About the Chiral Stability of Germacrene B and the Biomimetic Synthesis of Guaiane Sesquiterpenes.

The planar chirality of 15-hydroxygermacrene B (2) has been examined by means of the asymmetric Sharpless epoxidation, performed as a kinetic resolution. (1)H NMR experiments with Eu(hfc)(3) demonstrate that the recovered 2 is racemic. Consequently, 2 and most likely also germacrene B (1) are not enantiomerically stable at room temperature. The formation of the optically active cis-fused guaiane 4 of high ee with limited Sharpless reagent shows that the asymmetric epoxidation of 2 proceeds highly enantioselectively. The Sharpless epoxidation methodology applied on the (E,Z)-germacrane 3 results in the formation of the stable optically active epoxide 5. Acid-induced cyclization of 5, leading to a mixture of guaianes, probably proceeds via the trans-fused carbocationic intermediate A.