Derailmentof the Biosynthesis via an Acid-MediatedIntramolecular Cyclo-rearrangement Leads to a Novel Cytochalasin Skeleton

Cytochalasins are notable for their structural diversity and broad range of biological activities. The gene cluster responsible for the biosynthesis of cytochalasins bearing diverse polycycles was identified in <i>Phomopsis</i> sp. DHS-48. Characterization of the cluster indicated that only the 5/6/11-tricycle can be biosynthetically produced. The chemical space of cytochalasins was expanded by acid-mediated intramolecular cyclo-rearrangement of the 5/6/11-tricycle, and three new cytochalasins, phomoparagins D–F (<b>13</b>, <b>17</b>, and <b>18</b>, respectively), with diversified polycycles were obtained, among which compound <b>13</b> featured an unprecedented 5/6/5/7/6-pentacycle. Their structures and absolute configurations were established by spectroscopic analysis (1D, 2D NMR), electronic circular dichroism calculations, and a single-crystal X-ray diffraction experiment. The compounds inhibited the growth of HeLa and RKO cell lines with IC₅₀ values ranging from 0.8 to 47.3 μM. Cytoskeleton staining experiments and molecular docking models revealed that compound <b>13</b> showed cytotoxicity by targeting F-actin.