4-Pyrones with hydroxylamine

Yates and his co-workers have studied the reactions of 2,6-dialkyl-4-pyrones with hydroxylamine. Thus, 2,6-dimethyl-4-pyrone and 2,6-diethyl-4-pyrone give the corresponding 2,6-dialkyl-4-hydroxylaminopyridine-l-oxides in 17 to 20% yields (VIII-3). 

Sosnovskikh VY, Mel nikov MY, Pogozhikh SA (1999) Reactions of 3,3-dialkyl-2,3-dihydro-6-trifluoromethyl-4-pyrones with hydroxylamine. Synthesis tmd structures of 5-( 1, l-ditdkvl-2-hydroxyethyl)-5-hydroxy-3-trilluoromethyl-AMsoxazoUnes. Russ Chem BuU 48 1323-1328... 

However, chromones react differently, because the phenolic hydroxy group in the ring-opened intermediate is unreactive. Thus, isoxazoles (264) result from the reaction with hydroxylamine, and a pyrazole is formed with hydrazine. y-Pyrones also give pyrazoles with hydrazine. 

Although the xanthones contain a ketone oxygen atom, they, like the pyrones (see p. 385), do not react with hydroxylamine, or phenyl hydrazine. 

The reactions of 4-pyrones with hydrazines and hydroxylamine, can lead to recyclisations involving the second heteroatom of the attacking nucleophile, producing pyrazoles and isoxazoles, respectively, however in the simplest examples 4-pyrones react with hydroxylamine, giving either l-hydroxy-4-pyridones or... 

Ciclopirox may be produced as follows 2 g of 4-methyl-6-cyclohexyl-2-pyrone were heated with 1 g of hydroxylamine hydrochloride and 5 g of 2-aminopyridine to B0°C for B hours. 

Hydroxylaminopyridine-l-oxide (IX-7) was obtained from the reaction of hydroxylamine with 7-pyrone. Its structure was proved by hydrogenation to 4-aminopyridine and also by oxidation to 4-nitropyridine-l-oxide."... 

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