Abstract
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 pi e electrocyclisation of (1,3Z,5Z,7E)-nonatetraene 9 is substantiated for the biosynthesis of 7-methylcyclooctatriene 5, a product of the Mediterranean brown alga Cutleria multifida. Low temperature syntheses (-30 degrees 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 pi e electrocyclisation 9-->5 (E(a) = 59.4 kJ mol(-1)) are the lowest values currently known for natural pericyclic reactions.
