Pyrylium Salts, Pyrans and Pyrones

As oxygen is divalent, no strict equivalent of benzene exists, although the pyrylium cation does achieve aromaticity (see Chapter 1). Both 2/f-pyrans and 4/f-pyrans are known, but are encountered more frequently as their carbonyl analogues pyran-2-one and pyran-4-one (see Box 4.1). In addition, reduced forms such as 3,4-dihydro-2//-pyran and 3,4,5,6- 

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Pyrylium salts, pyrone derivatives, and their benzo derivatives show various interesting reactivities and appear in many natural products. Therefore, many organic chemists have an interest in the chemistry of pyrylium salts 1 and 2 and pyrones 3 and 4 (Scheme 1) (82AHC66 83AHC187). This review is a survey of the literature from 1980 onward, concentrating especially on nucleophilic reactions and carbocyclic annulation reactions that retain the resulting pyran ring. Related benzopyrone derivatives, coumarins (5) and chromones (6), are also included. 

The Interconversions of Pyrylium Salts, Pyrans, Pyrones and Their Open Chain Forms, Russ. Chem. Rev., 1985, 1167. 

Pyrans, pyrones and pyrylium salts and their benzologues 498... 

Pyrans, pyrones, pyrylium salts, and their benzologues 295... 

Pyrones also add Grignard nucleophiles at the carbonyl carbon, C-4 dehydration of the inunediate tertiary alcohol product with mineral acid provides an important route to 4-mono-substituted pyrylium salts." More vigorous conditions lead to the reaction of both 2- and 4-pyrones with two mole equivalents of organometallic reagent and the formation of 2,2-disubstituted-2H- and 4,4-disubstituted-4//-pyrans, respectively." Perhaps surprisingly, hydride (lithium aluminium hydride) addition to 4,6-dimethyl-2-pyrone takes place, in contrast, at C-b." ... 

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