Enzyme-catalyzed rearrangement of a diepoxy-germacrane compound into new 7-epi-eudesmane derivatives

Two new 7-epi-eudesmane derivatives, together with two new germacrane compounds, have been isolated from the microbial-transformation of a (1alpha,10beta),(4beta,5alpha)-diepoxygermacrane using the hydroxylating fungi Rhizopus nigricans. The rearranged skeleton and the stereochemistry of the chiral centers have been determined by means of their spectral data, and the absolute configuration has been confirmed by single-crystal X-ray analyses. A possible mechanism based on an enzyme-catalyzed isomerization to a 1alpha-hydroxy-(4beta,5alpha)-epoxygermacr-9(E)-ene intermediate and a subsequent cyclization process is proposed in order to explain the formation of the 7-epi-eudesmane compounds.