A total synthesis of (±)-faranal, the true trail pheromone of Pharaoh's ant, Monomorium pharaonis

Abstract

A relatively short total synthesis of the true trail pheromone of Pharaoh's ant, (±)-faranal [(3SR,4RS)-(6E,10Z)-3,4,7,11-tetramethyltrideca-6,10-dienal (2)] is reported. The carbon skeleton was assembled by a Wittig condensation between (Z)-6-methyloct-5-en-2-one (4) and the 5-carboxypentylphosphonium salt (15). The ketone (4) was prepared in ‘one-pot’ and in a stereoselective manner using vinyl cuprate chemistry, while the relative stereochemistry of the two methyl substituents in the phosphonium salt (15) was established by using the Diels–Alder adduct (16) of buta-1,3-diene and maleic anhydride, which, after reduction and oxidative cleavage of the resulting cis-1,2-dimethylcyclohex-4-ene (18), gave only meso-3,4-dimethyladipic acid (19); this in turn was converted into the salt (15) in six straight-forward steps.