A general synthesis of (2Z)-terpenoic acids

A three-step synthesis of (2Z), (4E)-polyenic acids 6 is described. Condensation of aldehydes 1 with potassium prenal enolate led to dihydropyranols 3 which were oxidized into dihydropyrones 5, precursors of (2Z), (4E)-polyenic acids 6. The procedure was applied to a synthesis of 13-cis-retinoic acids from β-ionylidene acetaldehydes.