Ring synthesis 2- benzopyrylium

The Cu-promoted enantioselective oxidative dearomatisation of aUcynylbenzaldehydes followed by a cycloisomerisation leads to azaphilones, fused 4//-pyrans (Scheme 1) <05JA9342>, while an alternative synthesis involves oxidation of a 1/7-benzopyrylium salt derived from a substituted benzaldehyde (Scheme 2) <05JOC4585>. Treatment of azaphilones with primary amines results in cleavage of the pyran ring and the formation of vinylogous y-pyridones. 

Controlled conditions are required for the production of simple adducts, for under more vigorous alkaline treatment, ring opening, then carbon-carbon bond cleavage via a retro-aldol mechanism takes place, and such processes, which are essentially the reverse of a route used for the synthesis of 1-benzopyryliums... 

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