1.3- Dicarbonyl compounds, oxidative dimerization

In the oxidation of anionized 1,3-dicarbonyl compounds (Table 8, numbers 1-7) at potentials between 0.6 and 1.4 V (see) and in the presence of butadiene, mainly the additive dimer (24) is obtained in the presence of ethyl vinyl ether chiefly the disubstituted monomers (28) or (29) arise. 

The primary adduct 53 (Eq. (117) ) of the anodically generated radical R undergoes a series of follow-up reactions a) hydrogen abstraction to 54, b) dimerization to additive dimers 55, c) coupling with R to 1,2-disubsti-tuted monomers56, d) le-oxidation to a carbonium ion that either solvolyzes to 57 or, when 1,3-dicarbonyl compounds are added cyclizes intramolecularly to tetra (58) - or dihydrofuran derivatives (59). Product control is possible in some cases by suitable choice of the anode potential. With a high anode potential,... 

A great variety of substituted radicals for dimerization can be generated by anodic oxidation of anionic species r5"Me5+, e.g., sodium salts of 1,3-dicarbonyl compounds, aliphatic nitro compounds, phenols, oximes, alkynes, thio-lates or organometallics (Eq. (157) ). 

Electrolysis of sodium salts of 1,3-dicarbonyl compounds often does not yield the wanted dimer, but ist methylene derivative 81 (Eq. (158) ) 351 <367) since the alcohol serving as solvent is oxidized to an aldehyde which reacts with the... 

A ruthenium-catalyzed three-component reaction between propargylic alcohols, 1,3-dicarbonyl compounds, and primary amines leading to fully substituted pyrroles was developed <07CEJ9973>. Cyclohexa[a]pyrroles ( azabicyclo[4.3.0] systems ) were formed by a three-component sequence involving allenic ketones, primary amines, and acryloyl chloride <07SL431>. An oxidative dimerization sequence involving arylpyruvates in the presence of ammonia was the key step in an approach to the pyrrole natural product, lukianol A <07S608>. 

The anodic oxidation of enol ethers at a graphite anode in methanol containing 2,6-lutidine and sodium perchlorate results in the dimerization of the enol ethers to acetals of 1,4-dicarbonyl compounds (equation 22). The mechanism of dimerization is thought to involve a tail-tail coupling of the cation radicals generated by the one-electron oxidation of the enol ethers. 

Following on from their previous work on the biomimetic synthesis of marine natural products, Steglich et al. proposed a biomimetic lamellarin synthesis in which an oxidative dimerization of an arylpyruvic acid and condensation of the resulting 1,4-dicarbonyl compound with a suitable 2-arylethylamine would be the key steps of the synthesis. Thus, the synthesis of lamellarin G trimethyl ether was achieved by coupling two molecules of 3-(3,4-dimethoxyphenyl)pyruvic acid and the appropriate 2-phenylethylamine <9579941, 97AG(E)155>. The use of a mixture of two different arylpyruvic acids afforded the unsymmetrical lamellarin L <00MI1147>. 

In the case of 1,3-dicarbonyl compounds, the solvent frequently interferes with the coupling reaction. So with diethyl sodio malonate in ethanol [212b], methanol [212b], dimethylacetamide [212b], or HMPTA [216], besides the dimer and the trimer, the compounds LIXa-c are obtained. They are presumably formed by oxidation of the solvent to the aldehyde and its condensation with the active methylene compound. No dimer was detected in the oxidation of sodio acetoacetate in ethanol, with the major product being LX [217]. Anodic oxidation of cyclic 1,3-diketones in aqueous methanolic sodium hydroxide does not yield the dimer but product LXI, formed by condensation of the starting compound with formaldehyde [218]. 

Satisfactory to good yields of adducts have been found for styrenes [Eq. (21a), Y = phenyl], conjugated dienes (Y = vinyl), enamines (Y = NR2), and enol ethers (Y = alkoxy), particularly if they are unsubstituted at the 6-carbon atom to Y. Nonactivated alkenes react less satisfactorily. In the oxidation of anionized 1,3-dicarbonyl compounds (Table 11, numbers 1-8) at potentials between 0.6 and 1.4 V (SCE) and in the presence of butadiene, only the additive dimer LXII is obtained in the presence of ethyl vinyl ether only the disubstituted monomers LXVI or LXVII arise, but with styrene both types of products LXII and LXVI are formed. This result indicates that the primary adduct LXIII is oxidized rapidly between 0.6 to 1.4 V to the carbenium ion in the case of an ethoxymethyl radical (Y = OEt), and slowly in the case of an allyl radical (Y = vinyl). 

P-Dicarbonyl compounds generally reacted with heteroaryllead reagents to afford the a-aryl derivatives in synthetically useful yields. However, in the reaction of 2-thienyllead triacetate with 2-ethoxycarbonylcyclopentanone (86), the C-arylation product was observed in a low yield (9%), together with the presence of a dimer resulting from a radical oxidative coupling induced by the organolead reagent. Use of 2-thienyllead tribenzoate, a weaker oxidant, restored the classical reactivity. When the reaction was performed in pyridine as solvent, the arylation product, the 2-(2-thienyl)cyclopentanone derivative, was obtained in 76% yield. 

A classical synthesis of alkyl- and arylpyrazines involves dimerization of a-amino carbonyl compounds, which may be produced by many methods such as reduction of a-oximino ketones, aminolysis of a-halogeno ketones (Section 6.0T11.2), oxidation of a-amino alcohols and reduction of a-amino acids . The condensation of 1,2-diamines with a-dicarbonyl compounds is available for the synthesis of particularly quinoxaline derivatives (Section 6.03.11.1). The synthetic methods which rely on self-condensation, however, provide only symmetrically substituted pyra-... 

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