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 b y means of the asymmetric Sharpless epoxidation, performed as a kineti c resolution. H-1 NMR experiments with Eu(hFc)(3) demonstrate that the recovered 2 is racemic. Consequently, 2 and mast likely also germacre ne B (1) are not enantiomerically stable at room temperature. The form ation of the optically active cis-fused guaiane 4 of high ee with limi ted Sharpless reagent shows that the asymmetric epoxidation of a proce eds highly enantioselectively. The Sharpless epoxidation methodology a pplied on the (E,Z)-germacrane 3 results in the formation of the stabl e optically active epoxide 5. Acid-induced cyclization of 5, leading t o a mixture of guaianes, probably proceeds via the trans-fused carboca tionic intermediate A.