The structure, stereochemistry, and biosynthetic origin of a diterpenoid fungal metabolite, traversianal, established by 1H, 2H, and 13C magnetic resonance

The structure of traversianal, a tricyclic diterpenoid fungal metabolite of Cercospora traversiana, has been elucidated through detailed 1H and 13C magnetic resonance studies, including homo- and heteronuclear correlation spectra of the natural product and examination of 13C-labelled material obtained by [1,2-13C2]acetate incorporation experiments. Its stereochemistry was established from a series of nuclear Overhauser effect difference spectra. The tricyclic carbon skeleton of traversianal is that of the fusicoccin/cotylenin and ophiobolane terpenes although the oxygenation pattern closely resembles the latter. Incorporation experiments with [2,2,2-2H3, 1-13C1]acetate revealed that traversianal arises by a sequence that differs substantially from that established for the fusicoccanes but rather resembles that previously shown for the ophiobolanes, in the retention of hydride at C-2, -10, and -14. However, the opposite configuration of the methyl group at C-3 suggests that the route to traversianal involves a terminal trans-geranylgeranyl unit instead of the cis unit implicated in ophiobolin generation.