Pyridines manganese dioxide

The oxidation of organic compounds by manganese dioxide has recently been reviewed. It is of limited application for the introduction of double bonds, but the advantages of mildness and simple workup make it attractive for some laboratory-scale transformations. Manganese dioxide is similar to chloranil in that it will oxidize A -3-ketones to A -dienones in refluxing benzene. Unfortunately, this reaction does not normally go to completion, and the separation of product from starting material is difficult. However, Sondheimer found that A -3-alcohols are converted into A -3-ketones, and in this instance separation is easier, but conversions are only 30%. (cf. Harrison s report that manganese dioxide in DMF or pyridine at room temperature very slowly converts A -3-alcohols to A -3-ketones.)... 

The procedure is experimentally simple, and the workup involves only the destruction of the traces of hydrogen peroxide with manganese dioxide and evaporation of the hexamethyldisiloxane. Pyridine additives serve to buffer the highly acidic rhenium species and to shut down the detrimental acid-catalyzed epoxideopening pathways. The scope of this transformation is best appreciated through the examples presented in Table 12.2 [28],... 

Maleic anhydride Manganese dioxide Alkali metals, amines, KOH, NaOH, pyridine Aluminum, hydrogen sulfide, oxidants, potassium azide, hydrogen peroxide, peroxosulfuric acid, sodium peroxide... 

Electrophilic substitution of 3-methoxy-4-methylaniline (655) by the complex 663 leads to the molybdenum complex 664. Oxidative cyclization of complex 664 with concomitant aromatization using activated commercial manganese dioxide provides 2-methoxy-3-methylcarbazole (37) in 53% yield (560). In contrast, cyclization of the corresponding tricarbonyliron complex to 37 was achieved in a maximum yield of 11 % on a small scale using iodine in pyridine as the oxidizing agent (see Scheme 5.49). 

Other chemical reagents that have been used to dehydrogenate are diphenyl disulfide for 1,2,3,4-tetrahydrocarbazole itself, N-bromosuccinimide in pyridine for 1-ethoxycarbonyl-1,2,3,4-tetrahydrocarbazole, selenium dioxide for 9-methyl-1,2,3,4-tetrahydrocarbazole (a 1 5 mixture of 9-methyl-carbazole and 1-oxo-1,2,3,4-tetrahydrocarbazole was obtained ), and manganese dioxide to aromatize 1-methyl- and l,4-dimethyl-6-alkoxy-3-formyl-1,2,3,4-tetrahydrocarbazoles and 1,9-diprenyl-1,4-dihydrocarba-zole. ... 

Carbodiimides undergo a [4-1-2] cycloaddition reaction to form pyrrolo[3,4-f]pyridine derivatives <1998J(P1)3065>. Dehydrogenation of the cycloadduct in the presence of manganese dioxide gives improved yields (Scheme 8). 

Activated manganese dioxide, which is generally used for the selective oxidation of allylic alcohols, permitted the synthesis of an unsaturated (3-keto-arabino-hexopyranosyl)pyridine.13 New oxidative systems for alcohols, involving molecular sieve-assisted... 

Treatment of the unsaturated C-nucleoside 69 with activated manganese dioxide afforded 2-(l,5-anhydro-2,6-dideoxy-L-m/thro-hex-l-eni-tol-l-yl-3-ulose)-8-nitro-u-triazolo[l,5-fl]pyridine (70) in 40% yield.13... 

The side chain in fluorinated 2-(hydroxymethyl)pyridines is selectively oxidized using manganese dioxide, giving the corresponding pyridine-2-carbaldehydes 4.259... 

A very mild Oppenauer oxidation using acetone and aluminium isopropoxide allows the obtention of the desired ketone, while pyridine-chromic acid or manganese dioxide produce, aromatization of the ring on the left. 

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