Sesquiterpenoids from Cis,Trans-Farnesyl Pyrophosphate with Initial Closure at the 10,11-Double Bond

When considering sesquiterpenoids, the number of possible reaction pathways increases significantly over those available in the monoterpenoid series. This results from the increased size of the precursor, farnesyl pyrophosphate, and the fact that it contains three double bonds. Both the 2,3- and 6,7-double bonds are initially formed in the trans configuration, but the 2,3-bond is part of an allylic carbocation system and therefore it can isomerise to the cis form. Initial ring closure can occur at either of the two remote double bonds and with the 2,3-bond in either of the cis or trans configuration. 

This carbocation can be trapped by the terminal olefin to give the acorane skeleton. Subsequent trapping of that carbocation by the remaining double bond leads to the cedrane skeleton. Three successive 

2-carbon shifts then give the framework of the khusane family of sesquiterpenoids. 

8 SESQUITERPENOIDS FROM CISJRANS-FARNESYL PYROPHOSPHATE WITH INITIAL CLOSURE AT THE 10,11-DOUBLE BOND 

The most chemically obvious cyclisation is that which forms the germacrane skeleton. However, this system also contains a strained 

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