Diketones and a-Ketoesters

The released quinoline is a mild catalyst for the fragmentation reactions. Many carbonyl compounds, such as PhC(CN)C=0, pyranones and unsaturated ketones, react similarly. In the reaction of diphenylketene with a-diketones and a-ketoesters, only one carbonyl group participates in this reaction. In contrast, acetoacetic esters and jS-diketones do not react with diphenylketene. ... 

The fluorination of P-diketones and p-ketoesters with N-/luorobis(trifluo-romethanesulfonyl)imide (Table 3a, B) can be controlled to give either mono-fluorination or difluorination. Monofluorination occurs when the strong acid, bis(trifluoromethanesulfonyl)imide, a reaction product, is removed by addition of water, which prevents further enolization and fluormation of the monofluoro adduct [83] (equation 38)... 

Sodeoka and coworkers reported a chiral Pd-catalyzed enantioselective Michael reaction of 1,3-diketones and (3-ketoesters to enones, [Eq. (13.18)]. It was proposed that the catalyst 29 exists in equilibrium with 30 through liberating H2O... 

TA NaBr-MRNi has been found to be an effective catalyst for enantio-differentiating hydrogenations of ketones which have a general structure of R—CO—CH2—X—O— as shown in Table XVII (52c) and methyl ketones as shown in Table XXVI (52d). Among all, /i-diketones and /i-ketoesters are the most favorable substrate for this catalyst. Specific rotations [a] 0 of (R, R )-diols produced from /3-diketones by hydrogenation with this catalyst are summarized in Table XXVII (44). 

Table 8.6 shows that the equilibrium mixture consists of almost entirely keto form in the case of simple aliphatic and aromatic ketones, whereas significant amounts of enol tautomer are present in /J-diketones and /J-ketoesters. In these latter cases, the enol contains a conjugated tt electron system and an intramolecular hydrogen bond (30). Phenol exists entirely in the enol form, as the alter-... 

Addition occurred in 2-trimethylsilyloxy-furan upon treatment with PhlO-BF3-Me3SiN3, with direct formation of 5-azidofuranone [99]. Some / -diketones and /j-ketoesters underwent a-azidation by PhIO-Me3SiN3 [100],... 

The a-functionalization of /i-diketones and j3-ketoesters has been effected through IOB.BF3. At room temperature methanol with 2,4-pentanedione gave its a-methoxy derivative, whereas methanesulphonic acid in refluxing chloroform afforded the a-mesylate [35] ... 

Another analogue of HTI which was used with either ketones or silyl enol ethers was [hydroxy(mesyloxy)iodo]benzene, PhI(OH)OS02Me [25]. A related reagent formed in situ from iodosylbenzene and trimethylsilyl triflate, probably PhI(OSiMe3)OTf, reacted similarly with silyl enol ethers to afford a-ketotriflates (see Table 5.3). /1-Diketones and /1-ketoesters underwent tosyloxylation by HTI the reaction was very effective in substrates with a perfluoroalkyl moiety and gave their hydrates [26] ... 

Phenylation and arylation have also been performed in bifunctional compounds such as malonates, /1-diketones and /1-ketoesters (Table 8.4). Diethyl malonate was arylated at room temperature affording mixtures of mono- and bis-arylated products, whereas isopropylidene malonate (Meldrum s acid) underwent bis arylation directly. Dimedone, the all carbon analogue of Meldrum s acid, was also mono-and bis-phenylated, with some concomitant O-phenylation [33], Generally, / -diketones show often ambident reactivity but the O-arylated product is normally the minor one an exception was noted in the triketone (PhCO)2CHCOPh which underwent mainly O-phenylation (68%) [33], Several dianions from /1-diketones have been arylated in high yields at the a-position, in a procedure superior to other methods [35]. 

The basicity of LDA is so high that it is even possible to generate bisenolates from /3-diketones and /3-ketoesters (Figure 10.7). Even carboxylates can be deprotonated at the a carbon if the strongest organic bases are employed (Figure 10.8). In contrast, the twofold deprotonation of phenylacetic acid by ethylmagnesium bromide is not com-... 

In this experiment proton NMR spectroscopy is nsed in evalnating the equilibrium composition of various keto-enol mixtures. Chemical shifts and spin-spin splitting patterns are employed to assign the spectral features to specific protons, and the integrated intensities are used to yield a quantitative measure of the relative amounts of the keto and enol forms. Solvent effects on the chemical shifts and on the equilibrium constant are investigated for one or more j8-diketones and j8-ketoesters. 

Haddadin and Issidorides first reported an elegant method for the synthesis of quinoxaline 1,4-dioxides (47) from the reaction of benzofurazan 1-oxide (46) and an enamine or an active methylene compound, such as a -diketone or a -ketoester, in the presence of base. Quinoxaline 1,4-dioxide formation formally involves loss of secondary amine in the enamine reaction and loss of water when an active methylene compound of the type R CHjCOR is used. This reaction is now commonly referred to as the Beirut reaction. The isolation of the dihydroquinoxaline 1,4-dioxide 48 from the reaction of 46 and dimethylisobutenylamine (Me2C=CHNMej), which is unable to aromatize by amine loss, suggests that 2,3-dihydroquinoxalines are likely intermediates in all these reactions. ... 

Due to the lower acidity of the a-hydrogen of malonic acid derivatives compared to the acidity of the corresponding hydrogens of P-diketones and p-ketoesters, the reactivity of malonic acid derivatives towards aryllead compounds is very dependent upon the nature of the substrate. 

Allylation of acylsilanes and a-ketoesters proceeds normally. Monoallylation of a-diketones is also easily realized, whereas glyoxal A7Ai-monohydrazone gives 1,7-octa-diene-4,5-diol. However, sequential reactions of the glyoxal monohydrazone with RLi and then allylindium reagent lead to unsymmetrical diols. ... 

This method is also applicable to a-alkenylationf f (Scheme 10) and the synthesis of 1,4-diketones " and y-ketoesters (Scheme 11). As in a-arylation, favorable resnlts are largely limited to the a-snbstitntion of methyl ketones. 

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