New gas-phase domino processes leading to benzopyranones and benzofurans

A new domino approach to flavones by gas phase pyrolysis of β,γ-dioxophosphonium ylides containing a 2-methoxyphenyl group is frustrated by unexpected and novel decarbonylation of the intermediate flavon-3-yl radical leading to 2-phenylbenzofuran. Alternative approaches based on dioxolane protection of one carbonyl, or selective elimination in β,β'-dioxo or β-oxo-β'-thioxo ylides were not successful, but pyrolysis of a β-oxo-β'-phenylimino ylide did give the required domino reaction leading to a protected benzopyranone in moderate yield.