Stemona alkaloids represent a unique class of natural products exclusively isolated from the monocotyledonous family Stemonaceae comprising three genera mainly distributed in southeast Asia. Structurally the
alkaloids are characterised by a pyrrolo[1,2- a]azepine nucleus usually linked with two
carbon chains mostly forming terminal
lactone rings. Based on biosynthetic considerations and their various distribution the present review describes 82
Stemona alkaloids grouped into three skeletal types. Due to different
carbon chains attached to C-9 of the pyrroloazepine nucleus they were classified into stichoneurine-, protostemonine- and
croomine-type
alkaloids. The genera Croomia and Stichoneuron only accumulate
croomine or stichoneurine derivatives, respectively, whereas the genus Stemona produces all three types of
alkaloids. However, species-specific accumulation trends towards certain structural types represent valuable chemosystematic criteria. Bioassays with larvae of Spodoptera littoralis exhibited very high insect toxicity for the roots of Stemona species containing certain protostemonine derivatives, especially
didehydrostemofoline, whereas those with dominating stichoneurine or
croomine derivatives showed low toxicity but sometimes remarkable repellence due to an accumulation of
tuberostemonine.
Tuberostemonine also showed effects on the motility of helminth worms and reduced the excitatory transmission at the crayfish neuromuscular junction. Significant
antitussive activity was shown for the stereoisomeric
neotuberostemonine in guinea-pig after
cough induction by
citric acid aerosol stimulation. Studies on structure-activity relationship with seven related compounds revealed that the saturated tricyclic pyrrolobenzazepine nucleus of tuberostemonines is the prerequisite for
antitussive activity.