Enolates oxidative dimerization

Diketones are intermediates for synthesis of perfumes and natural products, and several preparative methods have been developed (327) in the simplest preparative methods, ketone enolates ate oxidatively dimerized (328) ... 

The isolation of calycanthine (9) in 1888 by Eccles [28] and the subsequent proposition for its origins in the oxidative dimerization of tryptamine by Woodward [29] and Robinson [30] had prompted several key synthetic studies based on a biomimetic approach. Hendrickson was the first to experimentally verify the plausibility of forming the C3-C3 linked dimers through an oxidative radical dimerization strategy (Scheme 9.2a). He demonstrated that the sodium enolate of a tryptamine-derived oxindole could be oxidized with iodine to afford a mixture of three possible stereoisomers. The racemic product was isolated in 13 % yield, while the meso product was isolated in 8 % yield. Global reduction of the oxindole and carbamates afforded the first synthetic samples of chimonanthine (7) [9a],... 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Oxidation of arylmethyl ketoximes by phenyliodoso diacetate in glacial acetic acid was second order overall, first order each in substrate and oxidant.145 Iodine allowed the oxidative dimerization of glycine ester enolates with low to moderate diastereoselec-tivity that is consistent with kinetic control.146 Although malonic acid is not oxidized by iodate under acidic conditions, oxidation proceeds in the presence of catalytic ruthenium(III). A mechanism is put forward to account for the observed orders of reaction.147 The rate of periodate oxidation of m-toluidine in acetone-water increases with ionic strength.148... 

Another carbon-carbon bond forming reaction mediated by (difluoroiodo)benzene-boron trifluoride was reported using silyl enol ethers which underwent oxidative dimerization to 1,4-diketones [58] (see also Sections 5.2.2 and 12.2.2). 

The total synthesis of (+)- -onocerin via four-component coupling and tetracyclization steps was achieved in the laboratory of E.J. Corey. The farnesyl acetate-derived acyl silane was treated with vinyllithium, which brought about the stereospecific formation of a (Z)-silyl enol ether as a result of a spontaneous Brook rearrangement. In the same pot, the solution of I2 was added to obtain the desired diepoxide via oxidative dimerization. 

Oxidative dimeriztUiou. Enols and enolizable ketones in which a,/3-unsaturation is not possible are oxidatively dimerized by DDQ Thus 2-phenylindane-l, 3-dione is converted into the tetrone. 

This oxidative dimerization of ketone enolates has been extended to an intramolecular oxidative coupling of dilithium enolates of 2,4-pentanediones to give... 

Phenols and enolizable ketones that cannot undergo Q , -dehy-drogenation may afford intermolecular products arising from either C-C or C-0 coupling on treatment with DDQ in methanol. 2,6-Dimethoxyphenol, for example, results predominantly in oxidative dimerization (eq 21), while the hindered 2,4,6-tri-r-butyl-phenol generates the product of quinone coupling (eq 22). Various other unusual products have been observed on DDQ oxidation of phenols and enolic compounds, their structure being dependent on that of the parent compound. ... 

Diones have been prepared by oxidative dimerization of ketone enolates using copper(ll) chloride, but copper(ii) trifluoromethanesulphonate has been... 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

Dichloro-5,6-dicyanoquinone as oxidizing agent Oxidative dimerization of enols and enolizable ketones... 

Cupric chlorideHithium diisopropylamide -Dicarbonyl compds. by oxidative dimerization or cross-coupling via enolates... 

As in the case of enolates and organic anions in general65 it may be expected that the oxidation of —C—H systems strongly activated by sulphonyl groups in basic solvents may produce C—C dimer bonds. A very recent66 first example concerns the oxidation of... 

Ketones are oxidatively cleaved by Cr(VI) or Mn(VII) reagents. The reaction is sometimes of utility in the synthesis of difunctional molecules by ring cleavage. The mechanism for both reagents is believed to involve an enol intermediate.206 A study involving both kinetic data and quantitative product studies has permitted a fairly complete description of the Cr(VI) oxidation of benzyl phenyl ketone.207 The products include both oxidative-cleavage products and benzil, 7, which results from oxidation a to the carbonyl. In addition, the dimeric product 8, which is suggestive of radical intermediates, is formed under some conditions. 

The reaction of camphor enolate with selenium in the presence of methyl iodide,19,20 followed by areal oxidation, affords 1,3-diselenetanes formed by dimerization of the putative selenoketone intermediate (Scheme 22). These... 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Reaction of compound 134, either with sodium carbonate or potassium /tz -butoxide, leads in moderate yields to the enolized bicyclic compound 135 along with a dimer resulting from the oxidative coupling of the initial enolate of substrate 134 (Scheme 24) <2005T1693>. 

The dimeric dialkoxy-bridged compounds [L2FeOR]2 (L = enolate of acac or dipivaloylmethane R = Me, Et, or Pr ) have been prepared by the oxidation of Fe compounds in alcoholic solutions containing the free ligand and base or directly from Fe compounds. The structure (57) is proposed. ... 

Full details of Barton s selenofenchone (212 X = Se) and fenchylidenefeachane (212 X = 2-fenchylidene) synthesis (Vol. 6, p. 40) have been published" and Wynberg has discussed the antipodal interaction effect in the reductive dimerization of (+)- and ( )-camphor to bornylidenebornanes (Vol. 7, p. 41)," sensitized photo-oxidation of which has also been reported." The CuCl2-promoted dimerization of camphor-lithium enolate yields the expected mixture of dia-... 

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