Structure and Absolute Configuration of Kongiidiazadione, a New Phytotoxic 3-Substituted-5-Diazenylcyclopentendione Produced by Diaporthe Kongii

A new 3-substituted-5-diazenylcyclopentendione named kongiidiazadione was isolated from culture filtrates of Diaporthe kongii, associated with stem cankers on sunflower in Australia. Kongiidiazadione was characterized by spectroscopic (essentially nuclear magnetic resonance [NMR] and high-resolution, electrospray ionization, mass spectrometry [HRESIMS]) methods as (-)-5-diazenyl-3-hydroxymethyl-cyclopent-3-en-1,2-dione. The stereochemistry of the diazenyl group was determined by IR spectroscopy, while the (R) absolute configuration at C(5) was assigned by computational analysis of its electronic circular dichroism (ECD) spectrum. When assayed on leaf disks of different plant species at 5 mM, the kongiidiazadione had a differential impact, causing clear necrosis, in particular to Helianthus annuus. Moreover, kongiidiazadione proved to have a weak antibacterial activity against gram-positive Bacillus amyloliquefaciens.