Astragalus lentiginosus (Fig. 55) [64]. Compound 80 possesses a 12/6-membered
ring system. A proposed cyclization mechanism for generating the 12/6-membered
ring system originating in geranylfarnesyl diphosphate (8) is shown in Fig. 55, and
the formation of a 14-membered ring might be the first step in this reaction.
5.2 Bicarbocyclic Sesterterpenoids Constructed by the
Type 2 Terpene Cyclases
The majority of bicarbocyclic sesterterpenoids constructed by the type 2 terpene
cyclases possess 6/6-membered ring systems. An example of the 6/6-membered ring
formation starting from geranylfarnesyl diphosphate (8) is shown in Fig. 56.
Salvimirzacolide (81), with a 6/6-membered ring system, was isolated from the
aerial parts of the plant Salvia mirzayanii (Fig. 57) [65]. Another example is
salvileucolide methyl ester (82), which reportedly exists in the aerial parts of two
Iranian Salvia species plants (Fig. 57) [66]. The structures of both 81 and 82 have
been confirmed by X-ray crystallography.
In some cases, an alkyl shift occurs in the middle of the cyclization reaction. For
example, the basic carbon skeleton of halisulfate-3 (83) is different from those of 81
and 82 (Fig. 58) [67]. Compound 83 is one of the metabolites of a sponge, Ircinia
sp., which was collected in the Philippines. The cyclization reaction for the formation of the basic carbon skeleton of 83 starting from geranylfarnesyl diphosphate (8)
is shown in Fig. 59.
OPP
H
OHC
O
80
8
formation of
14-membered ring
12/6-membered ring system
Fig. 55 Structure of
mericellene A (80), and
proposed cyclization
mechanism for the
formation of the 12/6membered ring system
Sesterterpenoids
31
ring system. A proposed cyclization mechanism for generating the 12/6-membered
ring system originating in geranylfarnesyl diphosphate (8) is shown in Fig. 55, and
the formation of a 14-membered ring might be the first step in this reaction.
5.2 Bicarbocyclic Sesterterpenoids Constructed by the
Type 2 Terpene Cyclases
The majority of bicarbocyclic sesterterpenoids constructed by the type 2 terpene
cyclases possess 6/6-membered ring systems. An example of the 6/6-membered ring
formation starting from geranylfarnesyl diphosphate (8) is shown in Fig. 56.
Salvimirzacolide (81), with a 6/6-membered ring system, was isolated from the
aerial parts of the plant Salvia mirzayanii (Fig. 57) [65]. Another example is
salvileucolide methyl ester (82), which reportedly exists in the aerial parts of two
Iranian Salvia species plants (Fig. 57) [66]. The structures of both 81 and 82 have
been confirmed by X-ray crystallography.
In some cases, an alkyl shift occurs in the middle of the cyclization reaction. For
example, the basic carbon skeleton of halisulfate-3 (83) is different from those of 81
and 82 (Fig. 58) [67]. Compound 83 is one of the metabolites of a sponge, Ircinia
sp., which was collected in the Philippines. The cyclization reaction for the formation of the basic carbon skeleton of 83 starting from geranylfarnesyl diphosphate (8)
is shown in Fig. 59.
OPP
H
OHC
O
80
8
formation of
14-membered ring
12/6-membered ring system
Fig. 55 Structure of
mericellene A (80), and
proposed cyclization
mechanism for the
formation of the 12/6membered ring system
Sesterterpenoids
31