Structure and Biosynthesis of the Diterpene Hydrocarbon, Casbene
The stereochemical course of the enzyme catalyzed cyclization of geranylgeranyl pyrophosphate (GGOPP) to casbene, (1S,14R)-(E,E,E)-3,7,11,15,15-pentamethylbicyclo{14.1.0}pentadeca-2,6,10-triene, was defined in the castor bean plant (Ricinus communis L.,) by resolving the four stereochemical ambiguities in the cyclization. The stereochemical course of the cyclization is defined as follows: (1) the re face of the C-14 carbon of GGOPP is attacked by the C-1 carbon, (2) the E geometry of the double bonds of GGOPP is retained in casbene, (3) the pro-S hydrogen is removed from the C-1 carbon of GGOPP, and (4) the configuration of the 14,15-double bond of GGOPP is retained in the cyclopropane ring of casbene.
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