Diterpenoids from Streptomyces sp. SN194 and Their Antifungal Activity against Botrytis cinerea

Botrytis cinerea is a serious phytopathogen affecting a wide range of crops around the world. Many fungicides targeting Botrytis cinerea have failed due to the pathogen's genetic plasticity. In an effort to search for new fungicides from natural products, two new diterpenoids, named chloroxaloterpin A, 1, and B, 2, were isolated from culture broth of Streptomyces sp. SN194 along with four known diterpenoids, viguiepinol, 3, and oxaloterpins C-E, 4-6. Their structures were elucidated based on extensive MS, NMR, and X-ray crystallography analyses. Both the [(2-chlorophenyl)amino]carbonyl carbanic acyl group in 1 and the 2-[(2-chlorophenyl)amino]-2-oxo-acetyl group in 2 are discovered in natural products for the first time. All six compounds were tested against Botrytis cinerea, and chloroxaloterpin A, 1, and B, 2, demonstrated strong inhibitory activity against spore germination with EC₅₀ of 4.40 and 4.96 μg/mL, respectively.