Total Synthesis of the Meroterpenoid Manginoid A as Fueled by a Challenging Pinacol Coupling and Bicycle‐forming Etherification

Terpene-Shikimate conjugated meroterpenoids from the endophytic fungus Guignardia mangiferae

Nine undescribed shikimate-conjugated meroterpenes, as well as nine known compounds, were isolated from solid cultures of the fungus Guignardia mangiferae, an endophyte obtained from the leaves of Dendrobium nobile. The structures of these undescribed compounds were characterized by analyses of their 1D and 2D NMR and HRESIMS data, and their absolute configurations were assigned by single-crystal X-ray crystallography, electronic circular dichroism (ECD) calculations, modified Mosher's method, and Mo₂(OCOCH₃)₄-induced ECD experiments. Of these compounds, mangnardone A represents the first example of terpene-shikimate-conjugated meroterpenoid with a hydroxy group at C-5. In addition, the X-ray diffraction analysis of mangnardone I is the first example to confirm the structure of bicycloalternarene (BCA) meroterpenoid by single-crystal data. Nine undescribed meroterpenes inhibited nitric oxide (NO) production in LPS-induced RAW 264.7 cells with IC₅₀ values in the range of 4.7-40.0 μM.

Enantioselective Total Syntheses of Manginoids A and C and Guignardones A and C

An enantioselective synthetic approach for preparing manginoids and guignardones, two types of biogenetically related meroterpenoids, is reported. This bioinspired and divergent synthesis employs an oxidative 1,3-dicarbonyl radical-initiated cyclization and cyclodehydration of the common precursor to forge the central ring of the manginoids and guignardones, respectively, at a late stage. Key synthetic steps include silica-gel-promoted semipinacol rearrangement to form the 6-oxabicyclo[3.2.1]octane skeleton and the Suzuki-Miyaura reaction of vinyl bromide to achieve fragment coupling. This synthesis protocol enables the asymmetric syntheses of four fungal meroterpenoids from commercially available materials.