3.1. Abietane
The abietane diterpenoids are the main components of conifers oleoresins, which play crucial roles in plant to resist pests and pathogens. Abietane diterpenoids isolated in recent years can be divided into five sub-classes, namely, classical abietanes, rearranged abietanes,[78] aromatic abietanes,[79] abietane lactones,[80] and abietane dimers.
In previous studies, more than 90 diterpenoids belonging to abietane have been reported from the genusIsodon .[81] During ongoing investigation onIsodon rugosiformis , rugosiformisin A, a skeleton-rearranged abietane-type diterpenoid with potent NO inhibitory activity, has been obtained (58 ).[82] The stereochemistry of58 has been determined using single-crystal X-ray diffraction analysis.
Officinalins A (59 ) and B (60 ),[83] which are a pair of unique C23 terpenoid epimers possessing an unprecedented tetracyclic 6/7/5/5 carbon skeleton with a tetracycline[9.6.0.03,8.012,16]heptadecane core, have been isolated from the leaves of Salvia officinalis . Another rearranged abietane diterpenoid, salviyunnanone A (61 ),[84] has been isolated fromSalvia yunnanensis . 61 possesses an unprecedented 7/5/6/3 fused ring system comprising an unusual 9,11-epoxy moiety. Recently, a series of highly oxidized abietane diterpenoids with potent inhibitory activity against cancer cells were identified fromEuphorbia fischeriana .[85]
A unique abietane diterpenoid dimer, fischdiabietane A (62 ),[86] possessing 2-oxaspiro[4.5]decane-1-one and 2-oxabicyclo[3.2.2]nonane framework, has been isolated from Euphorbia fischeriana . The absolute configuration of 62 has been determined by single-crystal X-ray diffraction analysis. 62 has induced apoptosis in T47D cells through the activation of caspase-3 and the degradation of poly (ADP-ribose) polymerase.