Silphiperfol-5*ene

Another possible mechanism to account for the formation of 3 was suggested on the basis of the preferential coordination of the Lewis acid with the carbonyl oxygen.83,84 In practice, this method has been utilized efficiently as pivotal steps in the total synthesis of ( )-5-oxosilphiper-fol-6-ene,83 ( + )-silphiperfol-6-ene,83 and ( )-3-oxosi]phinene.84... 

The essential oil of Echinops giganteus var. lelyi, a culinary herb from Cameroon, contains many sesquiterpenoids. The most abundant (26.9% of the oil) is silphiperfol-6-ene, (P24.1). The biogenesis of this material from farnesyl pyrophosphate is enzyme mediated but the chemistry of the enzymatic reactions follows the basic principles of carbocation mechanisms (Figure P24). Initial cyclisation of farnesyl pyrophosphate gives the cation (P24.2). Suggest a sequence of cationic reactions which could lead from (P24.2) to (P24.1). This is a particularly difficult problem. If you would like a hint (without having to look at the solution) one will be found at the end of the problems section. 

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