Carboxylic acids, esters oxidative rearrangement

Electron-attracting substituents should assist this reaction. However, with acetic anhydride in acetonitrile, 2-picolinic acid 1-oxide gives mainly pyridine 1-oxide and carbon dioxide, whilst the same reaction carried out under ultra-violet irradiation proceeds similarly but with the formation of a little more 2-hydroxypyridine. The methyl esters of the pyridine-carboxylic acid 1-oxides behave normally, but there is no evidence that the methoxycarbonyl group promotes the reaction, and 2-cyanopyridine 1-oxide does not react with acetic anhydride . The results of a kinetic study of the rearrangement of pyridine 1-oxide in acetic anhydride exclude the intramolecular rearrangement of the free cation (113) and also a free radical process. There remain the two possibilities of nucleophilic substitution by reaction between... 

Carboxylic acids with an electron donating substituent in the a-position decarboxylate in a two-electron oxidation to carbocations (see chap. 7). These can react with the solvent (alcohol, acetic acid, water) or the unreacted carboxylate to ethers, esters, or alcohols (Eq, 14). In some cases the carbon skeleton rearranges, which is a clear indication of the cationic pathway. 

The main synthetic application of the Wolff rearrangement is for the one-carbon homologation of carboxylic acids.242 In this procedure, a diazomethyl ketone is synthesized from an acyl chloride. The rearrangement is then carried out in a nucleophilic solvent that traps the ketene to form a carboxylic acid (in water) or an ester (in alcohols). Silver oxide is often used as a catalyst, since it seems to promote the rearrangement over carbene formation.243... 

The benzotriazolyl derivative of acrolein acetal, compound 882, is lithiated, treated with chlorodiphenylphosphine, and the obtained intermediate is oxidized with hydrogen peroxide to phosphine oxide 883 (Scheme 145). The relatively acidic proton in derivative 883 is easily removed by a base, and the obtained anion adds to a carbonyl group of aldehyde or ketone. Subsequent rearrangement and elimination of the phosphorane group generates diene 884. For the derivatives of aldehydes (884, R2 = H), (E)-(E) stereoselectivity of the elimination is observed. Acidic alcoholysis of dienes 884 affords esters of P,y-unsaturated carboxylic acids 885 < 1997JOC4131>. 

The acidity of the propargylic proton of the starting compound 18 allows the equilibration with the allene 19 induced by bases such as tertiary amines or alcoholates (Scheme 7.4). Such prototropic rearrangements furnish the title compounds 19 with at least one proton at the terminal carbon atom, often in good yields. The EWG group involves carboxylic acids [33], esters [34], ketones [35, 36], isonitriles [37], sul-fones [38], sulfoxides [39, 40] and phosphonates [41], The oxidation of easily accessi-... 

Oxidative hydrolysis transforms the intermediate ozonide into ketone(s) and/or carboxylic acid(s) in good yields. H202 in water, in sodium hydroxide solution, or in formic acid is the best proven oxidant.582,584,592 Peroxy acids and silver oxide are also employed. Rearrangement and overoxidation may be undesirable side-reactions. A simple two-step ozonation in MeOH yields methyl esters without added oxidizing agent.627... 

Activation by addition of a carboxylic acid to a triple bond occurs with ethyl ethynyl ether,which forms amides via reactive enol esters. The reaction is catalyzed by mercury(II) oxide under almost neutral conditions. Push-pull alkynes exert higher reactivityThe intermediate enol esters (Scheme 4) rearrange and react with the amino function of a second amino acid. Hydroxy, thiol and imidazole functional groups do not have to be protected. The degree of racemization is low, and yields are good in the case of small peptides. 

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