6- Hydroxy-2-pyrone, tautomerism

Hydroxy pyridine 1-oxides are also tautomeric the 4-isomer exists in about equal amounts of forms (783) and (784). 4-Hydroxy-pyrones and -pyridones exhibit a different type of tautomerism the a-one (e.g. 785) structure is favored relative to the y-one structure. (3-Hydroxy-4-pyrones such as kojic acid (786) show phenolic properties (CHEC 3.28, Scheme 30). a- and 3-Hydroxy cations (e.g. 787) are the conjugate acids of pyridones and pyrones and are considered in the next section. 

UV spectral data have been instrumental in determining structures of pyrones related to 1, which can exist in two different tautomeric forms (such as la and lb), rapidly interconverting in solution. In general, alkylations of these types of hydroxy pyrones occur at C4, but treatment of 1 with diazomethane affords both 21a and the 2-methoxy-4-pyrone 21b (60JCS502). 

Diacetyl-substituted pyran-2,4,6-trione 220 exists in the 2-pyrone tautomeric form shown in the solid state. This tautomer is also the predominant form in solution however, the symmetrical 2,6-dioxo-4-hydroxy tautomer was also detected. Mono-Schiff... 

Bu Lock, J.D., and H.G. Smith Pyrones I. Methyl Ethers of Tautomeric Hydroxy-pyrones and the Structure of Yangonin. J. Chem. Soc. (London) 1960, 502. 

The utility of cyclopentadienes as reactive dienophiles continues to be exploited. Glutaconic anhydride has been shown to exist largely as the anhydride rather than the tautomeric hydroxy-pyrone in non-polar solvents, and as such reacts smoothly with cyclopentadiene to give the useful prostanoid precursor... 

In this section the synthesis of coumalic acid (92), representing the a(2)-pyrone system, is described. a-Pyrones readily undergo a simple lactone-amide transformation, and the conversion of coumalic acid into the tautomeric 6-hydroxy-nicotinic acid (93) provides a further illustration of the conversion of a six-membered oxygen heterocycle into the corresponding nitrogen system. 

Tautomeric with the 2-hydroxy-4-pyrone struct. Isol. from the gametophytes of Equisetum arvense. 

Although ketones, including ethyl acetoacetate and several related compounds, react with malonyl chloride to form chloropyranodioxins (tMs section, p. 661), 1,3-diketones give pyrones. It was proposed originally that benzoylacetone forms the 4-hydroxy-2-pyrone that tautomerizes to the 2-hydroxy-4-pyrone... 

---