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Avermectins with a wide range of novel C-25 substituents have been prepared by feeding carboxylic acids or their biosynthetic precursors to a Streptomyces avermitilis mutant strain ATCC 53568. This organism lacks the ability to form isobutyric and S-2-methylbutyric acids from their 2-oxo acid precursors and thus is unable to produce natural avermectins unless supplied with these acids. The novel avermectins produced by mutational biosynthesis possess broad-spectrum antiparasitic activity.

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Various strains of D. concentrica have been studied in laboratory cultures and a series of aromatic metabolites has been isolated, including benzene derivatives with a C, or C, side-chain, such as the chromanone (V) (shown to be derived from acetate or a related compound), and mono-and di-methyl ethers of 1,8-dihydroxynaphthalene. Non-enzymic oxidation of 1,8-dihydroxynaphthalene has also been investigated. The combined evidence suggests that in the fungus the C , , benzene derivatives and 1,8-dihydroxynaphthalene are alternative metabolites of a common acetate-derived precursor, and that alternative metabolic fates of the 1,s-dihydroxynaphthalene are methylation and oxidation, by way of the binaphthyl (I) to polymeric quinones and the perylenequinone (II), as previously described.