Enaminoketones protonation

Isoxazoles are susceptible to attack by nucleophiles, the reactions involving displacement of a substituent, addition to the ring, or proton abstraction with subsequent ring-opening. Isoxazolium salts are even more susceptible to attack by a variety of nucleophiles, providing useful applications of the isoxazole nucleus in organic synthesis. Especially useful is the reductive cleavage of isoxazoles, which may be considered as masked 1,3-dicarbonyl compounds or enaminoketones. 

Finally, we note that conjugated enaminoketones ( enaminones ) are virtually always protonated on oxygen59. Reaction 12, observed both by UV60 and NMR spectroscopy61, is an example. It is possible, however, for the preferred site of protonation to be carbon, as reaction 13 illustrates62. 

The structure of compounds 218 as (Z)-enaminoketones is confirmed by the presence in the IR spectra of absorption bands of the amide (1710cm ) and endocyclic carbonyl groups (1650 cm ), as well as by a characteristic chemical shift of the methine proton (S 9.2 ppm) in the H NMR spectrum. 

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