Total synthesis of (-)-2-oxo epimesembranol and (+)-dihydromaritidine via a key Johnson-Claisen rearrangement

A general approach to Sceletium alkaloids of the family Aizoaceace following a key Johnson (orthoester)-Claisen rearrangement of an enantioenriched allylic alcohol has been disclosed. The tricyclic core (1c) of cis-3a-octahydroindoline skeleton was achieved via an ester-aminolysis followed by an intramolecular aza-Michael reaction with amine under elevated temperature. Utilizing aforementioned strategy, a collective total syntheses of Sceletium alkaloids, such as (-)-2-oxo-epimesembranol (1d) [the first total synthesis], (-)-6-epimesembranol (1b), and (-)-mesembrine (1a) were shown. Further this strategy was applied for total synthesis of (+)-dihydromaritidine (2c) sharing [5,11b]-ethanophenanthridine skeleton.