Oxidation by selenium dioxide

To meet the needs of the advanced students, preparations have now been included to illustrate, for example, reduction by lithium aluminium hydride and by the Meerwein-Ponndorf-Verley method, oxidation by selenium dioxide and by periodate, the Michael, Hoesch, Leuckart and Doebner-Miller Reactions, the Knorr pyrrole and the Hantzsch collidine syntheses, various Free Radical reactions, the Pinacol-Pinacolone, Beckmann and Arbusov Rearrangements, and the Bart and the Meyer Reactions, together with many others. 

CgHjCOCHj + SeOa —> CgHgCOCHO + Se + H O This is one example of the oxidation by selenium dioxide of compounds containing a methylene group adjacent to a carbonyl group to thecorresponding a-ketoaldehyde or a-diketone (see also Section VII,23). 

Methyl groups at positions 2 and 4 in quinoline are selectively oxidized by selenium dioxide (76BSF789). For example, 2,3,8-trimethylquinoline gives 3,8-dimethylquinoline-2-carbaldehyde (Scheme 61). 

The reaction of phenol with isoprene dibromide and sodium affords ( )-2-isopropenyl-2,3-dihydrobenzofuran (219) in high yield (78CL253) an additional formylation reaction led to fomannoxin (220), a toxin isolated from Heterobasidion annosum (77TL651). ( )-Tremetone (221) was synthesized by the same route and oxidized by selenium dioxide in acetic anhydride to give ( )-5-acetyl-2-(l-acetoxymethylvinyl)-2,3-dihydrobenzofuran. 

Aminofiavones are also available from chalcones derived from 5-acetamino-2-hydroxyacetophenone (57JOC304). Oxidation by selenium dioxide leads to 6-acetaminoflavones which may be deacetylated to the 6-amino compound, whilst alkaline hydrogen peroxide affords the corresponding flavonols (461) in a typical Algar-Flynn-Oyamada reaction (64CB610). 

The 2-methyl-5-n-butylpyridine was oxidated by selenium dioxide in pyridine to 5-n-butyl-2-pyridine carboxylic (fusarinic) acid, melting point 100-101°C. 

Oxidation of pyrazolines, Hydroxyaryl pyrazolines (1) are oxidized by selenium dioxide in aqueous dioxane to 2 -hydroxychalcones (2) in about 50% yield. Use of other common oxidants (LTA, MnOj, KMn04, etc.) results in the corresponding pyrazoles. 

The methylene group adjacent to a ketone may be oxidized by selenium dioxide to give 1,2-dicarbonyl compounds. It is important to carry out base-catalysed condensations of ketones wherever possible under an atmosphere of nitrogen. The reason is that enolate anions are readily autoxidized by the oxygen in air to form hydroperoxides These may then undergo further reaction including decomposition to form diketones. 

Woggon, W. D., Ruther, F., Egli, H. The mechanism of allylic oxidation by selenium dioxide. J. Chem. Soc., Chem. Common. 1980, 706-708. 

Diagnosis of unsaturation types. A -Cholestenyl acetate is oxidized by selenium dioxide in acetic acid-benzene to give 7a-acetoxy-A -cholestene (2), probably by allylic hydroxylation at C14, allylic rearrangement, and acetylation. Since oxidation occurs readily at room temperature whereas cholesterol is attacked only at 55-60°, the reaction can be used to detect small amounts of A -cholestenol... 

Methyl-5,6-dihydropyrido[2,3- /]pyrimidin-7(8//)-one has been oxidized by selenium dioxide in acetic acid.210-211... 

Pyndo[2,3-()]pyrazines with a partially hydrogenated pyridine moiety have also been aromatized. 7,8-Dihydro-7-methylpyrido 2,3-6]pyrazin-6(5//)-one (7) oxidized by selenium dioxide in acetic acid affords 7-methyl pyrido[2,3-A]pyrazin-6(5//)-one (8),63 and 5,6-dihydro-5-methylpyrido-[2,3-6]pyrazine (9) is oxidized by air to give 5-methylpyrido[2,3-6]pyrazin-6(5//)-one (10).14... 

A second proof of the ring system was the formation of anhydro-methylpseudolycorine (LVIIIa) in the pyrolysis of galanthine. This product and its phenanthridone (LVIIa) were identical with material obtained in a similar fashion from methylpseudolycorine. Galanthine was oxidized by selenium dioxide to a quaternary base (LXIV) which... 

Oxidation by selenium dioxide [128], tellurium(IV) chloride [129], and ruthenium(IV) tetrakis(trifluoroacetate) [130] have also been reported for a limited number of substrates. 

Allylic oxidation by selenium dioxide 61.3.3-5.2 Oxidations by tellurium-oxo compounds... 

Many earlier methods for the preparation of tropones involved carbene addition to a benzene or reduced benzene derivative [129], but with the more ready availability of cycloheptatriene the latter compound is now the most usual starting material. The first methods involved its oxidation by selenium dioxide and provided... 

Scheme 3.23 shows another variation 1-acetonaphthone oxidized by selenium dioxide to afford the corresponding 1,2-diketone, which is called 1-naphthyl glyoxal. Subsequently, the prodnced compound was used as a precursor for preparing the new quinoxaline. 

A cheaper version of oxidation by selenium dioxide uses concentrated sulphuric acid and a small quantity of selenium which is repeatedly oxidized and reduced during the reaction. By this means, picolines and lutidines have been oxidized, and 3-picoline gives nicotinic acid 293,... 

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