Chemistry Reference
In-Depth Information
Figure 1.11
Some representative examples of diterpenoids in plants.
considered as being derived from that of squalene. Triterpenoids are found mostly
in the plant kingdom, whereas steroids occur in plants, animals, and microor-
ganisms. The chemical diversity of plant triterpenoids results from the ability
of the C
30
precursor, squalene, to undergo various modes of cyclization and
subsequent “decoration” reactions. The plant triterpenoids belong to two main
groups, the tetracyclic and pentacyclic. The tetracyclic triterpenoids, which con-
sist of dammarane (
K1
) and tirucallane (
K2
) among others, are regarded by some
authors as methylated steroids. The group of pentacyclic triterpenoids is by far
the most diverse and is divided into five main groups: friedelane (
K3
), lupane
(
K4
), ursane (
K5
), oleanane (
K6
), and hopane (
K7
) (Fig. 1.12). The steroids
are modified triterpenoids that contain the tetracyclic ring system present in
lanosterol. Chemical diversity represented by steroids depends mainly on the
nature of the side chain attached to the steroid nucleus. Most prevalent in the
plant kingdom are stigmastane (
K8
) and cycloartane (
K9
) classes of steroids
(Fig. 1.12).
Triterpenoids and steroids frequently occur in many plant species as their
glycosides called saponins (33). The chemical diversity of saponins is depen-
dant therefore on both the nature of the 30-carbon moiety and the carbohydrate
residue. Some saponins contain carbohydrate residues attached to several differ-
ent positions of the aglycone (triterpenoid or the steroid) skeleton. Saponins are
classified into 11 main structural classes based on the carbon skeletons of their
