Esters selenium dioxide

A number of esters of arsonic and arsinic acids have been prepared. One method involves the oxidation of dialkyl alkylarsonites with selenium dioxide ... 

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 is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

Cinnamyl alcohols.1 These alcohols can be prepared by oxidation of 3-arylpropenes with selenium dioxide in dioxane. The yield compares favourably with that oblained by LiAlH4 reduction of esters of cinnamic acids. 

T he oxidation of olefins by selenium dioxide has received much atten-- tion because of the unique characteristics of the reaction that produces an allylic derivative of the olefin (ester, alcohol, or ether, depending upon the solvent) and elemental selenium as the final reduced state of the oxidant. 

It has been generally accepted (5, 6) that the oxidation of olefins with selenium dioxide proceeds through attack on the olefinic double bond by selenium dioxide followed by rearrangement and solvolysis of the intermediate selenium (II) ester formed. 

Synthetic methods for 2(5jF/)-furanones have been developed in the preparation of cardenolides (65MI31200). The ketone (171) when reacted with lithium ethoxyacetylide gives the carbinol (172) which undergoes acid catalyzed rearrangement to the a,/3-unsaturated ester (173). Allylic oxidation of (173) with selenium dioxide under mild conditions gives digitoxigenin acetate (174) (Scheme 38). 

To obtain these products selenium dioxide is allowed to react with the hydrogenated cinchona alkaloids or their derivatives in the presence of concentrated sulphuric acid and the products obtained are diluted with water and boiled. Selenohydroquinine is prepared from hydro-quinine sulphate or hydroquinine sulphuric ester, and forms yellow needles which remain unchanged below 235° C. selenoethylhydro-cupreine forms yellow needles, 5l.pt. 233° to 23-1° C., and selenohydro-cupreine separates as small, orange-coloured needles, w hich are unmelted below 235° C. The products are of use therapeutically. 

Although the oxidation of RAs(OR>2 with selenium dioxide leads to the esters of the corresponding arsenic(V) acids, Wieber et al. (192) have shown that oxidation of the cyclic esters with SeC>2 in the presence of a diol results in compounds of type XXVII ... 

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

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). 

Derivatives of phosphonic acids, RP==O(0H)2, can be prepared by several different oxidative methods. Primary phosphines RPH2 are oxidized to phosphonic acids by hydrogen peroxide or by sulfur dioxide thus, phenylphosphine gave benzenephosphonic acid (96%) on reaction with sulfur dioxide at room temperature in a sealed tube. Phosphinic acids, RI sO(OH)H, can also be oxidized to the corresponding phosphonic acids with hydrogen peroxide. Ozone oxidized the dioxaphosphorane (54) to the phosphonic ester in 73% yield. Ozone is also capable of stereospecific oxidation of phosphite esters to phosphates. For example, the cyclic phosphite (SS) was oxidized to the phosphate (56) with retention of configuration. Peroxy acids and selenium dioxide are other common oxidants for phosphite esters. 

Selenium dioxide oxidation of alkenes with a hydrogen in an a-position involves the formation of the allyl selenic ester (X = OH) by an ene reaction. [2,3] Sigmatropic rearrangement of the allyl selenic ester to the selenium(II) ester and its hydrolysis also resulted in the formation of allylic alcohols. The oxidation of alkenes with selenium dioxide is covered in Section D.4.10. 

Compounds having methylene groups situated between two activating groups—ketone, acid, or ester—are readily oxidized with selenium dioxide to furnish triketones, keto diesters, a,/3-diketo esters,or a-keto acids. ... 

Selective oxidation. In a synthesis of d/-sirenin (3) Rapoport el al. found that the oxidation of the unsaturated ester (1) with selenium dioxide in ethanol proved highly selective and gave a 55% yield of the ira/>s- ,/l-unsaturated aldehyde (2). This product was converted into (//-sirenin in 86% yield by rednction with mixed hydride. 

Some 16-substituted cardenolides have also been prepared.16) -Methyl-3/ -acetoxy-17a-pregnan-20-one (514) was converted into the unsaturated ester (515) by a Reformatsky reaction followed by dehydration. Selenium dioxide oxidation then gave the 16)S-methyl-17a-cardenolide (516). In the same way, 16a-cyano-3/3-acetoxy-17/l-pregnan-20-one (517) was transformed to the 16a-cyano-17)S-cardenolide (518). Failure to achieve the Reformatsky reaction on (519) was explained as a result of steric interference by the 16a-methyl group. 

Before the discovery of the applications of selenium dioxide and dimethyl sulfoxide, oxidations of methylene groups in 3-diketones or p-keto esters were achieved with nitroso compounds. Thus acetylacetone boiled with p-nitrosodimethylaniline in alcoholic sodium hydroxide gives trike-topentane in 55% yield [986],... 

In chemical oxidation or reduction the redox reagent and the substrate often form a covalent or ionic bond, for example, an ester in chromic acid oxidation [8], a sulfonium methylide in the Swern oxidation [9], cyclic esters in the svn dihydroxylation with OSO4 [10], or in the selenium dioxide oxidation of ketones and aldehydes [11]. In electrochemical processes the substrate must diffuse from the bulk of the solution to the electrode and compete there with other components of the electrolyte by competitive adsorption for a position at the electrode surface [12]. The next step is then generation of the reactive intermediate by electron transfer at the electrode that reacts with a low activation energy to the products. In chemical oxidations or reductions one finds a reductive or oxidative elimination of the intermediate with a higher activation energy. 

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