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Abstract:
Several cyclic and alicyclic CI1 hydrocarbons have been shown to act as gamete releasing and/or attracting
pheromones during sexual reproduction of brown algae (Phaeophyceae). The same compounds are also found in
the essential oils of various plants, of which the occurrence of the cycloheptadiene-pheromone ectocarpene in
Senecio isatideus (Compositae) is noteworthy. Administration of [3H]dodeca-3,6,9-trienoiacc id to cuttings of this
plant leads to incorporation of radioactivity into ectocarpene. Double-bond-deuterated nona-3,6-dienoic acid is
converted to fucoserratene, the pheromone of several Fucales, which is certainly not present among the
hydrocarbons of Senecio. This proves that the pool of medium-chain, multiply unsaturated fatty acids includes
precursors of all types of highly unsaturated hydrocarbons. Appropriately labelled (deuterium markers) fatty acid
homologues were synthesized and applied to Senecio plantlets to unravel the mechanistic aspects. The results
strongly suggest radical initiation of the pheromone biosynthesis by abstraction of a single hydrogen from a
1 ,bpentadienyl segment of the fatty acid followed by oxidation to the corresponding cation. This causes
fragmentation of the reactive intermediate into an olefine and carbon dioxide by neighbouring-group participation
of the flanking double bonds. A tentative biosynthetic scheme is deduced from the experimental results which
also sets the stereochemistry of the algal pheromones into a uniform mechanistic concept.
