Another example of an alkyl shift is found in the formation of thorectandrol A
(84) and thorectandrol B (85) (Fig. 60) [68]. Compounds 84 and 85 were isolated
from the sponge Thorectandra sp. collected in Palau, and both 84 and 85 inhibited
the growth of the MALME-3M and MCF-7 cancer cells. A proposed cyclization
mechanism for the formation of the basic carbon skeletons of 84 and 85 starting from
geranylfarnesyl diphosphate (8) is shown in Fig. 61. During this reaction, the alkyl
shift occurs twice.
5.3 Other Bicarbocyclic Sesterterpenoids
Even though most of the carbocyclic moieties of sesterterpenoids are formed by the
terpene cyclases, some carbocyclic structures are generated in a different manner.
For example, the bicarbocyclic ring systems of (+)-wistarin (86) [69, 70] and (À)wistarin (87) [71] would not be formed by the typical terpene cyclases (Fig. 62).
Compound 87 is an enantiomer of 86. Compound 86 was found in the sponge Ircinia
84
O
O
OH
O
O
OH
OAc
85
Fig. 60 Structures of
thorectandrol A (84) and
thorectandrol B (85)
OPP
H
+
8
OPP
H
H
OPP
H
OPP
Fig. 61 Proposed
cyclization mechanism for
generating the basic carbon
skeleton of 84 and 85
Sesterterpenoids
33