Herbidomicins, two pairs of polyketide tautomers produced by an actinomycete of the genus Herbidospora

Herbidospora is one of the underexplored actinomycete genera from which only a limited number of secondary metabolites are reported. In our continuing investigation on less explored actinomycetes, a liquid culture of Herbidospora sp. RD 11066 was found to contain unknown metabolites that had no match in our in-house UV database. Chromatographic separation and following structural analysis using NMR and MS identified these metabolites to be chromanone and chromene derivatives, which were respectively composed of an inseparable mixture of two isomeric forms. The former polyketides, designated to be herbidomicins A1 (1) and A2 (2), are positional isomers in terms of a methyl substituent on an aromatic ring that mutually interconvert by acetal exchange by two phenolic hydroxy groups. The latter pair, herbidomicins B1 (3) and B2 (4), is Z/E-isomers regarding an enol ether double bond. Herbidomicins 1–4 were weakly antifungal against a dermatophytic fungus Trichophyton rubrum and were moderately cytotoxic against murine leukemia P388 cells.