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1.2 Classification of the abietane diterpenoids
Abietane diterpenoids consist of approximately twenty carbon atoms and commonly have
three tertiary methyl groups (two at C-4 and one at C-10), and one isopropyl group at C-
13. Their numbering system is based on the nomenclature of natural product hydrides as
recommended by the I.U.P.A.C. in 1993 (Figure 3). These abietanes can be further
classified into eight types based on structural differences through reductions (addition of
hydrogen across a double bond) or oxidations such as hydroxylations and dehydrations as
well as cyclisations between different oxygenated groups.
20
13
12
11
C
14
1
2 10
A
3 5
4
9
B
6
8
7
19
18
Figure 3: Numbering system for abietanoids
Acidic abietanes (e.g. abietic acid (19), figure 4) are characterised by the presence of a
carboxylic acid in the compound. In levopimaric acid (20) there is a double bond in ring
C (figure 4). In ferruginol (21) and carnosol (22) type abietanes, ring C is phenolic in
character. Due to the presence of a lactone ring which extends across ring B from C-7 to
C-10, carnosol type abietanes are differentiated from ferruginol type abietanes.
Callicarpone (23) abietanes are based on the epoxide ring at ∆ 12 and the presence of a
hydroxy isopropyl group attached to C-15 as well as α,β-unsaturated ketone groups in
rings B and C. The royleanones (24) have a benzoquinone ring C. Tanshinone (25)
abietanes consist of highly oxidised rings where two of the rings, A and B are aromatic,
ring C contains two adjacent α,β-unsaturated systems and a fourth ring, a furan ring is
present as ring D. Fichtelite (26) closely resembles the abietane skeletal structure and
17
15
16
5