Geranyl diphosphate allylic cation

An exactly analogous process can then occur, in which geranyl diphosphate provides the allylic cation, and a further molecule of isopentenyl diphosphate adds on, giving farnesyl diphosphate this can subsequently yield geranylgeranyl diphosphate. 

The compounds geranyl diphosphate, farnesyl diphosphate, and geranylgeranyl diphosphate are biochemical precursors of monoterpenes, sesquiterpenes, and diterpenes respectively, and virtually all subsequent modifications of these precursors involve initial formation of an allylic cation through loss of diphosphate as the leaving group. 

Before cyclization can occur, however, there has to be a change in stereochemistry at the 2,3-double bond, from E in geranyl diphosphate to Z, as in neryl diphosphate. It should be reasonably clear that geranyl diphosphate cannot possibly cyclize to a six-membered ring, since the carbon atoms that need to bond are not close enough to each other. The change in stereochemistry is achieved through allylic cations and linalyl diphosphate (see Box 6.4). 

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