2.4. Germacrane
The traditional Chinese medicine Anisodus tanguticus , usually used for promoting blood circulation, contain four novle norsesquiterpenoids anisotanols A−D (1518 ) with a congested tricyclic 6/3/5 ring system, which have been assumed to be biosynthesized by W−M 1,4-hydride shift and cyclization of germacane-type carbon skeleton (Fig. 6).[38]Another unusual germacrane derivative tomenphantadenine (19 ), with an adenine-substituted unit, has been isolated from the whole plant of Elephantopus tomentosus L, and 19 showed potent antibacterial activity against the gram-positive Staphylococcusaureus .[39]
Four rearranged sesquiterpenoids eugenunilones C−F (2023 ), possibly originated from the carbon skeleton of germacrane, have been isolated from the fruits of Eugenia uniflora ,[40] an evergreen shrub native to South America.[41-43] Structurally, eugenunilones C and D (20 and 21 ) possess new skeletons assembled on caged tricyclo[4.3.1.03,7]decane and bicyclo[3.2.1]octane scaffolds, respectively, while eugenunilones E and F (22 and 23 ) possess a rare tricyclo[4.4.0.02,10]decane ring system.
Fig. 6. Proposed biosynthetic pathway for1518.