2-pyrone

Structural formula
General
Surname	2-pyrone
other names	α-pyrone 2 H -pyran-2-one
Molecular formula	C 5 H 4 O 2
External identifiers / databases
CAS number 504-31-4 EC number 207-990-5 ECHA InfoCard 100.007.264 PubChem 68154 ChemSpider 61462 Wikidata Q209475
properties
Molar mass	96.08 g mol −1
Physical state	liquid
density	1.198 g cm −3 (20 ° C )
Melting point	5.0 ° C
boiling point	209 ° C
Refractive index	1.5270 (25 ° C)
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

In addition to the 4-pyrone, 2-pyrone is one of the simple 6-membered heterocycles with one oxygen atom and two double bonds in the ring. In contrast to the pyrans , the two hydrogen atoms of the -CH ₂ group are also replaced by an additional oxygen atom. This type of heterocycle is called a pyron . Simple natural derivatives of 2-pyrone with only one ring are so far unknown. Natural derivatives with two rings are coumarin (1,2-benzopyron) and the hydroxy compounds of coumarin such as umbelliferone (7-hydroxycoumarin), esculetin (6,7-dihydroxycoumarin) and daphnetin (7,8-dihydroxycoumarin).

Extraction and presentation

2-pyrone can be prepared from malic acid by the action of concentrated sulfuric acid via condensation, splitting off of carbon monoxide and carbon dioxide with cumalic acid as an intermediate stage. Presumably, formyl acetic acid is formed first with the elimination of carbon monoxide . In the enol form , this is converted into 2-pyrone with condensation and splitting off of carbon dioxide via cumalic acid. The equations for this synthesis are as follows:

With a modern synthesis, 2-pyrone can be prepared directly by a reaction of carbon dioxide with ethyne . Using a catalyst based on transition metals such as titanium, cobalt or palladium, 2-pyrone is formed in a solvent by a [2 + 2 + 2] cycloaddition . The reaction takes place via a metal-cycloheptadiene complex. The metal is removed from this complex by reduction. The reaction proceeds according to the following equation:

${\displaystyle \mathrm {\ 2\ HC{\equiv C}H+\ CO_{2}\ {\xrightarrow {(ML_{n})}}\ C_{5}H_{4}O_{2}} }$

properties

2-pyrone has the properties of a δ-lactone and reacts when hydrogenated to form 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 . In contrast to 4-pyrone, stable resonance structures for the formation of a pyroxonium cation are not possible. 2-pyrone therefore does not form oxonium salts and cannot substitute aromatically.

Hydroxy derivatives of 2-pyrone can be alkylated with the Gogte synthesis. In the reaction of 4-methyl-6-hydroxy-2-pyrone with acetyl chloride , the hydroxyl group is exchanged for the methyl group with elimination of hydrogen chloride and carbon dioxide and 4,6-dimethyl-2-pyrone is formed.

Individual evidence

1. ↑ K. von Auwers, Liebigs Annalen der Chemie 422 ( 1921 ) 133-159.
2. ↑ ^{a b c d e} LF Fieser, M. Fieser; In: Textbook of organic chemistry , 3rd edition, Verlag Chemie, 1957, p. 943.
3. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-448.
4. ↑ Data sheet 2-Pyron at Sigma-Aldrich , accessed on November 4, 2016 ( PDF ).
5. ↑ LF Fieser, M. Fieser; In: Textbook of organic chemistry , 3rd edition, Verlag Chemie, 1957, p. 945.
6. ↑ Shibata, Tsuchikama: In: Org. Biomol. Chem. 2008, 6, 1317-1323.
7. ↑ LF Fieser, M. Fieser; In: Textbook of organic chemistry , 3rd edition, Verlag Chemie, 1957, p. 944.
8. ↑ CR Gogte, Proc. Indian Acad. Sci. 7A, ( 1938 ) 214.