Pericyclic Reactions in Nature: Evidence for a Spontaneous -Hydrogen Shift and an 8?e Electrocyclic Ring Closure in the Biosynthesis of Olefinic Hydrocarbons from Marine Brown Algae (Phaeophyceae).

Article abstract of DOI:10.1016/S0040-4020(01)81756-7

The stereochemistry of the algal C11H16 hydrocarbon giffordene 3 results from a spontaneous <1.7>-sigmatropic hydrogen shift of the thermolabile (1,3Z,5Z,8Z)-undecatetraene 8.An 8?e electrocyclisation of (1,3Z,5Z,7E)-nonatetraene 9 is substantiated for the synthesis of 7-methylcyclooctatriene 5, a product of the Mediterranean brown alga Cutleria multifida.Low temperature syntheses (-30 deg C) of the thermolabile precursors 8 and 9 are described.The activation energies of the <1.7>-sigmatropic hydrogen shift 8 -> 3 (E_a = 67.4 kJ mol^-1) and of the 8?e electrocyclisation 9 -> 5 (E_a = 59.4 kJ mol^-1) are the lowest values currently known for natural pericyclic reactions.