Biosynthesis of monoterpenes: Conversion of the acyclic precursors geranyl pyrophosphate and neryl pyrophosphate to the rearranged monoterpenes fenchol and fenchone by a soluble enzyme preparation from fennel ( Foeniculum vulgare)

Abstract

(+)-(1 S)-Fenchone, an irregular bicyclic monoterpene ketone thought to be derived via rearrangement of a bicyclic precursor, is one of the major terpenoids of the volatile oil of fennel ( Foeniculum vulgare M.). A soluble enzyme preparation obtained from homogenates of young fennel leaves converted the acyclic precursors geranyl pyrophosphate and neryl pyrophosphate to fenchol in the presence of Mn 2+, and in the presence of pyridine nucleotide, dehydrogenated the fenchol formed to fenchone. Particulate enzyme fractions were inactive in the synthesis of fenchane-type monoterpenes. The identities of the biosynthetic products as fenchol and fenchone were confirmed by radiochromatographic analysis and by the synthesis of crystalline derivatives. Differential crystallization of the fenchyl hydrogen phthalates and the fenchyl p-nitrobenzoates demonstrated that the initial cyclic product was endo-[ 3H]fenchol and not the exo-epimer, while resolution of [ 3H]fenchone oxime via the diastereomeric l-menthyloxymethyl ethers confirmed the stereochemistry of the labeled cyclic products as (−)-(1 S)- endo-fenchol and (+)-(1 S)-fenchone. Biosynthetic l-endo-[ 3H]fenchol derived from the [1- 3H]-labeled acyclic precursors was chemically converted to (+)-(1 R)-α-fenchocamphorone, and exchange of the α-protons of this derivative established that 3H was located specifically on the bridge carbon (C-7) of the original fenchane nucleus. Thus, the bicyclic fenchane skeleton was shown to be derived by rearrangement of a pinane-type intermediate. These results provide strong evidence that fenchone is derived by the cyclization of geranyl pyrophosphate or neryl pyrophosphate to endo-fenchol followed by dehydrogenation of this bicyclic alcohol, and they constitute the first demonstration of the biosynthesis of a rearranged monoterpene in a cell-free system.