Enols Selenium oxide oxidation

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... 

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. " The 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. ... 

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... 

The reaction of camphor enolate with selenium in the presence of methyl iodide,19,20 followed by areal oxidation, affords 1,3-diselenetanes formed by dimerization of the putative selenoketone intermediate (Scheme 22). These... 

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... 

In another simple procedure, deprotonation of methoxy bis(trimethylsilyl)methane with butyl lithium and addition of the resulting anion to aldehydes induces Peterson elimination (Scheme 27). The product methyl enol ethers could be hydrolysed to the parent acyl silanes with hydrochloric acid-THF or could be treated with electrophiles such as M-halosuccinimides to give a-haloacyl silanes105. Alternatively, treatment with phenyl selenenyl chloride, oxidation at selenium and selenoxide elimination afforded a,/3-unsaturated acyl silanes. 

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). 

The most versatile method for introduction of the selenenyl moiety is low tenqperatuie reaction of the enolate anion or an enolic derivative with a suitable selenium species, the precise conditions being dqpendent on the reactivity of both the carbtmyl compound and the selenium species. - Like sulfur, seloiium may be introduced dther in the divaloit state and subsequently oxidize or, more recently, as the selenoxide directly (Schone 14)7 The choice of method is determined by the subsequent reactions that need to be carried out. 

The aromatization of cyclohexenones is an important process that can be easily accomplished by the use of selenium-based reagents using similar techniques to those previously discussed for other carbonyl species. Thus, enolates derived from a,3-enones readily undergo selenenylation at the a -position and on oxidation and elimination afford the corresponding phenols. ... 

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). 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

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). 

The first total synthesis of the marine polycyclic ether toxin (-)-gambierol was accomplished in the laboratory of M. Sasaki. The introduction of the a,(3-unsaturation into the seven-membered H ring of the FGH tricyclic subunit proved to be problematic, because both the conventional selenium-based method and the Nicolaou oxidation with IBX failed. However, when the seven-membered ketone was treated with LiHMDS in the presence of TMSCI and EtsN, the corresponding silyl enol ether was formed, which was oxidized under Saegusa conditions to give the desired cyclic enone in high yield. Because of the small scale of the reaction, a large excess of Pd(OAc)2 was used in acetonitrile so the presence of a co-oxidant was not necessary. 

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... 

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