Bisaspochalasins A-C: Three Cytochalasan Homodimers with Highly Fused Ring System from an Endophytic .

Three unprecedented cytochalasan homodimers, bisaspochalasins A-C (-), and two known monomers, aspochalasins B and D ( and ), were isolated from an endophytic . Bisaspochalasin A () contains a 13-hydroxy-3,24-dioxatricyclo[11.10.1.0]tetracos-4-one cross-linkage, representing an unprecedented carbon skeleton. Bisaspochalasins B () and C () share a thioether bridge, while has a peroxy modification at C-7, which may be generated by Schenck-ene photooxygenation. Their structures, including their absolute configurations, were elucidated by HRESIMS, NMR, chemical transformation, and X-ray crystallography. Bisaspochalasin A showed inhibitory activity against human T cell proliferation with an IC value of 15.8 μM while maintaining low cytotoxicity to T cells.