Structure of Pyrones

2- and 4-Hydroxy-pyrylium salts are quite strongly acidic and are therefore much better known as their conjugate bases, the 2- and 4-pyrones. The simple 4-pyrones are qnite stable crystalline substances, whereas the 2-pyrones are mnch less stable 2-pyrone itself, which has the smell of fresh-mown hay, polymerises slowly on standing. There are relatively few simple pyrone natnral products, in great contrast with the widespread occnrrence and importance of their benzo derivatives, the coumarins and chromones (32.5.6), in nature. 

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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). 

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