Enamine enaminoketones

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). 

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). 

Another approach leading to pyrroles from enaminoketones was demonstrated [94H(37)487]. An enamine with the thiophene ring (15) was converted into an imido chloride vinylog, which interacts with esters of amino acids in the presence of sodium hydride in dimethylformamide and affords... 

In the case of enaminoketone (77) and /j-benzoquinone, there are two parallel reactions in the /3-position of the enamine 77 and in the 8-position of the tautomeric dienamine 77a [93MC40 94KFZ(2)36]. The first pathway (common in the Nenitzescu reaction) gives a dibenzofuran derivative (78), the second affords an unusual product—a jpiro[benzofuran-3,l cyclohexene] derivative (79). The likely pathways are indicated. The... 

Imidazo [1,2-fl]pyrimidines can be obtained from enediaminonitriles and A-acyliminoethers (96SC453). The unusual transformation of push-pull enamines into 4,6-dimethylamino-5-nitropyrimidine has been carried out (97KGS343). A facile synthetic method for pyrimidine derivatives based on fluorine-containing enaminoketones has recently been published (97H349). 

And finally, acylating agents can also be used as electrophiles to react with enamines. Following the hydrolysis of the enaminoketones (i.e., compounds with the substructure R2N-C=C-C(=0)-R ) or enaminoesters (i.e., compounds with the substructure R2N-C=C-C(=0)-0R ) the acylation products of the corresponding ketones are obtained. Figure 12.21 gives the mechanistic details of the acylation with an acid chloride, and Figure 12.22 shows the acylation with ethyl chloroformate. The first acylation yields /1-diketones, the second furnishes a /1-ketoester. 

Fig. 12.21. Acylation of an enamine with an acid chloride (—> enaminoketone E), followed by acidic workup (—> /J-diketone B). Since the enamine A is produced from cyclohexanone, the figure shows the second part of a two-step reaction which is an alternative to the one-step acylation of a ketone enolate (cf. Section 13.5.2).

The term enaminones is used in this section to designate enamines having at C(2) a carbonyl function conjugated with the carbon-carbon double bond. The two main groups of these compounds, 2-acylenamines (enaminoketones) and 2-alkoxycarbonyle-namines (enaminoesters), are discussed in the following subsections. 

Sodium bis[2-methoxyethoxy]alanate is a selective reagent for reducing the enamine double bond of linear enaminoketones, leaving the carbonyl group untouched165 (Scheme 116). 

Meyers and co-workers141 have recently reported an improved method for obtaining enaminoketones from j8-aminonitriles instead of j8-amino esters (Scheme 8). The enamines, which are easily obtained from ]8-aminopropionitriles, yield the amino dihydropyrindinium salt (210) on warming with anhydrous magnesium perchlorate or magnesium iodide in benzene or toluene. It was proposed that the role of magnesium... 

The enolic /J-diketone 89 with hydrazine and amines produced excellent yields of the pyrazolo derivative 91 and the enamines 92, respectively (Eq. 22).63,64 Similarly, the 3-acetyl compound 93 yielded the corresponding pyrazole when combined with hydrazine,64 and with amines it gave the enaminoketones 94.8-65... 

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