membered ring system. After a 6-membered ring is formed by the first cyclization
reaction, an epoxide might be generated by tailoring enzymes. The second cyclization reaction would then be initiated by the protonation of the epoxide.
6.3 Tricarbocyclic Sesterterpenoids Constructed by Both
Type 1 and 2 Terpene Cyclases
In some sesterterpenoids, the type 1 and 2 terpene cyclases seem to work together to
form the complex basic carbon skeleton, and many such sesterterpenoids have been
isolated from marine organisms [101].
One example is ansellone A (119), isolated from the nudibranch Cadlina
luteromarginata and a sponge, Phorbas sp. (Fig. 85) [102]. Analyses revealed that
119 activates the cAMP signaling pathway. In the proposed biosynthesis pathway, at
first the type 1 cyclization starting from geranylfarnesyl diphosphate (8) occurs, and
then the type 2 cyclization reactions form the basic carbon skeleton of 119
(Fig. 85) [101].
O
OH
OAc
O
HO
O
HO
H
+
PPO
OPP
8
H
H 2 O
HO
O
HO
119
OH
OH
type 1 cyclization
type 2 cyclization
oxidation
OH
Fig. 85 Proposed
biosynthesis pathway of
ansellone A (119)
Sesterterpenoids
45