Investigations of Bicyclo(3.1.1)heptane Terpenes in Higher Plants: Stereochemistry of Pinene Biosynthesis and Isolation of Bergamoten-12-Oic Acids
Cell-free enzyme preparations of (+)-pinene cyclase I and ($-$)-pinene cyclase II isolated from leaves of common sage (Salvia officinalis) are known to convert geranyl pyrophosphate (GPP) to the bicyclic monoterpenes (+)-$\alpha$-pinene (cyclase I) and ($-$)-$\alpha$-pinene and ($-$)-$\beta$-pinene (cyclase II). The stereochemical course of these cyclizations at the gem dimethyl position of the pinene products was investigated. (6E)- (8-$\sp3$H) GPP was synthesized in ten steps from geraniol and incubated with each cyclase to afford (+)- ($\sp3$H) -$\alpha$-pinene, ($-$)- ($\sp3$H) -$\alpha$-pinene, and ($-$)- ($\sp3$H) $\beta$-pinene. A 10-step reaction sequence, which involved oxidative conversion of the endo methyl group of each pinene to the carbonyl carbon of 2-hydroxy-2,6-dimethylbicyclo (3.1.1) heptane-6-carboxylic acid lactone, was used to determine the stereochemical fate of the distal methyl groups of GPP. Complete retention of tritium in all three pinene products established that the E methyl group of GPP became the exo methyl group at the gem dimethyl position of each pinene. It is concluded that the pinenes are formed via enantiomeric, least motion cyclizations of boat-like $\alpha$-terpinyl carbocation intermediates.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign