Selenium dioxide Enol oxidation

The preparation of Pans-1,2-cyclohexanediol by oxidation of cyclohexene with peroxyformic acid and subsequent hydrolysis of the diol monoformate has been described, and other methods for the preparation of both cis- and trans-l,2-cyclohexanediols were cited. Subsequently the trans diol has been prepared by oxidation of cyclohexene with various peroxy acids, with hydrogen peroxide and selenium dioxide, and with iodine and silver acetate by the Prevost reaction. Alternative methods for preparing the trans isomer are hydroboration of various enol derivatives of cyclohexanone and reduction of Pans-2-cyclohexen-l-ol epoxide with lithium aluminum hydride. cis-1,2-Cyclohexanediol has been prepared by cis hydroxylation of cyclohexene with various reagents or catalysts derived from osmium tetroxide, by solvolysis of Pans-2-halocyclohexanol esters in a manner similar to the Woodward-Prevost reaction, by reduction of cis-2-cyclohexen-l-ol epoxide with lithium aluminum hydride, and by oxymercuration of 2-cyclohexen-l-ol with mercury(II) trifluoro-acetate in the presence of ehloral and subsequent reduction. ... 

Selenium dioxide can be used to oxidize ketones and aldehydes to a-dicarbonyl compounds. The reaction often gives high yields of products when there is a single type of CH2 group adjacent to the carbonyl group. In unsymmetrical ketones, oxidation usually occurs at the CH2 that is most readily enolized.255... 

The oxidation is regarded as taking place by an electrophilic attack of selenium dioxide (or selenous acid, H2Se03, the hydrate) on the enol of the ketone or aldehyde. This is followed by hydrolytic elimination of the selenium.258... 

Oxidation of the sulfur- or selenium-bridged azepines (171 X=S or Se) with mercury(II) oxide in methanol yields ultimately the 4f/-azepine (68JCS(C)23ll) with hydrogen peroxide as oxidant, the sulfur compound furnishes the sulfoxide (171 X=SO). Selenium dioxide oxidation of 7,8-dimethyl-lf/-l-benzazepin-2-one affords the 2,3-dioxo derivative (173) that displays no evidence of enol tautomers or heteroaromaticity (7ici(L)1439). 

Selenium dioxide is a most useful reagent for the oxidation of ketones or aldehydes to a-dicarbonyl compounds along with a,)3-unsaturated carbonyl compounds as by-products.291,293 The carbonyl compound probably reacts in its enol form in a way similar to that of alkene oxidation (equation 130).358... 

Selenium dioxide is able to a-oxygenate ketones via their enol tautomers. As is demonstrated in Figure 12.10 by the reaction of selenium dioxide with cyclohexanone, the actual electrophilic substitution product C is unstable. The latter contains selenium in the oxidation state +2 that takes the opportunity to transform into selenium in the oxidation state 0, i.e., elemental selenium, by way of the fragmentation reaction indicated. Thereby, the a-C O single bond of the primary product C is transformed into the a-C=0 double bond of the final product B (which, however, is largely present as the tautomeric enol A). 

Mechanism The reaction of the enol form of the carbonyl compound A with selenium dioxide gives selenous enol ester B. The oxidative rearrangement of selenous enol ester B gives C. Loss of selenium and water from C gives the dicarbonyl compound (Scheme 7.16). 

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. 

Methyl and methylene groups adjacent to carbonyl groups are easily oxidized to carbonyls to yield a-keto aldehydes or a-diketones. The reagent of choice is selenium dioxide or selenious acid. The reaction is catalyzed by acids and by acetate ion and proceeds through transition states involving enols of the carbonyl compounds [518]. The oxidation is carried out by refluxing the ketone with about 1.1 mol of selenium dioxide in water, dilute acetic acid, dioxane, or aqueous dioxane [517]. The byproduct, black selenium, is filtered off, but small amounts of red selenium sometimes remain in a colloidal form and cannot be removed even by distillation of the product. Shaking the product with mercury [523] or Raney nickel [524] takes care of the residual selenium. The a-dicarbonyl compounds are yellow oils that avidly react with water to form white crystalline hydrates (equations 407 and 408). 

Another preparation of 2,5-dimethylfuran-3-ol (41) is included for its exceptional simplicity, even though it lacks versatility. Biacetyl (2,3-butan-dione) dimerizes in alkali to form an aldol converted by acid to the desired product in yields rising 60%.102 A very simple method has also been published for converting commercially available 2,3-dihydro-5-methylfuran to 5-methylfuran-3(2H)-one, tautomeric with 5-methylfuran-3-ol, by means of oxidation with selenium dioxide. According to the H-NMR spectrum, the product is 80%, ketone, 20% enol.103 The furanone is produced when 2-deoxy-D-ribose is treated with acid.104... 

Avoidance by choice of oxygenated starting materials Oxidation through Lithiation and Ort/ro-Lithiation Hydroxylation of Pyridines by ortho-Lithiation Synthesis of Atpenin B Introducing OH by Nucleophilic Substitution Part II - Oxidation of Enols and Enolates Direct Oxidation without Formation of a Specific Enol Selenium dioxide Nitrosation with nitrites Nitrosation with stable nitroso compounds Indirect Oxidation with Formation of a Specific Enol Enone Formation Pd(II) oxidation ofsilyl enol ethers Bromination of enols in enone formation Sulfur and selenium compounds in enone formation Asymmetric Synthesis of Cannabispirenones... 

Direct oxidation of carbonyls to 1,2-dicarbonyls can be carried out with selenium dioxide (Se02) though there are selectivity problems. The reaction starts with an ene reaction 73 forming the enol derivative 74. This undergoes a [2,3] sigmatropic shift 74 to form the C-0 bond and loss of selenium and water 75 gives the final product 76. 

A second major application of selenium dioxide is the oxidation of aldehydes and ketones to the corresponding l,2-dicarbonyl.533 7 55 conversion proceeds by reaction of the enol form of the carbonyl (386) with selenium dioxide to give the selenous enol ester, 387. Oxidative rearrangement to 388 is followed by loss of... 

Finally, acid-catalyzed enolization of aldehydes and ketones in the presence of selenium dioxide (SeO ) also results in oxidation a- to those carbonyl functionalities via the corresponding enol. As shown in Scheme 8.14, the reaction of the enol with Se02 followed by the loss of selenium(II) oxide (which subsequently... 

The homotropolone (327a) has been prepared by selenium dioxide oxidation of cyclo-octadienone. Its enol acetate (327b) has been shown to exist as an equilibrium mixture of 80% (327b) and 20% of the valence isomer (328). 

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