Enaminoketones

As masked dicarbonyl compounds, enaminoketones (158) or vinylamidinium salts (159 160) can be employed (Scheme 84) (79S385). Similarly, 3-phenacylpyridinium methiodides... 

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. 

The IR spectra of several enaminoketones have been reported, and a study of these spectra has shown strong coupling between the C=0 and C=C stretching vibrations and band splitting due to rotational isomerism (7SciJJb). 

Dimethyl acetylenedicarboxylate (80) undergoes initial 1,2 cycloaddition with acyclic enamines to form cyclobutene intermediates which immediately decompose into acyclic dienaminodiesters (94,95). When an acyclic n/c-enediamine is used instead of a simple acyclic enamine, a dienediamino-diester is produced via a cyclobutene intermediate (95a). A cyclization reaction of dimethyl acetylenedicarboxylate with an acyclic enaminoketone... 

Enaminoketones undergo 1,4 cycloadditions with sulfene (162a). This is illustrated by the reaction of enamine 120 with sulfene to form sulfone 121 in an 80 % yield (162,163). 

The potential 2-hydroxy-3-aminofurans exist as enaminoketones 177 (89T6631). 

In an alkaline medium the condensation of carbonyl and amino groups of the reactants seems to be more probable (pathway 1), although pathway 2, which is identical to the reactions of enaminoketones, is also possible. 

The formation of a stable imino-enole tautomer 358 is due to the conjugation of the C=C and C=N double bonds with the aromatic ring and hydroxyl group. The enaminoketone tautomer 357 is present in a negligible amount. 

The thionation-cycloaddition sequence is accompanied by the elimination of dimethylamine from the cycloadduct to afford thiopyrans. Similarly, when the thionation-cycloaddition methodology was applied to enaminoketones 98 and 99, obtained from thiochromanones, tricyclic compounds 100 and 101 were obtained (Figure 2.12). 

A similar strategy has been used to prepare pyrimidines, as well as pyra-zoles and isoxazoles by reacting the enamine intermediate with a variety of bidentate nucleophiles [78]. Microwave irradiation of a cyclic 1,3-diketone 49 and acetal 45 in water generated the corresponding enaminoketone 50 in situ which reacted with amidines, substituted hydrazines or hydroxylamine in only 2 min in a one-pot process to give 4-acylpyrimidines, pyrazoles or isoxazoles, respectively (Scheme 20). 

Enaminoketones undergo a clean oxidative cleavage to a-diketones, presumably through a dioxetane intermediate.180... 

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