Examination of Potential Exceptions to the F and S Biosynthetic Classification of Fused-Ring Aromatic Polyketides

The reported acetate-derived labelling of the fungal naphthalene gamma-pyrone fonsecin, two streptomycete dodecaketide alpha-pyrones TW93f and TW93g, and the streptomycete phenanthraquinones piloquinone, murayaquinone and haloquinone appear to be exceptions to the generalisation that fungi and streptomycetes produce fused-ring aromatic polyketides by different modes of cyclisation. A review of their 1) originally assigned formulae, 2) [C-13(2)] acetate-derived labelling patterns, and 3) modes of cyclisation leads to the recognition of feasible alternative chemical structures or biosynthetic pathways, which are in accord with the originally proposed classification system.