Pimarenyl cation

Abietic acid, a common component of conifer rosin, is formed from (-)-abieta-7(8),13(14)-diene ((-)-abietadiene) (Scheme 18 A) [144]. The cyclization of GGPP to abietadiene requires several discrete steps and enzyme-bound intermediates, yet is catalyzed by a single protein, abietadiene synthase. The mechanism of this enzyme involves protonation-induced cyclization of GGPP to enzyme-bound (-i-)-CPP, followed by typical ionization of (-i-)-CPP and cyclization to the transient olefin intermediate, pimaradiene, which undergoes reprotonation at the vinyl substituent, methyl migration and deprotonation to (-)-abietadiene [145]. Similarly, the route to (-)-kaurene involves protonation-initiated cyclization of GGPP to yield (-)-CPP, followed by ionization-initiated cyclization of (-)-CPP to a pimarenyl cation, methyl migration and deprotonation to (-)-kaurene (Scheme 18 B) [20]. 

The requisite pimarenyl cation might also be formed more directly from (245) (scheme 24) via a 14- 8 hydride shift. 

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