3.4. Pimarane
Pimarane, a type of tricyclic diterpenoids, generally obtained from plants and fungi but seldom from other biological resources.[113, 114] Pimarane could be further classified into pimarane, isopimrane, ent -pimarane, andent -isopimrane based on differed stereochemistry. Seven new diterpenoids (8995 ) have been isolated from the tuber of Icacina oliviformis .[115]Oliviformislactones A (89 ) and B (90 ) are the first examples of rearranged 3,4-seco -pimarane possessing a 6/6/5/5 tetracyclic ring system; secopimaranlactone A (91 ) and secocleistanthone A (92 ) are the first examples of 3,4-seco -pimarane and 3,4-seco -cleistanthane type diterpenoids, respectively, which have been obtained from the Icacinaceae family. Chromatographic purification of Premna fulvaextracts led to one undescribed isopimarane-type diterpenoid, named premnafulvol A (93 ),[116] and the plausible biotransformation pathway of 93 has been proposed (Fig. 14). Glabresides A−C (9496 ),[117] three undescribedent -pimarane diterpenoid dimers, have been isolated fromSigesbeckia glabrescens . 96 exhibited the most potent inhibition activity on nitric oxide (NO) production induced by LPS in BV2 microglia compared with the other two compounds. 96 also increased the protein expression level of heme oxygenase-1 (HO-1), and suppressed inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2) in LPS-stimulated BV2 cells.
Fig. 14. Proposed biosynthetic pathway for 93 .