Another monocarbocyclic sesterterpenoid with a 6-membered ring is
cyclolinteinone (70), isolated from the Caribbean sponge Cacospongia linteiformis
(Fig. 46) [54]. The positions of the methyl groups on the 6-membered ring differentiate
70 from 64–69. Compound 70 can downregulate the protein expression of an inducible NO synthase and cyclo-oxygenase-2 via the inhibition of NF-ĸB activation.
A cyclization reaction leading to the formation of the basic carbon skeleton of 70
is shown in Fig. 47. In this proposed mechanism, a 1,2-hydride shift and a 1,2-alkyl
shift occur to change the position of the methyl group on the 6-membered ring, and
then deprotonation finalizes the reaction.
The structures of cyclolinteinol (71) and cyclolinteinol acetate (72) are similar to
that of 70 (Fig. 48) [55]. They were isolated from the Caribbean sponge
Cacospongia cf. linteiformis.
70
O
O
O
Fig. 46 Structure of 70
8
OPP
H
+
H
OPP
H
1,2-hydride shift
1,2-alkyl shift
deprotonation
Fig. 47 Cyclization
reaction leading to the
formation of the basic
carbon skeleton of 70
71
O
OH
O
O
O
72
O
O
O
O
Fig. 48 Structures of
cyclolinteinol (71) and
cyclolinteinol acetate (72)
Sesterterpenoids
27