Rare Cladosporols and Spirobisnaphthalenes from the Chinese toad-derived Cladosporium cladosporioides F-10-2-A

Two novel cladosporol derivatives, cladosporols J−K (1−2), and three previously undescribed spirobisnaphthalenes, urnucratins D−F (3−5), as well as eleven known cladosporols (6−16), were characterized from Cladosporium cladosporioides, an endophytic fungus found on the skin of Chinese toad. Cladosporols J−K (1−2) with a single double bond have been rarely reported, while urnucratins D−F (3−5) featured an unusual benzoquinone bisnaphthospiroether skeleton, contributing to an expanding category of rare natural products. Their structures and absolute configurations were determined using extensive spectroscopic methods, including nuclear magnetic resonance (NMR), high-resolution electrospray ionization mass spectrometry (HRESIMS) analyses, as well as through electronic circular dichroism (ECD). Biological assays revealed that compounds 1 and 2 exhibited inhibitory activity against A549 cells, with half maximal inhibitory concentration (IC50) values of 28.22 μM and 32.77 μM, respectively.