Epoxidations with peroxybenzoic acid

Substrates of this type were utilized for the synthesis of alditols. Schneider and co-workers prepared from trans and cis 2-buten-1,4-diols 2,3-anhydro-DL-erythritol (54) and -threitol (55) by direct epoxidation with peroxybenzoic acid. This method of epoxide formation failed in the case of 2 ,4 -hexadiene-l,6-diol from the 2Z,4Z isomer the corresponding diepoxide (57) was obtained in a 2% yield only. However, from 1,6-dibromo-and 1,6-dibenzyloxycarbonyloxy-2 ,4 -hexadienes the corresponding derivatives of 2,3 4,5-dianhydro-DL-gal-actitol (56) could be obtained in moderate yields. Compounds of this type are active against some forms of sarcoma. 

Oxepane (1), as a typical ether, is susceptible to oxidation and yields oxepan-2-one (78) as the initial product. Adipic acid was the product finally isolated after oxidation with Ru04 and NaI04 in a two-phase system (80SC205) or oxygen in the presence of a Pt catalyst (76CB3707) (Scheme 9). Oxidation of 2,3,6,7-tetrahydrooxepin (79) has been reported with peroxybenzoic acid or osmium tetroxide to yield the epoxide (80) or the cis diol (81) respectively (Scheme 10) (58JA3132). 

Oxidations with peroxybenzoic acid are carried out in solutions in dichloromethane, chloroform, benzene, ether, or ethyl acetate at or below room temperature and include epoxidation of double bonds [295, 296, 297, 298, 299, 300, 301], oxidation of benzaldehydes to carboxylic acids or phenols [302], the Baeyer-Villiger reaction of ketones [303, 304, 305, 306, 307], and oxidation of sulfides to sulfoxides [308, 309]. Peroxybenzoic acid is also used for the anti hydroxylation of double bonds [310], the oxidation of pyrrolidines to pyrrolidones [377] and of pyrroles to succinimides [377], and the preparation of azoxy compounds from azo compounds [372]. 

Epoxidation takes place preferentially or more rapidly at electron-rich (i.e., tetraalkylated) double bonds [217]. The reaction is stereospeciflc cis alkenes give cis epoxides, and trans alkenes give trans oxides [211. 1,2-Dimethylcyclopentene is oxidized with peroxybenzoic acid to 1,2-di-inethylcyclopentene oxide in 85% yield [296], and dj-cyclooctene is transformed by hydrogen peroxide into c -cyclooctene oxide in 60-61% yield [1099]. 

Stereoselectivity in cyclic allylic alcohols depends on the ring size and on the reagent used. 2-Cyclohexen-l-ol, on treatment with peroxybenzoic acid, gives a cis epoxide, probably because of the hydrogen bond of the hydroxyl hydrogen in the transition state. If the acetate of the alcohol is treated with peroxybenzoic acid, the trans epoxide predominates over the cis epoxide in a ratio of 4 1 (equation 276) [298]. 

Similar oxidants are used for epoxidation of esters of unsaturated carboxylic acids. Methyl oleate is oxidized with peroxybenzoic acid [295] or peroxylauric acid [174] to methyl 9,10-epoxystearate acid in respective yields of 67 and 76%. Alkaline 50% hydrogen peroxide in methanolic solution transforms diethyl ethylidenemalonate at pH 8.5-9.0 and at 35-40 C over a period of 1 h into ethyl 2-ethoxycarbonyl-2,3-epoxybutyrate in 82% yield [145], A somewhat exotic oxidizing agent, dimethyldioxirane, converts ethyl tra/u-cinnamate into ethyl 2,3-epoxyhydrocinnamate in 63% isolated yield [210]. 

Monoperoxyphthalic acid can be used for preparation of epoxides in the same way as peroxybenzoic acid. In general, good yields are obtained under almost the same conditions as with peroxybenzoic acid, but monoperoxyphthalic acid has the advantage that it is more stable. Because of this stability monoperoxyphthalic acid is particularly suited for epoxidation of less reactive olefins when reaction is effected in chloroform solution the phthalic acid produced is readily separated because it is insoluble in this solvent. 

Substituted peroxybenzoic acids are used for epoxidation of tnfluorovinyl alkenes with attached functional groups [7] (equation 5)... 

Epoxidation of 25a with peroxybenzoic acid23 afforded a mixture (28) of stereoisomeric epoxides which, without separation, were hydrolyzed in mildly acidic medium to DL-lyxose and DL-xylose. 

TABLE 5. Calculated [B3LYP/6-311+G(d,p)] activation parameters (kcal mol-1 and eu) for the epoxidation of cyclohexene and isobutene with dimethyldioxirane (DMDO), peroxybenzoic acid (PBA), m-chloroperoxybenzoic acid (m-CPBA) and peroxyformic acid (PFA). Solvent corrections were performed with the COSMO model. The numbers in bold are experimental values95-136. Numbers in parentheses are at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory... 

Trifluoroacrylonitrile can be epoxidized by oxygen with 1,1,2-trichlorotrifluoroethane (CFC-113) as a solvent under pressure at elevated temperatures in moderate yield (Table 2).77 Substituted peroxybenzoic acids are used for the epoxidation of trifluorovinyl alkenes with attached functionalities such as ester, amide or dimethoxyphosphoryl groups (Table 2).7S Functional derivatives of perfluoro-2-methylprop-2-enoic acid are oxidized to the corresponding epoxy compounds in this reaction.78 In the case of ethyl ester 40, the epoxide 41 is contaminated with the adduct of 3-chloroperoxybenzoic acid to the C = C bond, compound 42, that is formed even at low temperatures.78... 

The presence of the substrate of functional groups capable of interacting with the metal directs the stereoselectivity of the epoxidation, as shown by the comparative reactivity of (82) towards TAHP/Mo(CO)6 and peroxybenzoic acid (PBA).241... 

Epoxidations and Baeyet Villiger oxidations. This peroxy acid resembles peroxytrifluoroacetic acid in activity, but buffers are not necessary. The report includes two successful epoxidations with the new peroxy acid (and also p-nitro-peroxybenzoic acid) for which peroxytrifluoroacetic acid was of no value. 

In a-ionone, peroxybenzoic acid and peroxyphthalic acid epoxidize solely the endocyclic double bond to give 3,4-epoxy-a-ionone in 96.5 and 66% yields, respectively [299]. Alkaline hydrogen peroxide, on the other hand, epoxidizes only the double bond conjugated with carbonyl to give a 28-31% yield of a-ionone-ot, p -epoxide [332] (equation 440). 

In comparison with the standard system for epoxidation which uses m-ehloro-peroxybenzoic acid as oxidizing agent, the MTO/H202/aromatic Lewis base system displays several advantages (a) it is safer but equal in price (b) due to the suppression of epoxide ring opening, it has a much broader scope (c) its selectivity is higher, and (d) it is more reactive, requires less solvent, the product work-up is easier, and the only by-produet formed is water. 

Sable and coworkers also synthesized all four of the predicted racemic forms of cyclopentanetetrol (131-134), and the meso diastereomer (130). The all-cfs, meso isomer is unknown. cis-Hydroxylation of the enediols (136 and 137) is effected with permanganate or with silver chlorate-osmium tetroxide Irans-hydroxylation is caused by peroxybenzoic acid. Ihe epoxy group enters ds to the neighboring, free hydroxyl group in the intermediates (136-A, 136-B, and 137-A), according to Henbest s rule. However, prior reaction of the diol structure in the intermediate (136-A) with acetone caused the epoxide group to enter trana,... 

Peroxybenzoic acid (often called simply perbenzoic acid) is the commonest reagent for preparation of epoxides. It is best obtained directly from benzoyl chloride and sodium peroxide29 or by cleavage of dibenzoyl peroxide with sodium alkoxide 30... 

Epoxides are usually prepared from olefins and peroxy acids (especially peroxybenzoic acid) according to the following general procedure The olefin is dissolved in chloroform, cooled, and treated with a slight excess of peroxy add in cold chloroform. The reaction mixture is set aside at 0°, or for less reactive olefins at room temperature. The course of the reaction is followed by titration of small samples when the reaction is ended, unchanged peroxy acid and the carboxylic acid formed are carefully extracted, and the chloroform solution is worked up. 

Benzaldehyde reacts readily with molecular oxygen with formation of peroxybenzoic acid (see the Section Autoxidation ). This tendency of benzaldehyde to autoxidation has been utilized in an epoxidation procedure that avoids the preparation of pure peroxy acid.34 It is only necessary to treat an olefin-benzaldehyde mixture with oxygen or air to obtain the epoxides in good yield. Yields of epoxide are, however, poor if an aliphatic aldehyde is used.35... 

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