Oxiranes with hydrogen peroxide

Direct phase-transfer catalysed epoxidation of electron-deficient alkenes, such as chalcones, cycloalk-2-enones and benzoquinones with hydrogen peroxide or r-butyl peroxide under basic conditions (Section 10.7) has been extended by the use of quininium and quinidinium catalysts to produce optically active oxiranes [1 — 16] the alkaloid bases are less efficient than their salts as catalysts [e.g. 8]. In addition to N-benzylquininium chloride, the binaphthyl ephedrinium salt (16 in Scheme 12.5) and the bis-cinchonidinium system (Scheme 12.12) have been used [12, 17]. Generally, the more rigid quininium systems are more effective than the ephedrinium salts. 

The conversion of 3-chloropentafluoropropene to 2-(chlorodifluoromethyI)-2,3,3-trifluoro-oxirane (33) can be carried out64 by heating the mixture of the alkene and oxygen in 1,1,2-trichlorotrifluoroethane (CFC-113) in an autoclave.64 The oxidation with hydrogen peroxide in alkaline solution is negatively influenced by the high nucleophilic reactivity of allylic chlorine.65 66 The reaction is performed at very low temperatures that favor the attack of the hydroperoxy anion in competition with the hydroxy anion. Acceptable yields of 31 -38 % are obtained in the presence of a phase-transfer catalyst.66... 

The observed stereochemistry of addition suggests that oxiranes (epoxides) may be intermediates. Oxiranes may, indeed, be isolated from the reaction of certain alkenes with hydrogen peroxide in the presence of sodium tungstate. °... 

Allylic alcohols may be derived from alkenes by metallation to give the allylpotassium species, followed by treatment with fluorodimethoxyborane. Oxidation of the resultant boronic ester with hydrogen peroxide gives the allylic alcohol (Scheme 15). ° - ° Some allylic rearrangement may be observed for example, metallation of a-pinene with potassium r-butoxide in petroleum ether solution and subsequent boration and oxidation gave myrtenol (42%) and /rons-pinocarveol (1%) (equation 24), while treatment of the allylpotassium with oxirane gave the alkylated prc ucts in a ratio of ca. 2 1. ° ... 

Oxiranes can be converted to dialdehydes with hydrogen peroxide in the presence of a boric acid ester or phosphoric acid ester.The base-catalyzed oxidation of oxirane and alkyl-substituted oxiranes with fcA-r-butylhydroperoxide has been reported. With increased substitution the molecule undergoes fragmentation. With DMSO in the presence of terf-butylhydroperoxide and strong acids, a-ketols are formed via a sulfonium salt (Eq. 156). ... 

Peroxo Mo and W complexes catalyzed oxidation of alcohols and olefins with hydrogen peroxide to form oxiranes and lactones 03MI105. 

Hydroxymethyl)oxirane (glycidol) is produced industrially by the oxidation of allyl alcohol with hydrogen peroxide in the presence of sodium hydrogen tungstate. It serves as a useful starting material in various syntheses [9]. 

The direct oxidation of alkenes to oxirans by hydrogen peroxide is, of course, only possible when a catalyst is used. In the past the main choice of catalyst has been oxides of the metals of Groups 5a, 5b, 6a, or 6b. In 1978, however. Sharpless put the use of arylseleninic acids as catalysts on a firm practical footing.Using the nitrophenylseleninic acids (25 R = H, R = NO2) and (25 R = R = NO2), 95 % preparative yields of cyclo-octene oxide were obtained from the alkene in CH2CI2 with 30% H2O2. 

Ketones.— Dioxolanes [involving use of (CFjCOjCO]." Epoxidation of Olefins with Hydrogen Peroxide [high yields ofepoxides are obtained in presence of RfCOR catalysts (Rf = R = CF, Rf = CFiCl, R = CFC1 Rf = CFQj, R = CClj]. " 3,3-Bis(trifluoromethyl)oxirans. 

Nucleophilic opening of oxiranes to give ultimately 1,2-diols is usually effected without isolation of the oxirane oxiranation (epoxidation) of alkenes with unbuffered peroxy-ethanoic acid or hydrogen peroxide in methanoic acid (Section 5.05.4.2.2(/)) tends to give monoesters of 1,2-diols (e.g. 53), which can be hydrolyzed to the diols (Scheme 46). 

The high degree of stereoselectivity associated with most syntheses and reactions of oxiranes accounts for the enormous utility of these systems in steroid syntheses. Individual selectivity at various positions in the steroid nucleus necessitates the discussion of a collection of uniquely specific reactions used in the synthesis of steroidal epoxides. The most convenient and generally applicable methods involve the peracid, the alkaline hydrogen peroxide and the halohydrin reactions. Several additional but more limited techniques are also available. 

Many functional groups are stable to alkaline hydrogen peroxide. Acetate esters are usually hydrolyzed under the reaction conditions although methods have been developed to prevent hydrolysis.For the preparation of the 4,5-oxiranes of desoxycorticosterone, hydrocortisone, and cortisone, the alkali-sensitive ketol side chains must be protected with a base-resistant group, e.g., the tetrahydropyranyl ether or the ethylene ketal derivative. Sodium carbonate has been used successfully as a base with unprotected ketol side chains, but it should be noted that some ketols are sensitive to sodium carbonate in the absence of hydrogen peroxide. The spiroketal side chain of the sapogenins is stable to the basic reaction conditions. 

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