3.3 Linear Sesterterpenoids with a Tetronic Acid Moiety
The tetronic acid moiety is present in numerous linear sesterterpenoids, and many of
them exhibit bioactivities. The tetronic acid moiety seems to be generated by an
oxidation of the 2-furanone moiety (Fig. 26).
For example, 32 was isolated from the Australian sponge Psammocinia
sp. (Fig. 27) [24] and has antimicrobial activity. A similar compound, isopalinurin
(33), was reported from the South Australian sponge, Dysidea sp. (Fig. 27)
[25]. Compound 33 is known as a moderate protein phosphatase inhibitor. In
addition to the tetronic acid moiety, 32 and 33 also possess a furan ring moiety.
Variabilin (34), an antimicrobial linear sesterterpene with a tetronic acid moiety
(Fig. 28) [26], was isolated from the Okinawan sponge, Amphidmedon
sp. Compound 34 possesses a stereocenter at the C-18 position, and the absolute
configuration of this position was determined as (S) by the synthesis of the degradation product of 34 [26].
29
30
2-furanone moiety
O
O
O
O
Fig. 24 Structures of 29
and 30. The 2-furanone
moieties are highlighted by
blue circles
OH
31
-butyrolactone moiety
2-furanone moiety
O
O
O
O
Fig. 25 Structure of
luffarin R (31), which
possesses not only the
2-furanone moiety but also a
γ-butyrolactone moiety
2-furanone moiety
tetronic acid moiety
oxidation
O
O
O
O
OH
Fig. 26 Proposed pathway
for the formation of the
tetronic acid moiety
16
T. Mitsuhashi and I. Abe