Based on the structural similarities of the recently isolated eremophilane-type sesquiterpenoids microsphaeropsisin B and C and the iso-eremophilane periconianone C, a revised biogenetic hypothesis for C8-C11-connected iso-eremophilanes is presented and corroborated by strong experimental evidence. The first enantioselective total syntheses of microsphaeropsisin B and C were achieved starting from a known intermediate, whose synthesis was elaborated previously in the total synthesis of periconianone A, and in a total of 15 steps starting from γ-hydroxy carvone. Mild reaction conditions for the subsequent α-ketol rearrangement not only resulted in the herein proposed conversion of microsphaeropsisin B into periconianone C, but also in the conversion of microsphaeropsisin C into 4-epi-periconianone C.
Keywords: biosynthesis; natural products; rearrangements; terpenoids; total synthesis.
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