Structural revision of stemoburkilline from an E-Alkene to a Z-Alkene

Semisynthesis studies starting from (11Z)-I',2'-didehydrostemofoline 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 to II(S), 12(S)-dihydrostemofoline followed by a stereoselective base-catalyzed ring-opening reaction to give (Z)-stemoburkilline. The same product was obtained using a similar synthetic protocol stariing from isostemofoline via a based-catalyzed ring-opening reaction of II (S), 12(R)-dihydrostemofoline. A re-examination of the crude root extracts of Stemona burkillii Prain and further NOE studies established stemoburkilline as the Z-isomer.