Diketones, hindered

Many stabilizers require costabilizers. Several organic costabilizers are quite useful with barium—zinc and calcium—zinc stabilizers, eg, P-diketones, epoxies, organophosphites, hindered phenols, and polyols (122). 

A variety of 2,5-dialkylfurans are available via the Paal-Knorr condensation cyclization is possible for both hindered and unhindered 1,4-diketones. Fleming prepared 2-cyclohexyl-5-methylfuran (31) in 91% yield via treatment of dione 30 with catalytic p-toluenesulfonic acid in refluxing benzene. Using the same methodology, Denisenko synthesized furan 33 in 35% yield from the corresponding dione (32). ... 

Sodium malonate attacks almost exclusively the sterically hindered site, whereas /i-oxo esters, 1,3-diketones, and the C-mcthylated analogs of dimethyl malonate lead exclusively to terminal attack. 

Intramolecular aldol condensation.1 This base can be effective for intramolecular aldol condensation of extremely hindered diketones that resist cyclization with the usual bases. 

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). 

In the second study, diketones were used as electrophiles and reacted with N-benzoylglycine to give a (Z/E) mixture of oxazolones 366 and 367 derived from condensation at the less hindered carbonyl group of the 1,2-dicarbonyl compound (Scheme 7.116). The ( )-isomers 367 were used as starting materials to prepare (Z) -5 - alky lidene- 3 - (benzoy lamino) -2(5//)-furanones 368... 

Addition of isopropyl lithium to the surviving ketone followed by oxidative rearrangement of the resulting tertiary ailylic alcohol and concomitant oxidation of the secondary allylic alcohol gave the diketone 10. Selective addition of methyl lithium to the less hindered of the two ketones, again from the more open face, then gave 3. 

Lanthanide and actinide complexes. Ln(L-L)) and An(L-L)4, of sterically hindered diketonates [e.g.. [Me3CC(0)CHC(0)CMej] fdpm) and [F CCF2CF2-CfOICHQOJCMeJ (fed)], are of considerable interest because of their volatility. Despite their high molecular weights, they have measurable vapor pressures at temperatures below the boiling point of water. This volatility has been exploited in... 

The preparation of furans by acid catalyzed dehydration of 1,4-diketones followed by cyclization of the monoenol has been comprehensively reviewed (69RCR389). This method of furan preparation is useful for all 1,4-diketones which are not sterically hindered with the highly hindered l,4-bis(2,4,6-triisopropylphenyl)-l,4-butanedione no cyclization occurs (50JA5754). Although sulfuric acid is normally used for cyclization, hydrochloric acid was employed in the conversion of a 1,2-diacylethylene to a furylacrylic acid derivative (59MIP31200). Other reagents used for this purpose include polyphosphoric acid, phosphorus trichloride, zinc chloride and DMSO. Both DMSO and phosphoric esters provide neutral... 

Carbon nucleophiles in general react at the least hindered position. Thus 1,3-ketones add to 2,6-diphenylthiopyrylium salts to give the expected 4-substituted 4H-products, but if the diketone is part of a six-membered ring it is further oxidized in situ to the thiopyrany-lidene product, while the other adducts require treatment with ferricyanide to convert them to the unsaturated products (76CB1549). 

The interaction of the metal alkoxides with the salts of carboxylic acids or with p-diketonates of other metals is especially attractive for the synthesis of bimetallic molecular precursors in the cases, when the preparation of the alkoxide of the other metal is somehow hindered or it is insoluble or irreactive under the conditions applied. This method has been widely used for the sol-gel preparation of HTSC materials (because of low solubility and reactivity of Cu(OR)2) and lead-containing ferroelectrics (in the view of difficult synthesis and low stability of Pb(OR)2). It should be mentioned that the reaction between a metal alkoxide and a functional derivative does far not always lead to the formation of a mixed-ligand bimetallic complex ... 

The delocalization in the donor and acceptor parts of case 2 systems leads to considerably hindered rotations around the formal single bonds. The acceptor part of a compound like 13 is similar to the anion of a /J-diketone. While the dynamics of the latter one is difficult to study without the intervention of a chelated cation, the acceptor part of 13 is more similar to an isolated /J-diketonate ion. A study of 13 by dynamic 11 NMR revealed exchange between three rotamers (EZ, ZE and EE, Scheme 2). The fourth rotamer, ZZ,... 

Data on such densely sterically hindered compounds that would hinder the attack by the electron have not been reported. There are two cases in which such an effect can be expected to operate. Whereas dial-kylperoxides are generally reducible at positive potentials, di-tert-butylperoxide has been reported as irreducible (139). Similarly for cyclic diketones (140), the unreducibility of 3,3,8,8-tetramethyl-l,2-cyclo-octanedione can arise from a similar effect, but the steric hindrance of coplanarity of the diketo grouping cannot be excluded in this case. A more detailed examination of these and similar systems would be necessary to reach a decision as to whether this type of steric effect can change the polarographic behaviour or not. 

With the more sterically hindered 2,4,6-trimethylbenzoyl chloride, the expected reaction occurred but it also gave an unexpected germa-/ -diketone which might be formed through a benzoylgermyllithium (Scheme 23)16. Digermyl diketones were synthesized in the same way (equation 104)9. 

Dehydration of 1,4-diketones results in cyclization, and this method is often employed for the synthesis of furans. This is feasible in the case of all those 1,4-diketones which are not sterically hindered. Thus, no ring closure occurs with l,4-bis-(2,4,6-triisopropylphenyl)-l,4-butanedione.49... 

As depicted in the following scheme, a homoallyl alcohol derived from a norbomyl a-diketone underwent a lead(IV) acetate reaction in MeOH, resulting in the formation of a novel methoxy substituted spirocyclic tetrahydrofuran <07CC4239>. It is believed that the addition of the methoxylead(IV) acetate species across the alkene from the less sterically hindered side to form a plumbonium cation leads to the major product after subsequent cyclization and reductive elimination. Moreover, construction of tetrahydrofurans by a Pd(II)/Pd(IV)-catalyzed aminooxygenation of homoallyl alcohols was also reported <07AGE5737>. 

Preussomerin I 697 and ( )-preussomerin G 698 were obtained from 620 with a five- and six-steps sequence in 15% and 12% overall yield, respectively, through modifications of substituents of the dioxocin ring. Thus, attack of lithium methoxide from the less hindered face of the enone 620, followed by protection of the phenolic oxygen as its methyl ether provided the methoxy adduct 692. The ketone 693 was obtained through a benzylic bromination-solvolysis-oxidation protocol, which required only a single purification. The C(2)-C(3) olefin was introduced by selective silylation of the C-l carbonyl of diketone 693 and oxidation of the silyl enol ether with Pd(OAc)2. Enone... 

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