6.2 Tricarbocyclic Sesterterpenoids Constructed by the Type 2
Terpene Cyclases
6.2.1 6/6/6-Membered Ring Systems
Many tricarbocyclic sesterterpenoids constructed by the type 2 terpene cyclases
exhibit 6/6/6-membered ring systems. For example, suvanine (114) [95] and
lintenolide F (115) [96] possess the 6/6/6-membered ring system (Fig. 78). Compound 114 was isolated from a sponge, Ircinia sp., and its chemical structure has
been confirmed by the X-ray crystallography of its degradation product [95], while
115 was isolated from a Caribbean sponge, Cacospongia cf. linteiformis [96].
A proposed cyclization mechanism for the formation of the carbon skeleton of
114 starting from geranylfarnesyl diphosphate (8) is shown in Fig. 79a, while the
cyclization reaction for that of 115 is illustrated in Fig. 79b.
HO
8
OPP
OPP
H
(Z)
110
111
113
H 2 O
ring expansion
11/6/6-membered
ring system
11/6/5-membered
ring system
Fig. 77 Proposed
cyclization mechanism for
the formation of the 11/6/6membered ring system of
113
O
O
O
HO
HO
O
OSO 3
114
115
Fig. 78 Structures of
suvanine (114) and
lintenolide F (115)
Sesterterpenoids
41