Novel C 9 and C 11 Hydrocarbons from the Brown Alga Cutleria multifida ; Sigmatropic and Electrocyclic Reactions in Nature. Part VI

In addition to the known C 11 H 16 hydrocarbons multifidene ( 4 ), aucantene ( 2 ), and ectocarpene ( 5 ), the marine brown alga Cutleria multifida produces trace amounts of the C 9 H 12 hydrocarbon 7‐melhylcycloocta‐1,3,5‐triene ( 8 ) and its valence tautomer 7‐methylbicyclo[4.2.0]octa‐2,4‐diene, A second novel C 9 H 12 hydrocarbon is 6‐vinyicyclo‐hepta‐1,4‐diene ( 9 ), a lower homologue of ectocarpene ( 5 ). Among the C 11 H 16 hydrocarbons, 7‐((1 E / 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 ), (6 S )‐ectocarpene ((6 S )‐ 5 ), and (4 R ,5 R )‐aucantene ((4 R ,5 R )‐ 2 ). Consecutive reactions of certain thermolabile primary products proceed via electrocyclic ring closure, 3,3‐sigmatropic rearrangement, or a 1,7‐sigmatropic H‐shift.