2-Pyrone

• α -pyrone
• 2H-pyran- 2-one

Liquid

1.198 g · cm -3 ( 20 ° C)

5.0 ° C

209 ° C

1.5270 (25 ° C)

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2 -pyrone is adjacent to the 4 -pyrone of the simple 6-membered heterocycles containing an oxygen atom and two double bonds in the ring. In contrast to the pyrans, the two hydrogen atoms of the -CH2- group are additionally replaced by a further oxygen atom. This type of heterocycles is called pyrones. Simple natural derivatives of 2- pyrone with only one ring are unknown. Natural derivatives with two rings are coumarin (1,2- benzopyrone ) and the hydroxy coumarin, such as umbelliferone (7- hydroxycoumarin ), esculetin ( 6,7- dihydroxycoumarin ) and daphnetin ( 7,8- dihydroxycoumarin ).

Production and representation

2- Pyrone can of malic acid by the action of concentrated sulfuric acid by condensation, with elimination of carbon monoxide and carbon dioxide are shown with coumalic acid as an intermediate. Presumably this formylacetate is first formed with elimination of carbon monoxide. This is converted to the enol form under condensation and elimination of carbon dioxide over coumalic to 2 -pyrone. The equations for this synthesis are as follows:

With a modern synthesis, 2- pyrone are represented directly by a reaction of carbon dioxide with ethyne. By using a catalyst based on transition metals such as titanium, cobalt and palladium in a solvent by a [2 2 2 ] cycloaddition of 2 -pyrone formed. The reaction proceeds via a metal - cycloheptadiene complex. The metal is removed from this complex by reduction. The reaction proceeds according to the following equation:

Properties

2 -pyrone has the properties of a δ - lactone and reacting in a hydrogenation to a mixture of δ - valerolactone, and valeric acid. The two double bonds in the ring form a diene system and are suitable for Diels- Alder reactions. Stable resonance structures to form a Pyroxonium cation are not possible in contrast to the 4 -pyrone. Therefore, 2 -pyrone does not form oxonium salts and can not be substituted aromatic.

With the Gogte Synthesis hydroxy derivatives of 2 -pyrone can be alkylated. In the reaction of 4-methyl -6-hydroxy -2-pyrone with acetyl chloride is formed by elimination of hydrogen chloride and carbon dioxide by replacing the hydroxy- methyl group and 4,6-dimethyl -2-pyrone.