Quinones cerium ammonium nitrate

Trisubstituted (3-lactams have been reported to be obtained through oxidative removal of the /V-alkyl group from more complex (3-lactams by treatment with cerium ammonium nitrate [260], Anologous methodology was employed for a general synthesis of cis- and trans-(3-lactams bearing a quinone moiety at the N-l, the C-3, or the C-4 position (I, II and III, respectively, Fig. 8), [261]. 

Despite the ionic reactions, other reactions (presented in Scheme 44) show that free-radical mechanisms can also take place. Catalyzed by ebselen, TBHP oxidation of alkylarenes to alkyl aryl ketones (141) [240], anthracene to anthraquinone (142), 1,4-dimethoxyarenes to 1,4-quinones (e.g., menadione 143) [244], and oxidative coupling of 2-aminophenol to phenoxazinone (144) [245] gave results similar to these when one-electron oxidants such as Ce(IV), Ag(II), or Mn(III) were the reagents. Moreover, oxidation of azine derived from 2-acetylpyridine gave a mixture of ketone (145) and condensed triazole (146) [240], The same result was found when cerium ammonium nitrate was used as the reagent. This suggests that the... 

The direct oxidation of arenes to quinones is a reaction with a limited scope [41], Only substrates that form stable quinones give good yields. For example, oxidation of anthracene to stable 9,10-anthraquinone with chromic acid is practiced on industrial scale. Such oxidations are believed to proceed through a series of one-electron oxidation/solvolysis steps. Yields and selectivity may be improved by using a strong one-electron oxidant such as cerium ammonium nitrate (CAN), as in the oxidation of phenanthrene to phenanthrenequinones (Eq. 9) [42]. 

Y butenolides 22 were obtained in good yields and as essentially single stereoisomers (Table 5.2). In these studies, the tat leucine derived SchifTbase ligand 18a gave rise to the best diastereo and enantioselectivities, and even with the less expensive valine derived Schiffbase ligand, the products were formed with 97% ee. Removal of the N aryl group was readily accomplished with cerium ammonium nitrate followed by acidic hydrolysis of the intermediate aza quinone. 

Oxidation of aromatic ethers to carbonyl compounds or of dimethoxy aromatics to quinones with cerium ammonium nitrate (see 1st edition). 

Irradiations should be conducted with a probe generator for an optimal rate. After an oxidative workup (with cerium ammonium nitrate in dilute acid), quinones are isolated in yields at least equivalent to those of conventional methods. No indication is given to the regioselectivity of the reaction in relevant cases. An interesting comparison is given with the results of irradiation with microwaves. 

Monofluorinated hydroquinone and quinone are obtained by stepwise oxidation of 2-fluorophenol(l) using different oxidation agents.167 In the first stage, potassium peroxydisulfate is applied and the resulting hydroquinone 2 is further oxidized to 3 by ammonium cerium(IV) nitrate.167... 

Electrophilic aromatic substitution of 5-hydroxy-2,4-dimethoxy-3-methylaniline by reaction with the iron complex salts affords the corresponding aryl-substituted tricarbonyliron-cyclohexadiene complexes. O-Acetylation followed by iron-mediated arylamine cydization with concomitant aromatization provides the substituted carbazole derivatives. Oxidation using cerium(IV) ammonium nitrate (CAN) leads to the carbazole-l,4-quinones. Addition of methyllithium at low temperature occurs preferentially at C-1, representing the more reactive carbonyl group, and thus provides in only five steps carbazomycin G (46 % overall yield) and carbazomycin H (7 % overall yield). 

Cerium(IV) ammonium nitrate has been used increasingly during the last decade in the synthesis of quinones, particularly as a means of effecting oxidative demethylation of methoxyarenes. 

The inhibition of thymidylate synthase is recognized as a viable tqiproach to the control of cancer and DNA viral infections. Mertes and coworkers proposed S-quinone derivatives of 2 deoxyuridine S -I iosphate as potential irreversible inhibitors of this enzyme and prepared the quinones (32a) by oxidative demethylation of the appropriate dimethoxyarenes using CAN. Interestingly, the quinones (32b) were also prepared from the corresponding dimethoxyarenes but using silveifU) oxide and nitric acid in aqueous dioxane to effect this transformation. Cerium(IV) ammonium nitrate could not be used, due to precipitation. 

Catechols and hydroquinones can be converted (91-98%) into quinones by cerium(IV) salts coated onto silica as free-flowing yellow powder from impregnation with cerium(IV) ammonium nitrate. This reaction is usually performed in the presence of magnesium sulfate. The same (NH4)2Ce(N03)6 SiQ2 reagent in the dry state effects oxidative nitrations of arenes. For example a-mq>hthol is converted to the Orth) (42%) and the para (38%) nitro compounds, while its metiiyl or ethyl ethers give exclusively the para nitration product (equation 4). In solution, the products are cmtaminated with the products of dinitration and of oxidation into quinones. ... 

Quinones can he prepared hy the oxidation of phenols, dihydroxy-henzenes, dimethoxyhenzenes and anilines. For example, 1,4-dihydroxy-henzene (hydroquinone) can he oxidized in good yield using sodium chlorate in dilute sulfuric acid in the presence of vanadium pentoxide and also hy manganese dioxide and sulfuric acid and hy chromic acid. Other reagents which convert hydroquinones to quinones include Fremy s salt [potassium nitrosodisulfonate, (KS03)2N0] and cerium(IV) ammonium nitrate [CAN, Ce(NH4)2(N03)J. 

Cerium is a member of the lanthanides in the Periodic Table and adopts tetra- and tripositive states in its electronic configuration. Among cerium reagents, ceric ammonium nitrate (CAN) is most widely used in organic synthesis. It is well known to convert phenol derivatives to quinones in high yields under mild conditions. An excellent review on cerium(IV) oxidation of organic compounds is available, and only a few examples will be described herein. 

A single case of a cycloproparene oxidation proceeding without destruction of the molecular skeleton has been reported. 3,6-Dimethoxy-l/f-cyclopropa[h]naphthalene reacted with ammonium cerium(IV) nitrate to give the quinone 2. ... 

N03)j, a newcomer to the arena of oxidants, is useful for the acetoxylation of aromatic side chains in benzylic positions [415, 416] and for the oxidation of methylene or methyl groups that are adjacent to aromatic rings to carbonyl groups [238, 415, 417]. The reagent also oxidizes alcohols to aldehydes [418, 419, 420, 421] and phenols to quinones [422, 423], cleaves vicinal diols to ketones and a-hydroxy ketones to acids [424, 425], and converts diaryl sulfides into sulfoxides [426]. A specialty of ammonium cerium nitrate is the oxidative recovery of carbonyl compounds from their oximes and semicarbazones [422, 427] and of carboxylic acids from their hydrazides [428] under mild conditions. 

Reactions of ammonium hexanitratocerate(IV) with organic substrates other than benzyl alcohol have also been examined, and 1,4-hydroquinone was quantitatively transformed into 1,4-quinone. Anisole and naphthalene can be nitrated. For the cerium-mediated oxidation reactions in ionic liquids, high reaction temperature is beneficial because of the formation of smaller amounts of by-products. 

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