Pyrylium salt
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A pyrylium compound is a conjugated 6 membered carbon ring system with one carbon atom replaced by a positively charged oxygen atom forming a salt with a negatively charged counterion [1]. It is the oxygen pendant of benzene and shares with it aromatic properties. The carbon to oxygen double bond is also an oxonium ion which due to aromatic stabilization is much less reactive than ordinary oxonium ions. However, the parent compound Pyrylium is unstable and not known. Pyrylium cations react with nucleophiles in the 2,4 and 6 positions and may result in ring-opening reactions.
Benzo-fused pyrylium compounds are also called flavylium compounds of which the pigment anthocyanin is an important representative.
A pyrylium cation with a hydroxyl anion substituent in the 2-position is not the zwitterionic aromat 1 but a neutral unsaturated lactone or a α-pyrone (2H-pyran-2-one) (2). Important representatives of this class are the coumarins.
Likewise a 4-hydroxyl pyrylium compound is a φ-pyrone or 2H-pyran-4-one (4) to which group belongs a compound such like maltol.
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[edit] 2-pyrones
2-Pyrones are known to react with alkynes in a Diels-Alder reaction to form arene compounds with expulsion of carbon dioxide, for example [2]:
[edit] See also
- 6-membered aromatic rings with one carbon replaced by another group: borabenzene,benzene, silabenzene, germanabenzene,stannabenzene, pyridine, phosphorine, pyrylium salt
- pyrans are pyrones lacking the ketone group.
[edit] External links
- 2,6-Di-tert-butyl-4-methylpyrylium trifluoromethanesulfonate Organic Syntheses, Coll. Vol. 7, p.144 (1990); Vol. 60, p.34 (1981). Article
[edit] References
- ^ Heterocyclic Chemistry T. L. Gilchrist ISBN 0-582-27843-0
- ^ An alkynylboronic ester cycloaddition route to functionalised aromatic boronic esters Patrick M. Delaney, Jane E. Moore and Joseph P. A. Harrity Chem. Commun., 2006, 3323 - 3325, DOI:10.1039/b607322k
