Craterstatins A-D: Terpenyl-p-aminobenzoic Acids Produced by an Australian Pasture Soil-Derived Streptomyces sp

An investigation into Australian pasture soil-derived microbes led to the prioritization of Streptomyces sp. S4S-00182A09 due to the media extract ability to inhibit the motility of dog heartworm Dirofilaria immitis microfilariae (Mf), and as a source of natural products with molecular formula unprecedented in the scientific literature. Fractionation of a scaled-up cultivation extract of S4S-00182A09 yielded the new terpenyl-p-aminobenzoic acids, craterstatins A-D (1-4), together with the known fatty acid, 2-hydroxy-12-methylmyristic acid (5). Structures were assigned to 1-5 on the basis of spectroscopic, ECD, and Marfey's analyses, as well as biosynthetic considerations and chemical transformation. Of note, craterstatin A (1) proved unstable to storage and handling, especially under acidic conditions, undergoing facile conversion to the stereoisomeric dehydration products craterstatins B (2) and C (3) that feature an unprecedented carbo-heterocyclic framework, with 3 undergoing quantitative conversion to 2 on storage. While the craterstatins 1-4 did not exhibit anthelmintic activity against D. immitis (EC50 > 100 μM), weak activity displayed by the hydroxylated fatty acid 5 prompted screening of a library of synthetic hydroxy fatty acids 6-11, which revealed modest inhibitory activity for (±)-3-hydroxyundecanoic acid (6), (±)-3-hydroxytetradecanoic acid (7), and (±)-3-hydroxypentadecanoic acid (8) (EC50 16.3, 28.2, and 19.0 μM, respectively).