Preparation 1,2-diketones

This approach leads directly to the enone needed for nootkatone. A diketone prepared from a natural terpene (Chapter 51) is also treated with HC1 and much the same reactions ensue except that the fragmentation now breaks open a four-membered ring. First, the intramolecular aldol reaction to make the second six-membered ring. 

Diketones Prepared from Silyl Enol Ethers and Nitroolefins... 

In this synthesis of 1,5-dicaibonyl compounds, 3-butenyl halide is behaving as a madced 3-oxobutyl reagent, and can be used as an equivalent of metiiyl vinyl ketone. These reactirais offer new anellation methods. Also 1,4-addition of the allyl group to enones, followed by oxidatitm. offers a conveiuent synthetic method for 1,5-diketone preparation. Lewis acid promoted Nfichael addition of allylsilane (48) to a,p-unsaturated ketones, followed by the palladium-catalyzed oxidation, affods 1,5-diketones (Scheme 17). 3... 

The dioxime of a cyclic 1,2-diketone (prepared in this example by successive nitrosation and oximation of the 1,3-cyclohexanedione) is oxidized to a furoxan (l,2,S-oxadiazole 2-oxide) the isomeric N-oxide is formed by mild treatment of the dioxime with thionyl chloride [2496]. 

Triketone mono-dithioacetals, 5-diketones prepared from 2-acyl-1,3-dithianes (Section... 

To a 200-mL round-bottom flask equipped with a stirring bar and an addition funnel was added 5 g (17.7 mmol) of the diketone prepared above dissolved in 60 mL of acetone and 12 mL of dimethyl sulfate (DMS). The mixture was stirred while a solution of 20% KOH in methanol was added at the rate of 1-2 drops/10 s. TLC indicated that as the starting material was being consumed, both a more polar intermediate and a less polar product began to appear. After the addition of an additional 10 mL DMS, the reaction mixture was poured into ether and washed with water (2 X). The organic extracts were concentrated and residual DMS was removed under high vacuum. The dark oily residue that remained was chromatographed on sihca gel (flash column... 

Synthesis. This next set of compounds more closely resemble a-T than the previously discussed class of compounds. These bithiophene derivatives, 2, can be synthesized by the two different routes shown in Scheme II. Each route has its advantages and disadvantages. The nickel catalyzed couplings were discussed above. Alternatively, these compounds can be synthesized by ring closure of a 1,4-diketone. Preparation of tri-arylthiophenes by this latter route leads to a mixture of thiophene and furan. These reactions are published and will not be discussed further in this chapter (16,12). 

Secondary Amines.—The conversion of primary amines into secondary amines, either directly or via intermediates such as imines, is of considerable synthetic value, and it is therefore not surprising that an abundance of methods for this conversion has again been reported. For example, lithium aluminium hydride reduces monoimines of a-diketones, prepared from a benzil and a primary amine under zinc chloride catalysis, to the corresponding jS-amino-alcohol. Several new reagents which effect the reduction of imines to secondary amines in high... 

Hojo reported a selective synthesis of 4-(trifluoromethyl)imidazoles (Scheme 35) [49], The diketones, prepared from aldehyde dialkyl hydrazones via the hydrolysis of 3-(dialkylhydrazino)-l,l,l-trifluoro-2-aIkanone intermediates, were treated with 1,1-dimethylhydrazine to afford the adduct. These adducts were generally unstable even at room temperature and, thus, were immediately dehydrated using POCI3-pyridine to afford 3-aryl-l,l,l-trifluoropropane-2-,3-dione-2-dimethylhydrazone. Intramolecular cyclization of these hydrazones in refluxing toluene afforded the desired 4-(trifluoromethyl)imidazoles. 

The synthesis of the precursors was carried out via two different approaches. In method A 66a,b with their substituents connected through the C-C bond were prepared by condensing 4,5-dicyano-l, 2-DAB with appropriately substituted vicinal diketones prepared similarly to the procedures (1995TL7305, 2007JPP(A)316). Acid was used as a solvent. In method B the intermediate 2,3-dichloroquinoxaline-6,7-dicarbonitrile 138 (2007EJOC4535) was used for the synthesis of the heteroatom-linked peripheral chain bearing precursors 66c-e. The chlorine atoms on the... 

This reaction applies to many i,2 diketones, and is termed the Benzilic Acid Rearrangement. It provides a ready method for the preparation of disubstituted a4iydroxy-carboxylic acids. When applied to a cyclic 1,2-diketone, the ring system is necessarily reduced by one carbon atom for example, cyclohexan-i,2 ... 

This synthesis of the pyrrole ring system, due to Knorr, consists in the condensation of an a-aminoketone with a 1,3-diketone or the ester of a p-keto-acid, a-Aminoketones are unstable in the free state, readily undergoing self-condensation consequently they must be prepared, by the reduction of an a-nitroso (or oximino) ketone, in the presence of the 1,3-diketone or p-ketoester, to ensure rapid interaction. 

The preparation of benzoylacctone Is another example of the acylation of a ketone (acetophenone) by ethyl acetate to a p diketone (Claisen condensation compare preceding Section) ... 

Several 1,4-dicarbonyl compounds are prepared based on this oxidation. Typically, the 1,4-diketone 10 or the 1,4-keto aldehyde 12 can be prepared by the allylation of a ketone[24] or aldehyde[61,62], followed by oxidation. The reaction is a good annulation method for cyclopentenones (11 and 13). Syntheses of pentalenene[78], laurenene[67], descarboxyquadrone[79], muscone (14 R = Me)[80]) and the coriolin intermediate 15[71] have been carried out by using allyl group as the masked methyl ketone (facing page). 

The o -diketone 865 can be prepared by the coupling of the acylstannane 864 with acyl chlorides[738,739]. The a-keto ester 868 is prepared by the coupling of (a-methoxyvinyl)tributylstannane (866) with acyl chloride, followed by ozo-nization of the coupled product 867[740,741],... 

The acylstannanes 874 and 875 are prepared by the reaction of acyl chlorides with (MciSnl - The symmetrical 1,2-diketones 877 can be prepared by the reaction of an excess of benzoyl chloride with (EtjSn) . Half of the benzoyl chloride is converted into the benzoyltin reagent 876, which is then coupled with the remaining benzoyl chloride under a CO atmosphere to afford the a-diketone 877[748], Triethyl phosphite is used as a ligand. 

Akylsilanes are more reactive than vinylskanes in Friedel-Crafts reactions, as shown in the selective acylation of 2,3-disilylalkenes. The akylsilanes, a-skyloxyakyltrialkylsilanes, have been used as enolate equivalents in the preparation of 1,4-diketones (178). The mild reaction conditions required for these reactions tolerate many other functional groups, providing valuable synthetic routes. 

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