Bipolaquinones A–J, Immunosuppressive Meroterpenoids from a Soil-Derived Bipolaris zeicola

Ten new meroterpenoids, bipolaquinones A–J (1–10), and one known congener, isocochlioquinone F (11), were isolated and identified from the fermented rice cultures of a soil-derived fungus, Bipolaris zeicola. The planar structures of 1–10 were elucidated based on extensive spectroscopic analyses (including HRESIMS and 1D and 2D NMR data), and their absolute configurations were determined by single-crystal X-ray diffraction analyses, comparison of experimental electronic circular dichroism (ECD) data, ECD calculations, and hydrolysis reaction. The immunosuppressive activity assay revealed that compounds 2, 3, and 7–10 showed significant inhibitory activity against concanavalin A (ConA)-induced T lymphocyte proliferation with IC₅₀ values ranging from 4.1 to 9.4 μM, which furnished potential lead molecules for the design and development of new immunosuppressants for treating autoimmune-associated diseases.