Stereoselective synthesis of 2Z, 4E-dienals by addition of organometallic reagents to pyrylium perchlorate

A facile and stereoselective approach to 2Z,4E-dienals and derived products is presented. The method involves addition of an organometallic reagent to a pyrylium salt, followed by electrocyclic ring-opening of the intermediate 2H-pyran, to give the desired Z,E-dienal, usually with greater than 95% stereochemical purity. These dienals may be trapped in situ with a second organometallic reagent, or oxidised or reduced, without loss of stereochemical integrity. The synthesis of (1,3Z,5E)-undeca-1,3,5-triene, a component of the essential oils of the brown Hawaiian seaweeds Dictyopteris plagiogramma and D. australis, is reported to illustrate the simplicity of the procedure.