Fragmentation reactions, loss ketone

The diketone first tautomerizes to the keto-enol. Protonation of the ketone gives a very stable carbocation, to which EtOH adds. Deprotonation of the nucleophile, protonation of the OH leaving group, and loss of H20 gives a new carbocation, which undergoes a fragmentation reaction with loss of H+ to give the enol ether product. 

There are three potential options for the mechanism of Grob fragmentation reactions, one concerted and two stepwise. The stepwise reactions, which result from initial loss of either the nucleofuge or electrofuge, are less useful in synthesis since they often promote side reactions. However, one highly efficient stepwise transformation was reported by Kato. Treatment of diol 6 with pyridinium chloride results in formation of benzyl carbocation 8. Cleavage of this alcohol yields monocycle 9 and, ultimately, the ketone product 10. ... 

Ketones constitute another important class that will yield radicals on irradiation. Nonconjugated ketones absorb weakly in the ultraviolet at 270 nm and decompose with cleavage at the carbonyl group as indicated in Equation 9.31. The carbonyl radical may then fragment further with loss of carbon monoxide according to Equation 9.32. Although this reaction can serve as a source of the... 

The reaction starts with formation of the tosylhydrazone from the epoxy-ketone. The tosylhydra-zone is unstable with respect to opening of the epoxide in an elimination reaction, and it is this elimination that sets up the familiar 1, 2, 3, 4 system ready for fragmentation. The push comes from the newly created hydroxyl group, and the pull from the irresistible concerted loss of a good leaving group (Ts ) and an even better one (N2). Notice how all the (green) bonds that break are parallel to one another, held anti-periplanar by two double bonds. Perfect ... 

When radical pairs are produced by a cleavage of ketones, and the loss of CO is prevented on energetic grounds, fragments of the reactions of the primary radical pair become important. Deoxybenzoins (15)30) or adamantyl ketones (20)25,30) provide examples of such systems. The photodecomposition of l,2-diphenyl-2-methyl-pro-panone (15)30) illustrates an example of a photofragmentation in reaction (Scheme VIII). The ratio of the major products are highly dependent on solvent environment. For example, the photochemistry of this compound in acetonitrile or benzene yields... 

The retro-ene reaction cleaves an unsaturated compound into two unsaturated fragments. A common example of a retro-ene reaction in organic synthesis is the acid-catalyzed decarboxylation of a (B-ketoester. The ester is hydrolyzed to the (3-ketoacid by the aqueous acid, which rapidly loses carbon dioxide to form enol. The loss of CO2 drives the reaction to the right-hand side. The enol rapidly tautomerizes to the methyl ketone (Scheme 8.17). 

Nature uses the two-carbon acetate fragment of acetyd CoA as the major building block for synthesis. Acetyl CoA can act not only as an electrophilic acceptor, being attacked by nucleophiles at the carbonyl group, but also as a nucleophilic donor by loss of its acidic a hydrogen. Once formed, the enolate ion of acetyl CoA can add to another carbonyl group in a condensation reaction. For example, citric acid is biosynthesized by nucleophilic addition of acetyl CoA to the ketone carbonyl group of oxaloacetic acid (2-oxob i-tanedioic acid) in a kind of mixed aldol reaction. 

Several 21,22-seco-diacids (150) have been prepared in the 20/S,28-epoxy-18a,19/8H-ursane series by oxidation of the corresponding 22-hydroxymethylene-21-ketones (151). Reaction of the 21-ketone (152) with oxygen in an alkaline medium afforded the a-hydroxy-acid (153) and the seco-diacid (150). ° Several interesting reactions were observed during this work. ° While pyrolysis of the a-acetoxy-acid (154) yielded the ketone (155) pyrolysis of the seco-diacid anhydride resulted in loss of carbon monoxide and formation of the lactone (156). Lead tetra-acetate oxidation of the ketone (155) [or the hydroxy-acid (153)] followed a Baeyer-Villiger pathway to the lactone (156). The ketone (155) was very susceptible to reduction in the presence of alcoholic alkali. The mass spectral fragmentation of a series of compounds based on 20/8,28-epoxy-18a,19/8H-ursane has been examined. ... 

Formaldehyde and other aldehydes and ketones formed during the atmospheric oxidation of more complex organic compounds are photolysed with a loss of a hydrogen atom or an alkyl radical. Photolysis of ketones (6.59) provides one way in which C-C bonds break another is the fragmentation of alkoxyl radicals mentioned earlier (reaction 6.40). Eventually, all organic compounds end up as carbon dioxide and water. 

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