Epoxides, from alkenes

Alkenes from epoxides. F.poxides in the hexapyranose series are converted into alkenes by reaction with KI, Zn, and P205 (all in large excess) in DMF at 90°. Yields in the two reported examples were 86 and 83%. 

J. K. Kochi, D. M. Singleton u. L. J. Andrews, Alkenes from Halides and Epoxides by Reductive Eliminations with Cr(lI)-Complexes, Tetrahedron 24, 3503 (1968). 

The application of 1,3-dipolar cycloaddition processes to the synthesis of substituted tetrahydrofurans has been investigated, starting from epoxides and alkenes under microwave irradiation. The epoxide 85 was rapidly converted into carbonyl ylide 86 that behaved as a 1,3-dipole toward various alkenes, leading to quantitative yields of tetrahydrofuran derivatives 87 (Scheme 30). The reactions were performed in toluene within 40 min instead of 40 h under classical conditions, without significantly altering the selectivi-ties [64]. 

In recent years, dioxiranes have become workhorses for a variety of selective transformations in organic synthesis, from epoxidation of alkenes to the conversion of alcohols into fee corresponding ketones <99CJC308>. Dioxirane-mediated epoxidation continues to be the method of choice for complex substrates wife acid-sensitive functionality. Thus, fee dimethyl-dioxirane (DMD)-mediated epoxidation of the silylated stilbene lactam 159 has been reported as a key step in fee synthesis of protoberberines <99JOC877>. 

Oxaspiropentanes have been generated and rearranged in a large variety of different environments. A series of alkylidene- and allylidenecyclopropanes, present in the structures of bicy-clo[3.1.0]hexanes or bicyclo[4.1.0]heptanes, were epoxidized and rearranged in situ to bicyclic ketones with the alkyl or allyl group preferentially to exclusively in the exo position (Table 4).51 This corresponds to a preferential to exclusive epoxidation of the corresponding alkenes from the sterically less demanding exo face. 

Epoxidation. 1,1,3,3-Tetrachloroacetone can mediate the epoxidation of olefins by hydrogen peroxide in the same way as hcxafluoroacctonc (9, 244-245). It has the advantage that it is probably less toxic, and it is inexpensive and commercially available. The actual oxidant presumably is the hydroperoxide 1, which is converted during the epoxidation into the unstable hydrate (2), from which the tctrachloroacetone can be recovered in >70% yield. Monosubstitutcd alkenes are epoxidized in low yields by this method more highly substituted alkenes are epoxidized in 65 85% yield (VPC).1... 

Epoxidation. Alkenes are epoxidized when treated with a reagent formed from dilute F2 (10% in N2) in acetonitrile containing some water. Both CH3CN and H20 are essential for formation of the oxidant, which is probably not peroxy-acetimidic acid (1, 470 8, 387). The new reagent effects epoxidation of alkenes rapidly, even at -15°. When used in excess it also effects epoxidation of a,p-enones in high yield.1... 

Epoxides (see also a,(3-Epoxy alcohols, etc., Glycidic acids, esters, nitriles) From alkenes by epoxidation Dimethyldioxirane, 120 Fluorine-Acetonitrile, 135 Potassium peroxomonosulfate, 259 From carbonyl compounds Alumina, 14... 

However, attempts to develop similar selective catalysts failed in the case of reactions that require one oxygen atom, like the oxidation of methane, ethane and other alkanes to alcohols, aromatic compounds to phenols, alkenes to epoxides, and many others. These mechanistically simple reactions assume one difficult condition the presence of active sites that upon obtaining two atoms from gas-phase 02 can transfer only one of them to the molecule to be oxidized, reserving the second atom for the next catalytic cycle with another molecule. This problem remains a hard challenge for chemical catalysis. 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

You saw earlier the epoxidation of a lactone-brldgsd alkene from the less hindered face. 

Irradiation of an alkene in the presence of molecular oxygen and an a-diketone furnishes the core-sponding oxirane in high yields. The reaction proceeds in the complete absence of nucleophiles, and thus can avoid formation of by-products arising from the reaction of nuclec hiles with sensitive oxiranes. The photoepoxidation proceeds via addition of an acylperoxy radical to the alkene. Photochemical epoxidation of cholesteryl acetate (176) has been carried out (equation 64a) the major epoxidation product is the sp,6 -epoxide (177a). In MCPBA epoxidation of (176) the major product is (177b). 

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