Although many examples of this group of natural terpenoids are now known, they are found principally in fungi and marine organisms, and span relatively few structural types. The origin of ophiobolene and ophiobolin A in the plant pathogen Helminthosporium maydis from cyclization of geranylfarnesyl PP (GFPP) is shown in Figure 5.52, and provides no novel features except for an experimentally demonstrated 1,5-hydride shift. GFPP arises by a continuation of the chain extension process, adding a further IPP unit to GGPP. Ophiobolin A shows a broad spectrum of biological activity against bacteria, fungi, and nematodes. The most common type of marine sesterterpenoid is exemplified by sclarin and this structure can be envisaged as the result of a concerted cyclization sequence (Figure 5.53) analogous to that seen with GGPP in the diterpenoids, and with squalene oxide in the triterpenoids (see below).
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