Peracids epoxidation with

Butadiene can also be readily epoxidized with peracids to the monoepoxide or the diepoxide (109,110). These have been proposed as important intermediates in the metaboHc cycle of butadiene in the human body (111). 

Epoxidation with peracid, and mild alkaline hydrolysis proceeds to give the 17a-hydroxy-20-ketone in a high overall yield. ... 

We have cited the case of epoxidation with peracids earlier in this book. A cross-linked polymethacrylic acid can, in principle, be treated with a sulphonic acid catalyst and H2O2 to give the peracid, which will allow epoxidation to be carried out in a convenient way, while the polymer can be reused. The key advantage is that the product stream does not contain an acid, which may be harmful in several ways. 

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

Epoxidation of allenes.1 The spirodioxides formed by epoxidation of allenes are unstable to acids, and only hindered ones have been obtained on epoxidation with peracids. They can be obtained, however, in 90-95% yield by epoxidation of allenes (even monosubstituted ones) with dimethyldioxirane in acetone buffered with solid K2C03. 

In a situation where severe steric hindrance (e.g., 16,16-dimethyl-20-keto-pregnanes) prevents enol acetate formation, an alternate scheme has been devised. Condensation of ethyl oxalate at C-21 produces, after hydrolysis, the 21-glyoxylic acid this on treatment with acetic anhydride and a strong acid catalyst such as perchloric acid gives both A17(20)-enol lactone acetates. Epoxidation with peracid, and mild alkaline hydrolysis proceeds to give the 17a-hydroxy-20-ketone in a high overall yield.257... 

Barton oxidation was the key to form the 1,2-diketone 341 in surprisingly high yield, in order to close the five-membered ring (Scheme 38). The conditions chosen for the deprotection of the aldehyde, mercuric oxide and boron trifluoride etherate, at room temperature, immediately led to aldol 342. After protection of the newly formed secondary alcohol as a benzoate, the diketone was fragmented quantitatively with excess sodium hypochlorite. Cyclization of the generated diacid 343 to the desired dilactone 344 proved very difficult. After a variety of methods failed, the use of lead tetraacetate (203), precedented by work performed within the stmcmre determination of picrotoxinin (1), was spectacularly successful (204). In 99% yield, the simultaneous formation of both lactones was achieved. EIcb reaction with an excess of tertiary amine removed the benzoate of 344 and the double bond formed was epoxidized with peracid affording p-oxirane 104 stereoselectively. Treatment of... 

Allylic bromides 383 and 384 were the relais substances for the synthesis of coriamyrtin (9) (Scheme 43). The allylic bromides were eliminated with potassium ferf-butoxide in toluene affording diene 387. 1,4-Addition with W-bromosuccini-mide in polar solvent at room temperature yielded the unsaturated a-bromo-5-hydroxy unit of 388, which formed the unsaturated epoxide 389 by base treatment. Epoxidation with peracid occurred as expected from the convex face. The cyclic ether moiety of bisepoxide 390 was oxidized to the lactone 368 with ruthenium... 

Mitsunobu reaction as well as by mesylation and subsequent base treatment failed, the secondary alcohol was inverted by oxidation with pyridinium dichromate and successive reduction with sodium borohydride. The inverted alcohol 454 was protected as an acetate and the acetonide was removed by acid treatment to enable conformational flexibility. Persilylation of triol 455 was succeeded by acetate cleavage with guanidine. Alcohol 456 was deprotonated to assist lactonization. Mild and short treatment with aqueous hydrogen fluoride allowed selective cleavage of the secondary silyl ether. Dehydration of the alcohol 457 was achieved by Tshugaejf vesLCtion. The final steps toward corianin (21) were deprotection of the tertiary alcohols of 458 and epoxidation with peracid. This alternative corianin synthesis needed 34 steps in 0.13% overall yield. 

The high proportion of axial attack on methylenecyclohexanes in epoxidation with peracids compared to peracid imides (see Section 4.5.1.1.3.) seems to indicate that the transition state of the peracid imide epoxidation is more crowded104-105 a similar, but somewhat less marked effect is observed in the epoxidation of 5-cholesten-3/ -ol (Table 3) with peracid imides and sterically demanding epoxidation reagents, such as (in situ generated) dimethyldioxirane (Table 3, entry 13 Table 4, entry 21), hydrogen peroxide/tungsten catalyst (Table 4, entry 16) or ferf-butyl hydroperoxide/molybdenum catalyst (Table 4, entries 14, 15). 

The effect is most likely more general and not restricted to chair cyclohexane systems. In alloaromadendrene (8), containing a seven-membered ring in a similar conformation, the calculated selectivity for axial attack is again somewhat less than found experimentally the preliminary structure assignment133 of the two diastereomers is confirmed by calculation168. The preference for axial attack in methylenecyclohexane systems is reversed in epoxidation with peracid imides (see Table 5). 

Even 16-membered ring lactones, one with only two stereogenic methyl groups120, the other with six stereogenic centers, but in rather remote positions (in y,t>-position of the dienone system of a protected carbonolide117), have been epoxidized with peracid with at least 90% diastereoselectivity, the former directly to a triepoxide. 

Less regularly used reagents are rerf-butyl hydroperoxide/terr-butyliithium,600,601 ozone.602 dioxiranes,603,604 fluorine/ water/acetonitrile,605 or A, A -diethylhydroxylaminc.606 Alkenes carrying a donor substituent can also be epoxidized with peracids.607-609 Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclectivity.610... 

Upon epoxidation with peracids, alkylidenecycloproparenes react at the exocyclic double bond and are converted to hydroxy ketones 15. Epoxidation of l-(diphenylmethylene)cyclo-propa[6]naphthalene with dimethyldioxirane at — 18 "C afforded the epoxide, 3, 3 -diphenyl-spiro l//-cyclopropa[2ft]naphthalene-l,2 -oxirane (16), which was characterized by H and CNMR. This is the first oxaspiropentene ever characterized. Under non-acidic conditions rearrangement of the epoxide 16 to 2,2-diphenylcyclobuta[6]naphthalen-l(2//)-one (17) occurred in 86% overall yield. 

The pattern you saw for epoxidation with peracids (more substituted aikenes react faster) is foiiowed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is attack of bromine. The scaie beiow shows the effect on the rate of reaction with bromine in methanoi of increasing the number of aikyi substituents from none... 

Epoxidation is carried out by four different procedures (i) epoxidation with peracids such as peracetic acid or perbenzoic acid in the presence of an acid catalyst (ii) epoxidation with organic and inorganic peroxides, including transition metal catalysts (iii) epoxidation with halohydrins using hypoha-lous acids and their salts and (iv) epoxidation with molecular oxygen. 

The direct epoxidation of olefins can be carried out with oxygen, per compounds, or alkaline hydrogen peroxide. In contrast to the above-mentioned processes, epoxidation with peracids occurs on the less sterically hindered side without Walden inversion. Direct epoxidation with oxygen... 

Neighboring group participation in addition reactions has been reviewed cf. Ref. 116 HO-3 assistance and stereochemical control in epoxidations with peracids has been reported, cf Ref 159. 

The Sharpless epoxidation provides good examples of both directed and asymmetric catalytic reactions. It has long been known that alkenes can be epoxidized with peracids, which deliver an electrophilic oxygen atom, as shown... 

Bartlett [206] in 1960 suggested a mechanism of olefin epoxidation with peracids, based on nonionized molecules and containing a three-atom cycle in the transition complex ... 

In the case of epoxidation with peracids formed in situ, the process occurs in a markedly heterogeneous environment, with oxygenated water and carboxylic acid to be found in an aqueous phase, and the unsaturated polymer and the solvent forming the organic phase. The peracid is formed in the aqueous phase, and it diffuses in the organic phase, where it attacks the double bond and the resulting carboxylic acid has a tendency to return the aqueous phase. This cycle continues until exhaustion of oxygenated water. 

Olefin inversion via epoxides is reported by two groups of workers. One sequence involves epoxidation with peracid, which occurs with retention of stereochemistry, followed by deoxygenation with hexamethyldisilane and potassium methoxide in HMPT at 65 °C. The alternative procedure utilizes reaction of the episode with triphenylphosphine dihalides to give vicinal dihalides. Zinc reduction of the dihalides is specifically trans, and thus the sequence epoxidation-bromination-reduction gives overall inversion of olefin configuration. ... 

Takl967 Takagi, T., Epoxidation with Peracid Type Polymer, J. Polymer Sci., Part B, Polymer Lett., 5 (1967) 1031-1035. 

Epoxidations with peracids can exhibit high degree of chemoselectivity and these generally display preferences for reaction with more nucleophilic alkenes. This phenomenon is illustrated in Vandewalle s work directed towards the total synthesis of the sesquiterpene estafiatin (6, Equation 3) [52]. The final step included selective epoxidation of the trisubstituted olefin from its more accessible convex face (dr =97 3). 

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