Stereochemical studies on algal pheromone biosynthesis. A model study with the flowering plant Senecio isatideus (Asteraceae)

Unsaturated C8 and C11 hydrocarbons act as chemical signals (chemotaxis) during sexual reproduction of many marine brown algae. One of these compounds, namely (+)-(6S)-6-(1Z-butenyl)cyclohepta-1,4-diene (= ectocarpene) is also formed as a major hydrocarbon by the flowering plant Senecio isatideus (Asteraceae). On administration of enantiospecifically labelled (8R)- or (8S)-[7,8-2H2]trideca-3,6,9-trienoic acid instead of the natural precursor dodeca-3,6,9-trienoic acid to this plant, the artificial C12 analogue of ectocarpene is formed. Mass spectroscopic analysis of the metabolites revealed this process to be enantiospecific for the C(8)-HRe atom of the precursor acid. The stereochemical course of the overall reaction is in agreement with a precise, U-shaped embedding of the trienoic acid into the active center of the enzyme(s) and Re-attack onto a hydrogen at the C(8) methylene group of this precursor. The reactive intermediate cyclizes by pi-orbital interaction between C(4) and C(6) of the acid and is accompanied by decarboxylation. The first product is a thermally unstable (1S,2R)-cis-1-(1E,3Z-hexadienyl)-2-vinylcyclopropane, which immediately rearranges to (+)-(6S)-ectocarpene (homo-Cope rearrangement). The present work provides first experimental evidence for a homo-Cope rearrangement as a naturally occurring electrocyclic reaction.