Abstract
Abstract The stereochemistry of the algal C 11 H 16 hydrocarbon giffordene 3 results from a spontaneous [1.7]-sigmatropic hydrogen shift of the thermolabile (1,3 Z ,5 Z ,8 Z )-undecatetraene 8 . An 8πe electrocyclisation of (1,3 Z ,5 Z ,-7 E )-nonatetraene 9 is substantiated for the biosynthesis of 7-methylcyclooctatriene 5 , a product of the Mediterranean brown alga Cutleria multifida . Low temperature syntheses (−30 °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 8πe electrocyclisation 9 → 5 ( E a = 59.4 kJ mol −1 ) are the lowest values currently known for natural pericyclic reactions.
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