Bridged Epipolythiodiketopiperazines from <i>Penicillium raciborskii</i>, an Endophytic Fungus of <i>Rhododendron tomentosum</i> Harmaja

Three new epithiodiketopiperazine natural products [outovirin A (<b>1</b>), outovirin B (<b>2</b>), and outovirin C (<b>3</b>)] resembling the antifungal natural product gliovirin have been identified in extracts of <i>Penicillium raciborskii</i>, an endophytic fungus isolated from <i>Rhododendron tomentosum</i>. The compounds are unusual for their class in that they possess sulfide bridges between α- and β-carbons rather than the typical α–α bridging. To our knowledge, outovirin A represents the first reported naturally produced epimonothio­diketopiperazine, and antifungal outovirin C is the first reported trisulfide gliovirin-like compound. This report describes the identification and structural elucidation of the compounds by LC-MS/MS and NMR