Structural Revision of Stemoburkilline from an E-Alkene to a Z-Alkene

Semisynthesis studies starting from (11Z)-1',2'-didehydrostemofoline (4) indicated that the known Stemona alkaloid stemoburkilline is the Z-isomer and not the E-isomer as initially reported. The semisynthesis involved conversion of (11Z)-1',2'-didehydrostemofoline (4) to 11(S),12(S)-dihydrostemofoline (3) followed by a stereoselective base-catalyzed ring-opening reaction to give (Z)-stemoburkilline (8). The same product was obtained using a similar synthetic protocol starting from isostemofoline (6) via a based-catalyzed ring-opening reaction of 11(S),12(R)-dihydrostemofoline (1). A re-examination of the crude root extracts of Stemona burkillii Prain and further NOE studies established stemoburkilline as the Z-isomer (8).