2.4 Triterpenoids
Triperpenoids, generally are obtained by biogenesis from six isoprene units, They are found to share commonly the acyclic precursor squalene (C30). Based on the various possible modes, whereby ring closure in squalene takes place may ultimately give rise to a large number of triterpenoids having a variety of skeleton structures. In actual practice, more than 4000 naturally occurring triterpenoids have been isolated and identified, and over 40 varying skeleton types have been established.
The triperpenoids may be categorized into two major groups, namely: the tetracyclic and the pentacyclic compounds: the former ones of the steriodal types with C-27 carbon atoms present in the skeleton while the latter are of the triterpenoid types with C-30 carbon atoms as shown below:
However, the steroidal types (C-27) and the triterpenoid types(C-30) may be distinguished by virtue of the fact that the former yields Diel’s hydrocarbon on dehydrogenation with Se at 360oC, while the latter gives either naphthalene or pinene end products. Interestingly, both these types of compounds may easily combine with sugar moieties at C-3 position to yield the corresponding glycosides. Nevertheless, the free triterpenoids are invariably associated with natural latex, resins, or cuticle of plants.
A few typical examples shall be discussed in the sections that follows:
