The difference between compounds 2 and 1 was the
configuration of the double bond C-19/20. The concurrence
of those geometry isomers might be involved in an elimination
reaction in the biosynthesis of geissoschizine to form a
doule bond, which affords both 19-E and 19-Z types.
Nevertheless, the 19-E type is dominant in all monoterpenoid
indole alkaloids because of its stability. Therefore, 2 should
be derived from an analogic pathway from Z-type geissoschizine,
although it has been isolated now. To our knowledge,
there are four more pairs of stereoisomeric monoterpenoid
indole alkaloids with an ethylidene group (E or Z) at
the 19-position, that is, 19-E/Z-16-epiisositsirikine,17 19-E/
Z-isositsirikine,17,18 19-E/Z-vallesamine,19 and 19-E/Z-picralinal.
20