Epoxidation of enol

Hydroxyl groups are stable to peracids, but oxidation of an allylic alcohol during an attempted epoxidation reaction has been reported." The di-hydroxyacetone side chain is usually protected during the peracid reaction, either by acetylation or by formation of a bismethylenedioxy derivative. To obtain high yields of epoxides it is essential to avoid high reaction temperatures and a strongly acidic medium. The products of epoxidation of enol acetates are especially sensitive to heat or acid and can easily rearrange to keto acetates. 

Dimethyldioxirane DMDO discovered by Murray and coworkers, is a superior choice for the epoxidation of most olefins, giving comparable or higher yields than m-CPBA-based epoxidation [21]. Proceeding rapidly under neutral and mild conditions, it is especially well suited for the synthesis of sensitive epoxides of enol esters, enol lactones [22], and enol ethers [23]. The reaction is stereospecific, gen-... 

Following their success with chiral ketone-mediated asymmetric epoxidation of unfunctionalized olefins, Zhu et al.113 further extended this chemistry to prochiral enol silyl ethers or prochiral enol esters. As the resultant compounds can easily be converted to the corresponding a-hydroxyl ketones, this method may also be regarded as a kind of a-hydroxylation method for carbonyl substrates. Thus, as shown in Scheme 4-58, the asymmetric epoxidation of enol silyl... 

Monoacelais of a-dicarbonyl compounds. Frimer s epoxidation of enol ethers (8, 7) has been extended to a synthesis of monoacetals of a-dicarbonyl compounds.1 Examples ... 

Product studies of the stereoselective cycloaddition and epoxidation of enol ethers by a-peroxylactone were interpreted in terms of 5n2 processes.156 Steric and stereoelec -tronic effects are important. 

It should finally be pointed out that the mild reaction conditions typically employed in dioxirane-mediated oxidations enable the asymmetric epoxidation of enol ethers and enol esters. With the silyl ethers, work-up provides enantiomeri-cally enriched a-hydroxy ketones. As summarized in Table 10.1, quite significant enantiomeric excesses were achieved by use of catalyst 10 at loadings ranging from 30 [30] to 300 mol% [31]. Enol esters afford the intact acyloxyepoxides enantiomeric purities are, again, quite remarkable. 

Epoxides of enol silyl ethers or acetates can be obtained in high yield by reactions with dimethyldioxirane at low temperatures.3 Dimethyldioxirane is generally... 

Epoxidation of enol ethers.1 This dioxirane can effect epoxidation of such hindered enol ethers as 2. A similar reaction of 2,3-dihydropyran with 1 affords the epoxide in >92% yield. 

The diene (117) has been epoxidized with 3,5-dinitnq)erbenzoic acid (equation 43). Attack at bodi the double bonds is stereoselective. Epoxides of enol ethers are nmmally difficult to isrdate, but the epoxide (118) is quite stable. This stabiliQr is due to the attachment of the electron-withdrawing carb-... 

Some of the early uses [13] of MCPBA indicate the initial objectives that one would consider when evaluating new potential substitutes. The epoxidation of alkenes such as 1-octene (Eq. 1) and ethyl crotonate (Eq. 2), and the epoxidation of enol ethers followed by cleavage (Eq. 3) [14] are illustrative. 

Enantioselective isomerization. Epoxides of enol esters provide chiral oi-acyloxy ketones and chiral starting materials. 

Disubstituted vinyl silanes are epoxidized in high ee s and enantiomerically enriched 1,1-disubstituted epoxides can be obtained via the desilylation of these epoxides e.g., 18). Allylic alcohols and conjugated dienes and enynes are effective substrates (e.g. 19 and 20). ° The epoxidations of enol ethers and enol esters were also studied. The resulting epoxides (e.g., 21) from enol esters can undergo stereoselective rearrangement to give optically active a-acyloxy ketones, (5)-22 or (i )-23, under different acidic conditions. 

A more synthetically useful version of this reaction involves epoxidation of enol trimethylsilyl ethers of ketones. The methods for synthesis of these compounds were discussed in Section 1.2. Epoxidation of the silyl enol ethers followed by aqueous workup gives a-hydroxy ketones and a-hydroxy aldehydes.The oxidation... 

Figure 2 Chiral epoxidation of enol ethers 4a and 4b (% enol ether , % epoxy ether ).

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