Terpene biosynthesis. Part II. Biosynthesis of thujane derivatives in Thuja, Tanacetum, and Juniperus species

Abstract

(–)-Thujone, (+)-isothujone, and (+)-sabinene were biosynthesised from [2-¹⁴C]mevalonic acid by certain Thuja, Tanacetum, and Juniperus species with from 90 to more than 99% of the incorporated tracer in that part of the skeleton derived from isopentenyl pyrophosphate. The part hypothetically derived from 3,3-dimethylallyl pyrophosphate was essentially unlabelled. The tracer pattern in (+)-sabinyl acetate was similar, but the acetate group contained ten times more tracer than the terpenoid portion, presumably as a result of degradation of mevalonic acid releasing tracer to the C₂-pool. These results are not consistent with the accepted view that isopentenyl and 3,3-dimethylallyl pyrophosphates, which condense to form monoterpenes, are both directly derived from mevalonic acid. They may be rationalised by: (a) the existence of a metabolic pool of isotopically normal 3,3-dimethylallyl pyrophosphate (or its structural equivalent) that reacted with labelled isopentenyl pyrophosphate of mevalonoid origin; (b) compartmentation effects; or (c) the formation of 3,3-dimethylallyl pyrophosphate from a source such as the monoterpene pool. The locations of tracer in all compounds were in accord with Ruzicka's hypothetical schemes for the biogenesis of bicyclic monoterpenes but were inconsistent with alternative mechanisms that have been proposed on the basis of incomplete degradations of thujone and α-pinene biosynthesised in Thuja and Pinus species.