Pericyclic reactions in nature: Evidence for a spontaneous [1.7]-hydrogen shift and an 8πe electrocyclic ring closure in the biosynthesis of olefinic hydrocarbons from marine brown algae (phaeophyceae).

Abstract

The stereochemistry of the algal C 11H 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.