Selenium dioxide allylic oxidation with

Among eariy synthetic exanqiles of selenium dioxide allylic oxidation s is Rapopoit s synthesis of siienin (. S Oxidation of (7) with selenium dioxide in ethanol at 90 C for 13 h gave a mixture of... 

ALLYLIC OXIDATION WITH HYDROGEN PEROXIDE-SELENIUM DIOXIDE frans-PINOCARVEOL... 

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. 

This procedure represents an efficient method for the preparation of rearranged allylic alcohols from allies similar to allylic oxidations with singlet oxygen but complementary to those obtained from selenium dioxide. The reagent must be prepared in situf ... 

The cleavage of the intermediate allyl sulfenate can also be initiated under acidic conditions, e.g., by using catalytic amounts of 4-toluenesulfonic acid in ethanol or with thiophenol in dioxane5. By a combination of this method with the selenium dioxide catalyzed oxidation of allylic sulfides, a one-pot transformation of allylic sulfides to rearranged allylic alcohols was achieved. 

Considering the examples of allylic oxidation with the selenium dioxide derived reagents presented above it is of interest to note that the selective oxidation of primary allylic alcohols 60 to the corresponding a, -unsaturat-ed aldehydes 61 was achieved [23] using an Se02/(TBHP)/Si02 system, while secondary allylic, benzylic and saturated alcohols remained unaffected (Eq. 10). 

Rapoport, H., Bhalerao, U. T. Stereochemistry of allylic oxidation with selenium dioxide. Stereospecific oxidation of gem-dimethyl olefins. J. Am. Chem. Soc. 1971,93, 4835 840. 

Treatment of the fraw.Mrart.s-germacranolide, epitulipanolide (477) and two of its derivatives with selenium dioxide and t-butyl hydroperoxide gives the melampolide-type alcohol (478).218 A mechanism for this allylic oxidation with inversion of configuration at the A1-10 double bond is proposed. It should be noted, however, that this is not the first report of such a process (cf. Vol. 11, p. 55). 

The cuprous iodide procedure has also been used in a simplified synthesis of dl-sirenin (11) from the diazo compound (9). Thus, when (9) is slowly added to a stirred suspension of an equivalent amount of cuprous iodide in THF at 35°, the bicyclic ester (10) is obtained in about 50% yield.3 This ester has been converted into d/-sirenin (11) by Rapoport5 in two steps allylic oxidation with selenium dioxide followed by reduction of the aldehyde and ester functions with mixed hydride. 

N-Substituted 3)5-acetoxy-6/ -amino-5a-androstanes bearing histidine and related imidazole and triazole derivatives have been prepared. Allylic oxidation with selenium dioxide of 3a- and 3)8-dimethylamino-5-enes, c.g. (498) and similar steroids, gave 4/ -hydroxy-5-ene and the allylic rearrangement product... 

Allylic oxidation. Selenium dioxide has been widely used for allylic oxidation of olefins, but this reaction is often troublesome because of formation of selenium-containing by-products and colloidal selenium, which are not easily eliminated. The combination of Se02 with H2O2 to reoxidize selenium species to Set) , has been used to convert olefins into diols and epoxides (2, 362), but this iiicIIuhI is not general for allylic oxidations. Umbreit and Sharpless have effected allylic oxidations with an excess of /-butyl hydroperoxide (90"i.) in combination... 

Oxidation of an alkene may take place at the double bond or at the adjacent allylic positions, and important synthetic reactions of each type are known. Reactions at the double bond, such as epoxidation and dihydroxylation, are described in Chapter 5. Allylic oxidation is of value in synthesis and provides a method to access allylic alcohols, a,p-unsaturated aldehydes or a,p-unsaturated ketones. A common reagent for such transformations is selenium dioxide. For example, with... 

Allylic Alcohols. The allylic oxidation of alkenes mediated by selenium dioxide produces reduced selenium by-products which can be a disadvantage in the practical separation of the allyl alcohol products. A new procedure uses only catalytic amounts of selenium dioxide in the presence of t-butyl hydroperoxide to reoxidize and thus recycle the reduced selenium species, and leads to clean allylic oxidation with yields comparable to the stoicheiometric method. B-Alkenyl derivatives (27) of 9-BBN (3) react in a Grignard-like fashion with aldehydes to give allyl alcohols (Scheme 7) in which the alkene stereochemistry is retained from the vinyl-borane. 

The allylic oxidation with f-butyl hydroperoxide and catalytic selenium dioxide was used in the synthesis of a giberellic acid (Eq. 67).187 authors state that the reaction is inefficient under standard conditions. 

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... 

The isomerization of allyl ethers to 1-propenyl ethers, which is usually performed with potassium tert-butoxide in dimethyl sulfoxide, can also be carried out under milder conditions using tris(triphen-ylphosphine)rhodium chloride,208 and by an ene reaction with diethyl azodicarboxylate,209,210 which affords a vinyl ether adduct. Removal of an O-allyl group may be achieved by oxidation with selenium dioxide in acetic acid,211 and by treatment with N-bromosuccinimide, followed by an aqueous base.201,212... 

Vapor phase chromatography was used to analyze allylic oxidation products. The column, inch X 35 feet, was packed with 20% UCON 5100 on Celite (50-70). NMR analyses were performed using a Varian A-60 instrument. Molecular weights were determined by cryoscopic methods (benzene and acetic acid solvents). Organoselenium compounds were analyzed for selenium by the method of Gould (2), which involves wet digestion of the sample, followed by iodometric titration of the selenium dioxide produced. 

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