university of kwazulu-natal faculty of science and agriculture school ...

Chapter 1 Introduction to diterpenoids
1.1 Classification of diterpenoids
Diterpenes consist of four isoprene units (C20) and are by far the most diverse group of
compounds. They are placed into different classes (termed acyclic, monocyclic, bicyclic,
tricyclic, tetracyclic and macrocyclic) based on the number of carbocyclic rings in the
structure (figures 1a-d).
The acyclic and monocyclic compounds (figure 1a) do not have much variation in that
they are either straight chain structures or contain one ring and a side chain respectively,
such as phytane (1) and vitamin A (2).
Bicyclic compounds belonging to Class 3 (figure 1a), contain two six-membered
carbocyclic rings with alkyl groups attached at either C-8 or C-9 or both. Labdanes and
clerodanes are differentiated because of the different positions of the methyl groups. In
clerodanes these methyl groups occur at C-4, C-5, C-8, C-9 and C-13 while in labdanes
there are two methyl groups at C-4 and one each at C-8, C-10 and C-13.
Class 4 diterpenes (figure 1b) are the tricyclic diterpenoids and have three six-membered
rings but differ in the substitution pattern on these rings. Abietane and totarane
diterpenoids are fairly similar in structure, the only difference being the position of the
isopropyl group in ring C, which is situated at C-13 in abietanes and C-14 for totaranes
due to different alkyl shifts, abietane being the precursor to totarane (Dewick, 2002;
Nakanishi, 1974).
The structural difference between pimarane and cassane type diterpenes is the position of
the methyl group (CH3-17) on ring C, which occurs at C-13 in pimaranes and C-14 in
cassanes, pimarane being the precursor to the cassanes (Nakanishi, 1974) and occurs by a
methyl shift from C-13 in the pimaranes to C-14 in the cassanes when a carbocation at C-
14 is created by a hydride shift in the sandaracopimarenyl cation (Figure 2). Taxane type
1