Of a-diketones

The mechanism of the formation of an acyloin from an ester may involve the initial formation of a diketone the latter is reduced by the metal to give the sodium salt of the enediol form of the acyloin ... 

In an intramolecular aldol condensation of a diketone many products are conceivable, since four different ends can be made. Five- and six-membered rings, however, wUl be formed preferentially. Kinetic or thermodynamic control or different acid-base catalysts may also induce selectivity. In the Lewis acid-catalyzed aldol condensation given below, the more substituted enol is formed preferentially (E.J. Corey, 1963 B, 1965B). 

Noncatalytic oxidation of propylene to propylene oxide is also possible. Use of a small amount of aldehyde in the gas-phase oxidation of propylene at 200—350°C and up to 6900 kPa (1000 psi) results in about 44% selectivity to propylene oxide. About 10% conversion of propylene results (214—215). Photochemical oxidation of propylene with oxygen to propylene oxide has been demonstrated in the presence of a-diketone sensitizers and an aprotic solvent (216). 

Monooximes of a-diketones have found applicability in the synthesis of 2-aminopyrazine 1-oxides by condensation with a-aminonitriles, and this reaction was used by White and coworkers in an approach to the synthesis of Cypridina etioluciferamine (Scheme 66 R = 3-indoloyl) (73T3761). In this instance, the use of TiCU as a catalyst was essential, since the carbonyl group in 3-acylindoles is normally deactivated and the required amine/carbonyl condensation is impractically slow. Under normal circumstances the carbonyl group in simple alkyl-substituted monoximes of a-diketones is the more reactive site and the reaction is rapid, requiring no catalysis (69LA(726)loo). 

In practice, the scope of the benzilic rearrangement as a tool for the ring contraction of steroids is defined by the availability of a-diketones, or the corresponding diosphenols. 

Ring contraction by benzilic acid rearrangement of a-diketones... 

The reaction of (fluoroalkenyl)carboranes with potassium permanganate in acetone leads to formation of a-diketones [119. These compounds react by photochemical reaction in which the radical formation at boron is followed by addition to the double bond [120] (equation 92). 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

Isoxazole derivatives are stable toward peracids but can be ozonolyzed. This, as is well known, enabled the 0-benzoyloximes of a-diketones with a well established configuration to be obtained, which were used to investigate the Beckmann rearrangement mech-anism. ... 

In the case of a diketone (e.g., 3-tosyloxypentane-2,4-dione), the formation of 5-acetyl-4-methyl-2-aryl-l,3-thiazole derivatives can be realized in very good yields (86-89%) (Scheme 7). All these experiments where performed in a Sears Kenmore unmodified household microwave oven (990 W) equipped with a turntable. The average bulk temperature was estimated by inserting a thermometer in the alumina bath housing the reaction vessel. 

Certain ketoximes can be converted to nitriles by the action of proton or Lewis acids. Among these are oximes of a-diketones (illustrated above), a-keto acids, a-dialkylamino ketones, a-hydroxy ketones, p-keto ethers, and similar compounds. These are fragmentation reactions, analogous to 17-25 and 17-26. For example, ot-dialkylamino ketoximes also give amines and aldehydes or ketones besides nitriles. 

A detailed kinetic study of the reaction of trialkyl phosphites (44) with benzil has been carried out (see Chapter 2 for the reactions of a-diketones with trialkyl phosphites). The reaction is first-order in both phosphite and benzil and the rate constant increases with the dielectric constant of the solvent. The authors propose initial attack of phosphite at carbonyl carbon (45), in opposition to the original suggestion by Ramirez, who proposed initial attack at carbonyl oxygen. 

C Tracer Studies in the Rearrangement of Unsymmetrical a-Diketones Phenylglyoxal to Mandelic Acid. J. Amer. chem. Soc. 70, 3499 (1948). 146. — CM Tracer Studies in the Rearrangements of a-Diketones. III. The Rearrangement of Unsymmetrical Benzils. Brookhaven Conf. Rep., Chem. Conf. Nr. 4, 49—58 (1950). Vgl. U.S. Atomic Energy Commission Document, BNL 44 (1950). 

Selective reduction of ketones.1 This reagent can be used to effect selective reduction of the more hindered of two ketones by DIBAH or dibromoalane. Thus treatment of a 1 1 mixture of two ketones with 1-2 equiv. of 1 results in preferential complexation of the less hindered ketone with 1 reduction of this mixture of free and complexed ketones results in preferential reduction of the free, originally more hindered, ketone. An electronic effect of substituents on a phenyl group can also play a role in the complexation. This method is not effective for discrimination between aldehydes and ketones, because MAD-complexes are easily reduced by hydrides. MAD can also serve as a protecting group for the more reactive carbonyl group of a diketone. The selectivity can be enhanced by use of a more bulky aluminum reagent such as methylaluminum bis(2-f-butyl-6-( 1,1-diethylpropyl)-4-methylphenoxide). 

DioxaphosphoIens.—The condensation of a-diketones with tervalent phosphorus compounds has been extended to monothiobenzil.43 Hydrolysis of the resulting adducts (59) gave both P-O and P-S bond fission. Exchange reactions on the benzil-... 

Aminotriazoles and their derivatives can be prepared from bis hydrazones and from substituted bis hydrazones of a-diketones. The bis hydrazones may in some cases be prepared more conveniently from -bromoketones than from a-diketones. Oxidation with mercuric oxide or with manganese dioxide gives the 1-aminotriazoles directly (Scheme m-ns Although two isomeric triazoles would be ex-... 

Bis acylhydrazones or bis aroylhydrazones of a-diketones also give derivatives of 1-aminotriazoles on mild oxidation (Scheme 21).i n The product was originally assigned a dihydrotetrazine structure, and other possibilities were considered,but Curtin and Alexandrou proposed the isoimide structure (8) which was, for the product obtained from biacetyl bis(benzoylhydrazone), confirmed by X-ray crystallography. The mechanism given in Scheme 21 has been put forward for its formation ... 

Oxidation of a-diketone bis arylhydrazones using a variety of reagents results in the formation of 2-aryl-1,2,3-triazoles. This reaction, which is... 

By Dehydration of a-Diketone Hydrazone Oximes Another general method of synthesis of 2i/-triazoles involves intramolecular elimination of water from adjacent hydrazone and oxime functions (Scheme 32). The usual reagent is acetic anhydride, but others, including phosphorus pentachloride and urea, have been used. 

If the hydrazone is unsubstituted, dehydration with acetic anhydride gives a 2-acetyltriazole, which is readily converted into an unsubstituted triazole.Oxidation of a-diketone hydrazone oximes with cupric sulfate in aqueous pyridine leads to the formation of 2.ff-triazole-1-oxides. ... 

Oxidation of a-diketone hydrazone imines with cuprammonium salts gives 2/f-triazoles. With substituted imines (using W-bromosuccin-imide as oxidant) 1,2-disubstituted triazolium salts are obtained (Scheme 33). ... 

Ketones are obtained from a-diketones by reduction with hydrogen sulfide in a pyridine-methanol solution [237], by refluxing with 47% hydriodic acid in acetic acid (yield 80%) [916], and by decomposition of monohydrazones with alkali [923]. Reduction of a-diketones to hydrocarbons is achieved by decomposition of bis-hydrazones by alkali [923]. 

In enediones in which two carbonyl groups of a diketone are linked by an ethylenic bond tin [174] and chromous chloride [196] reduce only the double bond, and none of the conjugated carbonyl groups. A double bond conjugated with one carbonyl group only is not reduced. Refluxing cholest-4-ene-3,6-dione with chromous chloride in tetrahydrofuran yielded 49% of 5/S-cholestane-3,6-dione, and a similar reduction of cholesta-l,4-diene-3,6-one gave 5)S-cholest-l-ene-3,6-dione [196]. 

Nitro group in an oxime is reduced preferentially to the oximino group with ammonium sulfide [240]. In a monoxime of a diketone, the oximino group is reduced to amino group to the exclusion of the carbonyl group by catalytic hydrogenation over platinum oxide in methanolic hydrochloric acid 9-keto-10-oximino-1,2,3,4-tetrahydrophenanthraquinone afforded 10-amino-9-keto-l,2,3,4-tetrahydrophenanthrene in 78% yield [949]. 

Dioximes of a-diketones such as benzil on oxidation with IBTA are converted into 1,2,5-oxadiazole-A-oxides (furoxans) in high yields (75S445) (Eq. 35). Benzo- (Scheme 46) and pyrido-oxadiazoles (Eq. 36) are formed when o-nitroaniline and 3-amino-2-nitropyridine are subjected to similar oxidation. 

The imidazole ring is often found in biologically active molecules and is as such important in medicinal chemistry [16]. An interesting route for the production of substituted imidazoles is the Debus-Radziszewski reaction. This is a three-component reaction of a diketone, an amine and an aldehyde to form trisubstituted imidazoles (Scheme 3). 

The key intermediate in the synthesis of the derivatives of 19-F3-androstane is the trifluoro analogue of the Wieland-Miescher ketone. Its preparation involves a Diels-Alder reaction between a trifluoromethyi ketone and a siloxy diene. Another original step is the regioselective reduction of a diketone only the ketone function in P of CF3 (probably activated by this substituent) is reduced (Figure 4.6). " Then, a succession of classical reactions leads to derivatives of androstane from the trifluoro analogue of the Wieland-Miescher ketone (Figure 4.7). ... 

The results of further investigations strongly support this formation of a-diketones and aldehydes. Before this work is considered, it is pertinent to discuss the action of alkali on sugars. As has already... 

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