Pyryliums from alkenes

Pyrylium-3-olates, formally 3-hydroxy-pyryliums rendered overall neutral by loss of the phenolic proton, though this is not usually the method for their formation, undergo cycloadditions across the 2,6-positions and in so doing parallel the reactivity of pyridinium-3-olates (8.7). Even unactivated alkenes will cycloadd when tethered and thus the process is intramolecular. Usually, the pyrylium-3-olate is generated by elimination of acetic acid from a 6-acetoxy-2//-pyran-3(67/)-one (a pyranulose acetate ) (see 18.2). 

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