Isomerization 3-diketones

Compounds A and B are isomeric diketones of molecular formula CgHio02. The H NMR spectrum of compound A contains two signals, both singlets, at 8 2.2 (six protons) and 2.8 (four protons). The H NMR spectrum of compound B contains two signals, one at 8 1.3 (triplet, six protons) and the other at 8 2.8 (quartet, four protons). What are the structures of compounds A and B ... 

The contrast between thermal and photochemical reactions of vinyl cyclobutane-dione 36 is of interest60>. Photolysis of 36 proceeded quantitatively with bisdecarbon-ylation to tetrachlorodiene 37 while thermolysis (70°) of 36 resulted in quantitative isomerization to isomeric diketone 39 (the photochemical precursor of 36). Similar effects have been observed with related compounds. The most reasonable mechanism for isomerization 36 - 39 is homolysis to biradical 38 which can either revert to 36 or... 

A. Muller and co-workers [Ber, deut. Chem. Ges. 76, 865 (1943) Bcr. d ut. chem. Ges. 75, 891 (1942) Acta Chim. Acad. Sci. Hung., in press] observed similar behavior with some isobenzopyrylium salts. If warmed in aqueous solution they are transformed into isobenzop3napols and these change on warming in ethyl acetate into the isomeric diketones by ring opening. 

The aliphatic version of the classical aromatic Friedel-Crafts acylation produces, by loss of proton, a non-conjugated enone which can then undergo a second acylation thus generating an unsaturated 1,5-diketone. Clearly, if the alkene is not symmetrical, two isomeric diketones are formed. Under the conditions of these acylations, the unsaturated diketone cyclises, loses water and forms a pyrylium salt. The formation of 2,4,6-trimethylpyrylium, best as its much more stable and non-hygroscopic carboxymethanesulfonate, illustrates the process. 

Compounds A and B are isomeric diketones of molecular formula C6H10O2. The... 

A separation in the metabolic route occurs on the reduction of the double bond of A -androstene-3,17-dione (LV), affording two saturated diketones isomeric at Cs. These are rings A/B trans, androstane-3,17-dione (LX), and rings A/B cis, etiocholane-3,17-dione (LXI). Both of these diketones have been isolated from urine. " That these two diketones are true metabolites of testosterone has been shown by the isolation of the tagged substances after injection of deuterium-labeled hormone. Once the metabolic course has diverged upon the reduction of the a, unsaturated ketone to the isomeric diketones, there is no crossing over from one branch of the pathway to the other. Thus, after the administration of either deuterium-labeled or nonisotopic androstane-3,17-dione (LX), only... 

Acylation of 4 with phenylbenzoate 9 was performed with sodium hydride in a solution of THF at 0°C, 4 h to provide 11 in 90% yield. No evidence of the isomeric diketone structure or the 1,2-dihydro isomer was observed. The absence of this product may be due to the enol stability of 11 compared to other similar systems where such isomers are observed. 

The rt,/3-unsaturated linear carbonyl compound 39 is obtained by the decomposition of the cyclic hydroperoxide 38 with PdCl2,[35]. The a, 0-epoxy ketone 40 is isomerized to the /3-diketone 41 with Pd(0) catalyst[36]. The 1,4-epiperoxide 42 is converted into the /3-hydroxy ketone 43 and other products[37]. 

The industrial precursor to 2,4-pentanedione is isopropenyl acetate, produced from acetone and ketene (307,308). The diketone is formed by the high temperature isomerization of isopropenyl acetate over a metal catalyst (309—311). 

Scheme 4 also represents the classical route to isoxazoles, first studied in 1888 by Claisen and his coworkers (1888CB1149). Reaction of a 1,3-diketone with hydroxylamine gives, via the isolable monoxime (108) and the 4-hydroxyisoxazole (109), the isoxazole (110). Unsym-metrical 1,3-diketones result in both possible isomers (110) and (111), but the ratio of the isomeric products can be controlled by the right combination of the 1,3-dicarbonyl component and the reaction conditions used. These important considerations are described in Chapter 4.16, along with the variations possible in the 1,3-dicarbonyl component designed to yield diverse substituents in the resultant isoxazole. 

It was soon found that the reaction of unsymmetrtcal 1,3-diketones (290) or their derivatives with hydroxylamine results in both possible isomeric isoxazoles (291) and (292), a complication which not only reduces the yield of desired product but also often leads to separation problems, particularly when R and R are similar. However, the reaction does give one isomer, or predominantly one isomer, if the right combination of the CCC... 

Oxiranes have been isomerized by palladium compounds to allylic alcohols and enones (79JA1623), and to 1,3-diketones (80JA2095). 

A recent paper by Singh et al. summarized the mechanism of the pyrazole formation via the Knorr reaction between diketones and monosubstituted hydrazines. The diketone is in equilibrium with its enolate forms 28a and 28b and NMR studies have shown the carbonyl group to react faster than its enolate forms.Computational studies were done to show that the product distribution ratio depended on the rates of dehydration of the 3,5-dihydroxy pyrazolidine intermediates of the two isomeric pathways for an unsymmetrical diketone 28. The affect of the hydrazine substituent R on the dehydration of the dihydroxy intermediates 19 and 22 was studied using semi-empirical calculations. ... 

BFO reacted readily with 1,3-diketones to give 2,3-disubstituted quinoxaline 1,4-dioxides. In the case of unsymmetrical 1,3-diketones, mixtures of isomeric quinoxaline dioxides were obtained, and the ratio of isomers was influenced by the steric bulk of the carbonyl substituent. When BFO 1 was combined with 1,3-diketone compounds 18 in the presence of triethylamine, the isomeric quinoxaline 1,4-dioxides 19 and 20 were obtained. When R = Me, 19 was the only product observed. As the steric bulk of R increased, increasing amounts of isomer 20 were observed. When R = tBu, 20 was the only product detected in the reaction. 

Though not extensively proven, the reaction path leading to only one of the possible isomers is peculiar to this synthesis of 4-chloroisoxazoles. The not infrequent formation of a mixture of two isomeric 3,5-disubstituted isoxazoles, which are difficult to separate, is recognized as one of the chief disadvantages of the use of -diketones to synthesize isoxazoles. 

Several examples of conjugate addition of carbanions carried out under aprotic conditions are given in Scheme 2.24. The reactions are typically quenched by addition of a proton source to neutralize the enolate. It is also possible to trap the adduct by silylation or, as we will see in Section 2.6.2, to carry out a tandem alkylation. Lithium enolates preformed by reaction with LDA in THF react with enones to give 1,4-diketones (Entries 1 and 2). Entries 3 and 4 involve addition of ester enolates to enones. The reaction in Entry 3 gives the 1,2-addition product at —78°C but isomerizes to the 1,4-product at 25° C. Esters of 1,5-dicarboxylic acids are obtained by addition of ester enolates to a,(3-unsaturated esters (Entry 5). Entries 6 to 8 show cases of... 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

As a further illustration of the phenomenon of H-bond resonance coupling let us consider the intramolecular H-bond of (3-hydroxyacrolein (0=CHCH=CH0H), a prototypical enolone (2-en-3-ol-l-one, or enol isomer of (3-diketone).55 This molecule may be envisioned as existing in two distinct isomeric forms, according to the position of the proton in the O- H—O hydrogen bond ... 

Pd2(dba)3-CHCl3/Bun3P catalyzes the isomerization of alkynediols to 1,4-diketones in high yield. Importantly, alkenyl-substituted alkynediols chemoselectively isomerized to the corresponding a,/3-unsaturated 1,4-diketones (Scheme 55).94... 

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

A survey of Wacker-type etherification reactions reveals many reports on the formation of five- and six-membered oxacycles using various internal oxygen nucleophiles. For example, phenols401,402 and aliphatic alcohols401,403-406 have been shown to be competent nucleophiles in Pd-catalyzed 6- TZ /fl-cyclization reactions that afford chromenes (Equation (109)) and dihydropyranones (Equation (110)). Also effective is the carbonyl oxygen or enol of a 1,3-diketone (Equation (111)).407 In this case, the initially formed exo-alkene is isomerized to a furan product. A similar 5-m -cyclization has been reported using an Ru(n) catalyst derived in situ from the oxidative addition of Ru3(CO)i2... 

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