The coriolins (I, II, III) are structurally unique tricyclicsesquiterpene antibiotics which possess useful biological properties including antibacterial and anti-tumor activity.' The coriolins bear a formal structural relationship to another recently described sesquiterpene fungal metabolite, hirsutic acid C( V),' the structure of which was rigorously established by X-ray crystallography.
The biosynthesis of hirsutic acid was recently reported.3
The most reasonable biosynthetic pathway to the coriolins begins with the cyclization of all trans-farnesyl pyrophosphate to humulene (VI ).' From humulene to the tricyclic coriolin metabolites three possible cyclisation routes, which differ only in type and degree of concertedness of the cycliaation, could be considered. Each of the proposed pathways, 2, _b, and 2, involves the initial protonation of humulene at C-10 followed by formation of the C-2 and C-3 bond. The resulting C-3 cationic species could then react further by one of these pathways to give after a series of Wagner-Meerwein shifts the coriolin ring system.
The correct pathway can be determined without recourse to elaborate degradative experiments from the 13C nmr spectra of coriolin biosynthesised from 1,2-r3C-acetate as a precursor.
Coriolus consors (ATCC 11574) was fermented according to Umezawa's method.* The "C--enriched 5-dihydrocoriolin C (IV) was isolated as the triacetate VII which is more stable and soluble than, and more easily separated from, other co-metabolites.6
In the FT-'% nmr spectrum of the labeled coriolin derivative 20 of the carbon signals appeared with characteristic satellites caused by r3C-'aC spin-spin coupling. The assignment of all the carbon chemical shifts and the 10 'sC-r3C couplings was made with the aid of the off-resonance decoupled spectrum of the unlabeled metabolite which yielded the individual