Derailment of the Biosynthesis via an Acid-Mediated Intramolecular 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 Phomopsis 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 (13, 17, and 18, respectively), with diversified polycycles were obtained, among which compound 13 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 13 showed cytotoxicity by targeting F-actin.