Epoxidation selective

The selective epoxidation of ethylene by hydrogen peroxide ia a 1,4-dioxane solvent ia the presence of an arsenic catalyst is claimed. No solvent degradation is observed. Ethylene oxide is the only significant product detected. The catalyst used may be either elemental arsenic, an arsenic compound, or both. 

Many selective epoxidations are possible with polyunsaturated steroids. In general, oc, -unsaturated ketones are not attacked by peracid, although linear dienones react slowly at the y,5-double bond. Aw-Chloroperbenzoic acid is the reagent of choice for this reaction.When two isolated double bonds are present in the steroid nucleus, e.g. (27) and (30), the most highly substituted double bond reacts preferentially with the peracid. Selective epoxidation of the nuclear double bond of stigmasterol can likewise be achieved.However, one exception to this general rule has been reported [See (33) (34)]. ... 

In the third sequence, the diastereomer with a /i-epoxide at the C2-C3 site was targeted (compound 1, Scheme 6). As we have seen, intermediate 11 is not a viable starting substrate to achieve this objective because it rests comfortably in a conformation that enforces a peripheral attack by an oxidant to give the undesired C2-C3 epoxide (Scheme 4). If, on the other hand, the exocyclic methylene at C-5 was to be introduced before the oxidation reaction, then given the known preference for an s-trans diene conformation, conformer 18a (Scheme 6) would be more populated at equilibrium. The A2 3 olefin diastereoface that is interior and hindered in the context of 18b is exterior and accessible in 18a. Subjection of intermediate 11 to the established three-step olefination sequence gives intermediate 18 in 54% overall yield. On the basis of the rationale put forth above, 18 should exist mainly in conformation 18a. Selective epoxidation of the C2-C3 enone double bond with potassium tm-butylperoxide furnishes a 4 1 mixture of diastereomeric epoxides favoring the desired isomer 19 19 arises from a peripheral attack on the enone double bond by er/-butylper-oxide, and it is easily purified by crystallization. A second peripheral attack on the ketone function of 19 by dimethylsulfonium methylide gives intermediate 20 exclusively, in a yield of 69%. 

With regard to the use of tungsten-based systems, Payne and Williams reported on the selective epoxidation of maleic, fumaric, and crotonic acids with a catalytic amount of sodium tungstate (2mol%) in combination with aqueous hydrogen... 

Epoxides are often encountered in nature, both as intermediates in key biosynthetic pathways and as secondary metabolites. The selective epoxidation of squa-lene, resulting in 2,3-squalene oxide, for example, is the prelude to the remarkable olefin oligomerization cascade that creates the steroid nucleus [7]. Tetrahydrodiols, the ultimate products of metabolism of polycyclic aromatic hydrocarbons, bind to the nucleic acids of mammalian cells and are implicated in carcinogenesis [8], In organic synthesis, epoxides are invaluable building blocks for introduction of diverse functionality into the hydrocarbon backbone in a 1,2-fashion. It is therefore not surprising that chemistry of epoxides has received much attention [9]. 

SELECTIVE EPOXIDATION OF TERMINAL DOUBLE BONDS 10,11-EPOXYFARNESYL ACETATE... 

A variety of reactions have been conducted. Catalysts based on noble metals on Deloxan amino poly siloxane supports have been used. Hitzler et al. (1998) have reported alkylation of mesitylene with propylene or wopropanol in SC propylene or CO2 using a solid acid Deloxan catalyst. Pesiri et al. (1998) have carried out selective epoxidation in SC CO2 with transition metal catalysts (V, Ti, Mo) and tert-BHPO high conversion and selectivity have been reported. 

Li, X. and Kutal, C. (2002) Photocatalytic selective epoxidation of styrene by molecular oxygen over highly dispersed titanium dioxide species on silica. Journal of Materials Science Letters, 21 (19), 1525-1527. 

Alkene Catalyst Reaction time (h) Turnover (mol/mol Ti) H2O2 Conversion (%) Selectivity (%) Product selectivity (%) Epoxide Glycol Glycol ethers... 

Selective epoxidation of one of the double bonds in dialkenes is of practical interest (Table XVI). Although monoepoxides predominate at low H2O2 concentrations, the diepoxides are also formed at higher concentrations. The diallyl epoxides of bisphenol A are major intermediates in the adhesives industry, and their synthesis in solid-catalyzed reactions in an eco-friendly manner remains a challenge. 

Alkene conversion to epoxide (%) Epoxide selectivity (%) Epoxide conversion (%) Cyclic carbonate selectivity (%)... 

The POM also catalyzed the selective epoxidation of alkenes with only 1 atm of 02 as an oxidant [61] ... 

Similarly, heterogeneous catalyst prepared by immobilization of POMs on chemically modified hydrophobic Si02 has been applied to the selective epoxidation of various alkenes with 15% aqueous H202 without organic solvents [168],... 

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