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 (89 –95 ) 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
(94 −96 ),[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 .