Novel C9 and C11 Hydrocarbons from the Brown Alga Cutleria multifida; Sigmatropic and Electrocyclic Reactions in Nature. Part VI

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Abstract
In addition to the known C11H16 hydrocarbons multifidene (4), aucantene (2), and ectocarpene (5), the marine brown alga Cutleria multifida produces trace amounts of the C9H12 hydrocarbon 7‐melhylcycloocta‐1,3,5‐triene (8) and its valence tautomer 7‐methylbicyclo[4.2.0]octa‐2,4‐diene, A second novel C9H12 hydrocarbon is 6‐vinyicyclo‐hepta‐1,4‐diene (9), a lower homologue of ectocarpene (5). Among the C11H16 hydrocarbons, 7‐((1E/Z)‐prop‐l‐enyl)cycloocta‐1,4‐diene (10/11) is found for the first time. The structure of all new products is confirmed by synthesis and spectroscopic data. The biosynthesis of the new hydrocarbons 8–11 is obviously linked to the pathways which lead to the major products giffordene (7), (6S)‐ectocarpene ((6S)‐5), and (4R,5R)‐aucantene ((4R,5R)‐2). Consecutive reactions of certain thermolabile primary products proceed via electrocyclic ring closure, 3,3‐sigmatropic rearrangement, or a 1,7‐sigmatropic H‐shift.