FIGURE 5 Biosynthesis of C₁₁ hydrocarbons in higher and lower plants. The similar pattern of functionalization of dodeca-3,6,9-trienoic acid and 9-HPEPE is shown.

removal of the C(8) H_R as a radical by the active center of the enzyme terminates the sequence, and the disubstituted cyclopropane (1S,2R) is released as the first product. The compound is thermolabile and rearranges via a cis-endo transition state to (6S)-ectocarpene.

In contrast to the terrestrial plant S. isatideus, female gametes of the marine brown algae (model system: female gametes of E. siliculosus) do not utilize dodeca-3,6,9-trienoic acid for production of the C₁₁ hydrocarbons. Instead, the marine plants exploit the pool of unsaturated C₂₀ acids (20:4 → 20:6) for the production of their pheromones. [²H₈]Arachidonic acid is very effectively transformed into 6-[²H₄]butylcyclohepta-1,4-diene (dictyotene) by a suspension of female gametes of E. siliculosus. A synthetic sample of [²H₆]nonadeca-8-11,14,17-tetraenoic acid, which can be thought of as a 20-noranalogue of icosa-5,8,11,14,17-pentaenoic acid, gives the corresponding norectocarpene, together with a labeled norfinavarrene, in high yield (39, 40). Since the icosanoids are not cleaved to unsaturated C₁₂ precursors, their primary functionalization is assumed to be achieved by a 9-lipoxygenase yielding 9-hydroperoxyicosa(5Z,7E,11Z,14Z,17Z)-pentaenoic acid (9-HPEPE), which mimics the functionalization pattern of dodeca-3,6,9-trienoic acid (Figure 5). Assuming a homolytic cleavage of the hydroperoxide (41) as shown in Figure 6, once again, the disubstituted cyclopropane will be released from the active center as a thermolabile intermediate. Although this mechanistic hypothesis has not yet been experimentally confirmed, the concept nevertheless provides a valuable platform for the systematic derivation of all the known C₁₁ hydrocarbons from the fatty acid precursors. It is also conceivable that the oxidative decarboxylation/cyclization of dodeca-3,6,9-trienoic acid in higher plants could proceed via a peroxy acid.

The unusual stereochemistry of the acyclic undeca-(2Z,4Z,6E,8Z)tetraene (giffordene; cf. Figure 1), the major volatile from the brown alga Giffordia mitchellae, follows from the same concept (40, 42). If the 9-HPEPE