An enantiomer of 34, (18R)-variabilin (35), was isolated from the Caribbean
sponge Ircinia felix (Fig. 28) [27]. Together with 35, variabilin
11-methyloctadecanoate (36), a branched-chain fatty acid ester of 35, was also
isolated [28].
Compounds 32–36 possess not only a tetronic acid moiety but also a furan ring
moiety. Actually, many linear sesterterpenoids with a tetronic acid moiety have a
furan ring moiety, and some of them possess more than one furan ring moiety in their
chemical structures. For example, spongionellin (37) [29], dehydrospongionellin
(38) [29], ircinin-1 (39) [30, 31], and ircinin-2 (40) [30, 31] have two furan ring
moieties, in addition to a tetronic acid moiety (Fig. 29). Compounds 37 and 38 are
from a Japanese sponge, Spongionella sp., and both inhibit the cell division of
fertilized starfish (Asterina pectinifera) eggs. Compounds 39 and 40 were isolated
from the sponge Ircinia oros, collected in the Bay of Naples along the south-western
coast of Italy [30], and another sponge Ircinia sp., collected from the Island of Bora
Bora in French Polynesia [31].
32
33
tetronic acid moiety
furan ring moiety
O
O
O
O
O
O
OH
OH
Fig. 27 Structures of 32
and 33. The tetronic acid
and furan ring moieties are
highlighted by orange and
red circles, respectively
34
OH
OH
O
O
35
36
branched-chain fatty acid ester
(S)
(R)
(R)
O
O
O
O
O
O
O
O
O
Fig. 28 Structures of 34–
36. The tetronic acid and
branched-chain fatty acid
ester are highlighted by
orange and purple circles,
respectively
Sesterterpenoids
17