3.1 Linear Sesterterpenoids with a Furan Ring Moiety
A furan ring moiety is observed frequently in the structures of the linear
sesterterpenoids. However, in almost all cases, the enzymes responsible for the
formation of the furan moiety of the linear sesterterpenoids have not been identified.
One example of a possible pathway for the biosynthesis of the furan skeleton is
shown in Fig. 19. Other pathways for the formation of the furan ring could also be
proposed as shown in Fig. 20.
As examples of linear sesterterpenoids with a furan ring moiety, furospongin-3
(25) and furospongin-4 (26) were isolated from the marine sponge Spongia
officinalis (Plate 1) (Fig. 21) [17]. Another example is idiadione (27), which was
discovered in a different sponge, Spongia idia (Fig. 21) [20]. An
epoxyfuranosesterterpene carboxylic acid (28) was isolated from a Western
Australian sponge Spongia sp. [21]. These linear sesterterpenoids possess one
furan ring moiety in their structures. In addition, other tailoring reactions (e.g.,
oxidation, reduction, methyl ester formation) also seem to occur in their
biosynthesis.
O
HO
OH
H +
oxidation
furan ring moiety
O
O
OH
H
Fig. 19 One example of the
proposed pathways for the
formation of the furan
moiety
8
OH
OH
OPP
23
24
– OPP
+ H 2 O
Fig. 18 Putative
biosynthesis pathway of
geranylfarnesol (23) and
geranylnerolidol (24)
Sesterterpenoids
13