7.1.3 5/8/6/5-Membered Ring System
There are two types of 5/8/6/5-ring systems. One is exemplified by variecolin (122),
which has been isolated from some fungi, including Aspergillus variecolor MF138
[105], Emericella purpurea [106], and Emericella aurantio-brunnea [107] (Fig. 88).
Compound 122 possesses immunosuppressive activity, and the formation of its 5/8/
6/5-membered ring system is shown in Fig. 88. In this reaction, an 11/6/5-membered
ring system is first formed from geranylfarnesyl diphosphate (8), and then protonation occurs to start a second round of cyclization, and the 5/8/6/5-membered ring
system is generated.
The other type of 5/8/6/5-membered ring systems is exemplified by aleurodiscal
(123), from the corticioid fungus Aleurodiscus mirabilis [108], and nitidasin (124),
from the plant Gentianella nitida [109, 110] (Fig. 89). The formation of the 5/8/6/5membered ring systems of 123 and 124 is initiated by generating a 15/5-membered
ring system (Fig. 89), while that of 122 starts from the generation of the 11/5membered ring system (Fig. 88).
H
H
H
H 2 O
OH
OPP
8
121
Fig. 87 Structure of
asperterpenol A (121), and
possible cyclization reaction
for the formation of the 5/8/
6/6-membered ring system
Sesterterpenoids
47