Enantioselective Total Synthesis of Cannogenol-3-O-α-l-rhamnoside via Sequential Cu(II)-Catalyzed Michael Addition/Intramolecular Aldol Cyclization Reactions

A concise and scalable enantioselective total synthesis of the natural cardenolides cannogenol and cannogenol-3-O-α-l-rhamnoside has been achieved in 18 linear steps. The synthesis features a Cu(II)-catalyzed enantioselective and diastereoselective Michael reaction/tandem aldol cyclization and a one-pot reduction/transposition, which resulted in a rapid (6 linear steps) assembly of a functionalized intermediate containing C19 oxygenation that could be elaborated to cardenolide cannogenol. In addition, a strategy for achieving regio- and stereoselective glycosylation at the C3 position of synthetic cannogenol was developed and applied to the preparation of cannogenol-3-O-α-l-rhamnoside.