Semisynthesis and acetylcholinesterase inhibitory activity of stemofoline alkaloids and analogues.

Semisynthesis of the known Stemona alkaloids oxystemofoline (7) and methoxystemofoline (8) has been achieved starting from (11Z)-1',2'-didehydrostemofoline (6), which confirmed their structures and absolute configurations. The synthesis of (1'R)-hydroxystemofoline (9) helped establish this compound as a natural product from Stemona aphylla. (1'S)-Hydroxystemofoline (10) and a number of related analogues were also prepared. In a TLC bioautographic assay, 9 was found to be the most active acetylcholinesterase inhibitor, but it was not as active as galanthamine.