7.2 Tetracarbocyclic Sesterterpenoids Constructed by
the Type 2 Terpene Cyclases
7.2.1 6/6/6/6-Membered Ring System
Most tetracarbocyclic sesterterpenoids constructed by the type 2 terpene cyclases
exhibit a 6/6/6/6-membered ring system and are among the most common
sesterterpenoids. Scalarin (127) was the first of this type of compound to be isolated
[113, 114]. The chemical structure of 127 and a cyclization mechanism for the
formation of the 6/6/6/6-membered ring system starting from geranylfarnesyl
diphosphate (8) are shown in Fig. 92.
7.2.2 6/6/5/7-Membered Ring System
Salmahyrtisol A (128) [115] and hippospongide A (129) (Fig. 93) [116] possess a 6/
6/5/7-membered ring system. Compounds 128 and 129 were isolated from the
sponges Hyrtios erecta from the Red Sea and Hippospongia sp. from coral reefs
off the coast of Tai-tung, Taiwan, respectively. From these two sponges, 116 with a
O
AcO
HO
O
PPO
PPO
H
H
+
PPO
8
127
Fig. 92 Structure of
scalarin (127), and possible
cyclization reaction for the
formation of the 6/6/6/6membered ring system of
127
O
OH
O
O
128
O
O
HO
129
116
O
O
H
OH
Fig. 93 Structures of 116, 128, and 129. The structure of 116 is also shown in Fig. 80. Compound
116 is a possible intermediate of 128 and 129
50
T. Mitsuhashi and I. Abe