Interestingly, the compound known as “siccanol,” isolated from the fungus
Bipolaris sorokiniana NSDR-011, is also compound 77. Even though “siccanol”
was initially reported as 11-epiterpestacin (78), an epimer of 77 (Fig. 53) [60], the
total synthesis of 78 revealed that “siccanol” was not 78, but 77 [61].
Fusaproliferin (79), isolated from the fungus Fusarium proliferatum, also possesses the 15/5-membered ring system (Fig. 53) [62]. Compound 79 is an acetate
ester of 77, and the stereochemistry of 79 was established by a synthesis approach
[63]. A proposed cyclization mechanism for the formation of the 15/5-membered
ring system starting from geranylfarnesyl diphosphate (8) is shown in Fig. 54. This
cyclization involves a 1,5-hydride shift, which is seen frequently in the type 1 cyclization reactions of sesterterpenoids.
5.1.2 12/6-Membered Ring System
Emericellene A (80) and related compounds have been reported from an endophytic
fungus, Emericella sp. AST0036, collected from a healthy leaf of the plant
77
OH
11
O
HO
OH
11
O
HO
OAc
11
O
HO
OH
OH
OH
78
79
Fig. 53 Structures of 77–
79, which possess a 15/5membered ring system
8
OPP
H
H 2 O
HO
1,5-hydride shift
Fig. 54 Proposed
cyclization mechanism for
the formation of the 15/5membered ring system
30
T. Mitsuhashi and I. Abe