB) glycosides, by a simple and efficient oxidation. Treatment of the PSB donor and various acceptors with triflic anhydride provided the desired glycosides in good to excellent yields. The leaving group, which was activated by an interrupted Pummerer reaction, can be recycled (PS-BOH) and regenerated as the precursor (PTB-OH). A natural hepatoprotective glycoside, leonoside F, was efficiently synthesized in a convergent [3+1] manner with this newly developed method. The present total synthesis also led to a structural revision of this phenylethanoid glycoside.