Epoxidation with peroxyacid

Unsaturated polymers, prepared from butadiene or isoprene, can be epoxidized with peroxyacids. The peroxyacids used can either be preformed or prepared in situ by reacting hydrogen peroxide with lower aliphatic carboxylic acids. The epoxidized polymers can be reacted with diamines or dianhydrides to give a cross-linked resin useful for adhesive and coating applications. 

Alkenes can be epoxidized with any of a number of peroxyacids, " of which m-chloroperoxybenzoic has been the most often used. The reaction, called the... 

Another route to the preparation of a-hydroxy derivatives consists of the transformation of enolates into silyl enolates and their subsequent oxidation. Oxidation of triaLkylsUyl enolates with peroxyacids, most frequently cpba, has been applied for preparation of a-hydroxy- and a-acetoxy aldehydes or ketones. Reactions require mild conditions and generally give good yields of the expected compound . Mechanistic investigations suggest the intermediate formation of epoxides which evolve to the final products via 1,4-sUyl group migration. 

Conjugated dienes can be epoxidized (1,2-addition), although the reaction is slower than for corresponding alkenes, but a,p-unsaturated ketones do not generally give epoxides when treated with peroxyacids.The epoxidation of a,p-unsaturated ketones with hydrogen peroxide under basic conditions is known as the Waits-Schejfer epoxidation, discovered in 1921. This fundamental reaction has been extended to a,()-unsaturated ketones (including quinones), aldehydes, and... 

The 2, 3 -epoxypropyl jS-glycoside of di(N-acetyl-D-glucosamine) where R is N-acetyl glucosamine specifically inactivates hen lysozyme and several other bird lysozymes (Maron et al. 1972). The residue of hen lysozyme specifically modified by VIII is asp. 52 (Eshdat et al. 1973). X-ray analysis reveals that the two glucosamine residues of the affinity label occupy subsites B and C of the substrate binding cleft (Moult et al. 1973). The synthesis of the affinity label was accomplished by the most general procedure for the synthesis of epoxides, namely oxidation of alkenes with peroxyacids. 

During the first total synthesis of briarellin diterpenes, briarellins E and F, L.E. Overman and co-workers utilized the large reactivity difference between a triple and a double bond in peroxyacid oxidations to selectively epoxidize a trisubstituted double bond in the presence of a terminal alkyne." The epoxidation with mCPBA was carried out in DCM in the presence of a base to afford the a-epoxide in a 9 1 diastereomeric ratio. 

An interesting difference in stereoselectivity between epoxidations using peroxyacid and peroxide is revealed in the reaction of (9) with 3-ClCgH CO H (MCPBA) and Me COOH (TBHP)2. Only the B-epoxide was formed using the former whereas a mixture of both stereoisomers resulted frcm epoxidation with TBHP. 

Another synthetic example illustrates both the epoxidation capabilities of this reaction as well as the stereochemical influence of neighboring alcohol unit noted above with the transformation 163 164. In a synthesis of (-)-malyngolide by Ogasawara and co-workers, allylic alcohol 176 was treated with VO(acac)2 and t-BuOOH, in the presence of 2,6-lutidine, and a 66% yield of 177 was obtained.266 xhe hydroxyl unit directed the epoxidation to the exo face of the bicyclic system, as shown. Also notice that the more substituted alkene was epoxidized rather than the less substituted one, which is also typical of epoxidation reactions of this type, and will be discussed in more detail below, in connection with peroxyacid reactions. 

Perhaps the most common method for the epoxidation of simple alkenes is their reaction with peroxyacids, which has occasionally been referred to as the Prilezhaev (Prileschajew) reaction. O This reaction does not require transition metal catalysis and the yield of epoxide is often high. Peroxyacids such as 156 are prepared by reaction of carboxylic acids with hydrogen peroxide. An equilibrium is established during the reaction that favors the peroxyacid, although several alternative methods are available.279 in general, strong... 

More highly substituted alkenes are electron rich and react faster with peroxyacids than do less substituted alkenes. In general, the relative rate of epoxidation increases with the nucleophilic character of the alkene l (CH2=CH2 has a relative rate of 1, RCH=CH2 = 24, RCH CHR = 500, R2C=CH2 = 500, R2C=CHR = 6500,... 

The steric course of the epoxidation of alkyl 3,4-dideoxy-glyc-3-enopyranosides (281) with peroxyacids is dependent on the configuration and bulkiness of substituents at C-1 and C-2, in rough analogy with the course of cis-hydroxylation. 

Therefore, a tetrasubstituted alkene (R2C=CR2) should react faster with peroxyacids than a trisubstituted alkene (R2C=CHR), which reacts faster than a disubstituted alkene. Monosubstituted alkenes such as RHC=CH2 react slowest in this reaction. In fact, simple alkenes such as 1-pentene can be very difficult to epoxidize under normal conditions because the reaction is very slow. In order to simplify this reaction, assume that all alkenes given as problems in this book can be epoxidized. However, in reality, there are vast differences in the reaction rate among various alkenes. 

In recent years, peroxyacids have been less available than they once were. In addition, high-purity hydrogen peroxide is rather dangerous to handle and difficult to obtain. This fact has forced manufacturers to use dilute aqueous solutions of hydrogen peroxide to prepare peroxides this gives lower purity peroxide products and lower yields of epoxides when they react with alkenes. The by-product of such reactions is a carboxylic acid, which can sometimes react with the epoxides product unless a buffer is used. Remember also that terminal alkenes are difficult to epoxidize using peroxyacids. 

Oxidation of an alkene with a peroxyacid leads to an epoxide, with a carboxylic acid as the by-product. 

Oxidation of an alkene with a peroxyacid leads to an epoxide, with a carboxylic acid as the by-produet 18,37,38,40,42,44, 51. Oxidative cleavage of an alkene with ozone leads to an ozonide. Reductive workup with dimethyl sulfoxide or zinc and acetic acid gives ketones and/or aldehydes. Oxidative workup with hydrogen peroxide gives ketones and/or earboxylic acids 19,20, 21, 22,36,39,41,44. 

Alkenes are oxidized to give epoxides on treatment with a peroxyacid (RCO H), such as mefn-chloroperoxybenzoic acid. An epoxide, also called an oxirane, is a cyclic ether with an oxygen atom in a three-membered ring. For example ... 

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