Absolute Stereostructures and Biogenetic Relationshipsof Phomopsols A and B, Including the First Axially Chiral Polyketide-DerivedAlkaloid, from the Mangrove Endophytic Fungus Phomopsis sp. xy21

A polyketide-derived alkaloid featuring a rotationally hindered C-9–C-9a axial bond and a unique 3,4-dihydro-2H-indeno[1,2-b]pyridine-1-oxide motif, named phomopsol A (1), and a highly oxidized polyketide containing a new 3,5-dihydro-2H-2,5-methanobenzo[e][1,4]dioxepine moiety, named phomopsol B (2), were isolated from the Thai mangrove endophytic fungus Phomopsis sp. xy21, together with the known compound 3. The absolute stereostructures of 1–3 were unambiguously established by single-crystal X-ray diffraction analysis (Cu Kα). Biosynthetic origins of 1–3 are proposed.