6.2.3 3/6/6-Membered Ring System
Cacospongionolide (117), with a 3/6/6-membered ring system (Fig. 81) [98, 99],
was isolated as a potent antitumor and ichthyotoxic agent from the sponge
Cacospongia mollior, collected in the Northern Adriatic Sea. The chemical structure
of 117 has been confirmed by the X-ray crystallography of its acetyl derivative.
A proposed cyclization mechanism for the formation of the 3/6/6-membered ring
system starting from geranylfarnesyl diphosphate (8) is shown in Fig. 82. One
methyl group of 8, highlighted with a red color in Fig. 82, might be involved in
the formation of the cyclopropane ring of 117.
6.2.4 6/5/4-Membered Ring System
Lintenone (118) is a representative sesterterpenoid with a 6/5/4-membered ring system
(Fig. 83) [100]. Compound 118 was isolated from a Caribbean sponge, Cacospongia
cf. linteiformis, and possesses potent ichthyotoxicity and antifeedant properties.
O
O
O
OH
117
Fig. 81 Structure of
cacospongionolide (117)
PPO
PPO
H
H
H
PPO
H
+
H
+
8
117
Fig. 82 Proposed
cyclization mechanism for
the formation of the 3/6/6membered ring system of
117
Sesterterpenoids
43