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in the kauranes. From the kauranes, loss of a hydride ion in ring B followed by ring
contraction and further oxidations result in the gibberellins (figure 10) (Dewick, 2002).
H
4
H
COOH
GGPP
H
7
- H
H
ent-7-hydoxykaurenoic acid
H
H
CO2H H
OH
O
H
OPP
sequence of
oxidation
reactions
ring
contraction
W-M 1,2-alkyl
shift
O
4
A H B
H
H
copalyl PP
H 7
ent-kaurene
GA 12-aldehyde
D
O
OPP
-H
sequence of oxidation
reactions, lactone
formation
H
H
13
C
sandaracopimarenyl cation
(Intermediate I)
HO
H
H
O
C O
4
12
10 9
8
17
16
15
COOH
gibberellic acid
12
13
15
H
1,2-hydride
migration
16
OH
17
H
W-M 1,2-alkyl
shift
H
H
H
13
15
(Intermediate II)
Figure 10: Biosynthetic pathway for bi-, tri- and tetracyclic diterpenes (reproduced
partially from Dewick (2002), Nakanishi (1974) and Hanson (1968))
Loss of a proton at C-16 (in intermediate I) with retention of the methyl at C-13 would
lead to the beyeranes (figure 11). Formation of a carbocation at C-12 from intermediate II
followed by an alkyl migration of the methylene bridge gives rise to the atisane type
compounds (Figure 11). The macrocyclic type compounds, the trachylobanes and
aconanes form pentacyclic and tetracyclic diterpenes respectively. The bond between C-
12 and C-15 is formed with loss of a proton producing the trachylobanes and
rearrangement of the 8,9 bond to the 9,16 position with loss of the C-17 methylene group
(Hanson, 1968) result in the aconanes (Figure 11).
H
12