Isolation and Structure Elucidation of Fujikurins A–D: Products of the PKS19 Gene Cluster in <i>Fusarium fujikuroi</i>

<i>Fusarium fujikuroi</i> is a member of the <i>Gibberella fujikuroi</i> species complex and well known for the production of gibberellins and mycotoxins including fusarins and fusaric acid. A recent genome sequencing study revealed that the fungus has the genetic potential to produce many more secondary metabolites than have been reported. This paper describes the structure elucidation of the products of the cryptic and silent PKS19 gene cluster that were recently identified (fujikurins A–D). We present the complete NMR data for the structure elucidation of the main compound fujikurin D, which shows tautomeric 1,3-diketo elements. The different tautomeric structures could be confirmed using quantum chemical calculations. Additionally, the structures of the minor compounds fujikurins A–C were elucidated by high-resolution mass spectrometric fragmentation experiments. It emerged that fujikurin A was identical to the bioactive compound CR377 of the taxonomically unclassified <i>Fusarium</i> strain CR377, while fujikurins B–D have not been reported from other fungi