Investigations of Bicyclo(3.1.1)heptane Terpenes in Higher Plants: Stereochemistry of Pinene Biosynthesis and Isolation of Bergamoten-12-Oic Acids
Denissen, Jon Francis
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https://hdl.handle.net/2142/70384
Description
Title
Investigations of Bicyclo(3.1.1)heptane Terpenes in Higher Plants: Stereochemistry of Pinene Biosynthesis and Isolation of Bergamoten-12-Oic Acids
Author(s)
Denissen, Jon Francis
Issue Date
1987
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Abstract
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.
The sesquiterpenes (+)-(E)-endo-$\beta$-bergamoten-12-oic acid (1), ($-$)-(E)-endo-$\alpha$-bergamoten-12-oic acid (2), and (+)-(E)-$\alpha$-santalen-12-oic acid (3) were identified as major components of whole leaf hexane extracts of wild tomato (Lycopersicon hirsutum) accession LA 1777. Structural assignments of the corresponding methyl esters of these acids were initially based on $\sp1$H and $\sp $C NMR spectral data and were subsequently confirmed by chemical correlation with authentic samples. Acids 1 and 2 were converted to the respective bergamotene hydrocarbons via reduction to the corresponding alcohols, conversion to the chlorides and reductive elimination with Super-Hydride. The methyl ester of 3 was identical to ester derived from oxidation of commercial santalol.
Phytochemicals 1 and 2, not previously identified as natural products, and 3 have been implicated as important oviposition attractants of Heliothis zea moths. Furthermore, the endo configuration of acids 1 and 2, if these sesquiterpenes arise by cyclization of (E, E)-farnesyl pyrophosphate, implies a maximum-motion biosynthetic mechanism that is apparently in contrast to that identified in pinene biosynthesis.
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