Pyrylium salts in polyene natural product synthesis: Organometallic additions to 4-methylpyrylium tetrafluoroborate

The synthetic utility of organometallic additions to pyrylium salts as a procedure for the stereocontrolled preparation of 2Z,4E-dienals, and the use of these compounds in polyene natural product synthesis, is summarised. The extension of this methodology to 4-methylpyrylium tetrafluoroborate to give a new route to 3-methyl-2Z,4E-dienals via a six-carbon organometallic homologation procedure is then described. Finally, the utilisation of this methodology to provide a concise synthetic approach to 13Z-retinal, retinal and a range of retinal analogues is then elaborated.