Synthesis of the hexacyclic triterpene core of the jujuboside saponins via tandem Wolff rearrangement-intramolecular ketene hetero-Diels-Alder reaction.

The jujubosides are saponin natural products reported to have immunoadjuvant, anticancer, antibacterial, antifungal, and antisweet activities. The triterpene component, jujubogenin contains a unique tricyclic ketal motif comprising the DEF ring system. Herein, we describe our efforts toward the total synthesis of jujubogenin, using a sterically-demanding intermolecular Diels-Alder reaction to assemble the C-ring and a tandem Wolff rearrangement-intramolecular ketene hetero-Diels-Alder reaction to form the DF-ring system. Acid-catalyzed cyclization of the resulting bicyclic enol ether then closes the E-ring to provide the hexacyclic core of jujubogenin.