Pericyclic reactions in nature: Synthesis and Cope rearrangement of thermolabile bis-alkenylcyclopropanes from female gametes of marine brown algae (Phaeophyceae)

Abstract

The biosyntheses of the 6-substituted cyclohepta-1,4-dienes dictyotene ( 1), ectocarpene ( 2), desmarestene ( 3), vinylcycloheptadiene ( 4) and lamoxirene ( 5) involve a spontaneous Cope rearrangement of thermolabile bis-alkenylcyclopropane precursors like 16, 10, 26 and 30. The unstable precursors and the rearranged cycloheptadienes were synthesised from the chiral 2-iodovinylcyclopropane 12 using Pd 0 or Cu I catalysed approaches. Activation parameters of the Cope rearrangements were determined. Bioassays with pre-ectocarpene 10 established the thermolabile cyclopropane (1R,2R)- 10 , rather than the cyclohepta-1,4-diene (6S)- 2 , as the genuine pheromone of the brown alga Ectocarpus siliculosus.