For example, the mechanism
of 5-epi-aristolochene synthase proceeds via an initial 1,10-
cyclization forming the (E,E)-germacradienyl cation to germacrene
A. This neutral olefin is then protonated on the C6–C7 double bond
in Markovnikov orientation to reform a cation, which then undergoes
2,7-cyclization to form a eudesmane cation