Anodic oxidation of enaminones

In the recent study on the anodic oxidation of enaminones which possess an unsaturated chain susceptible to react intramolecularly with an electrogenerated radical cation, the evidence for an intramolecular reaction was provided on the basis of the dEp/dlogv slope of 30 mV and one-electron behavior of the voltametric wave [48], The reaction could involve similar mechanistic pathways as shown in Scheme 4 (e-c-dimerization and following chemical reactions). However, the authors were not able to isolate the products after preparative oxidation in order to confirm the possible mechanism. 

The radical cations generated at the anode surface can dimerize before they diffuse into solution. The dimeric compound formed can then be cyclized in solution via formation of a carbon-heteroatom bond. The anodic oxidation of enaminones reported earlier [46] and also recently [47] could serve as an illustration of this type of intermolecular cyclization. 

Anodic oxidation of enaminone 10 was performed at the platinum gauze electrode in methanol solution containing LiC104 in a divided cell at controlled potential. After passage of 1.2F/mol two major products 11 and 12 were isolated in 50 and 20% yields respectively (Scheme 12). Oxidation of the enaminone 10 is initiated by electron loss from the dimethylamino moiety to... 

Although the reactivity of enaminones is not always the same as that of typical enamines due to the additional conjugative interaction with the carbonyl group, the anodic oxidation of enaminones seems useful in organic synthesis since they yield dimerized or cyclized products upon anodic oxidation. In anodic oxidation of the enaminones or enaminoesters in methanol containing sodium perchlorate, for instance, derivatives of pyrrole are formed via initial dimerization (equation 4)5. 

In the case of the enaminonitrile XVIIa, coupling through the phenyl ring is observed [127a]. Enamines related to the key intermediate in thiamine diphosphate-dependent enzymatic pathways lead to dimers in a one-electron oxidation via a thiazolium cation radical as intermediate [127b]. Anodic oxidation of enaminones, prepared from / -substituted acetophenones and A,A-dimethylformamide dimethylacetal, afford dimers and substituted furans in fair yield [127c]. 

Anodic oxidation of the enaminone 215, prepared from o-hydroxyacetophenone and dimethylformamide dimethyl acetal, results in the bichromone 216 (equation 91)113. 

Intramolecular cyclization is a general reaction in the anodic oxidation of substituted amino alkenes. Thus, the already mentioned A-methyl-A-phenyl-1,2,2-triphe-nylvinylamine leads to a new 3/7-indole by anodic oxidation in the presence of 2,6-lutidine [149,150]. The corresponding enediamine undergoes an electrolytic double cyclization to form an indolooxazolidine [149]. The formation of isoquinolines, benzaze-pines, and tetrahydrocarbazoles may also be obtained by anodically initiated intramolecular cyclization of A-benzyl, A- S-phenethyl, and anilino enaminones [Eq. (30)] [158]. 

The mechanism for the transformation of enaminones 1 via anodic oxidation to bischromones 72 was discussed in Sect. 2.1. Since the enaminones can be easily prepared from available o-hydroxyacetylarenes the reaction shown in Scheme 36 represents a general method for the synthesis of the corresponding bischromones which can be obtained in good yields [21]. 

Two successive anodic oxidations, the first one of the enaminone moiety and the second one of the aryl group, allowed intramolecular cyclization to occur in V-benzyl and 7V-/ -phenethyl enaminones with formation of isoquinolines and benzazepines, respectively97 (Scheme 70). 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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