Radical epoxidation

The following mechanism can be assumed. Initially, the ketyl intermediate 3-159 is formed, which leads to 3-160 by a radical epoxide opening. Reaction with the second molecule of Sml2 gives samarium-chelated hydroxy-ketyl 3-161, which cyclizes to afford the products 3-157 and 3-158. 

Aldehydes are easily oxidized (19) and are well-known sensitizer compounds benzaldehyde enhances oxidation of linear alkenes producing the corresponding epoxides (15). An interesting feature of radical epoxidation is the generation of the alkoxy radical RO, which is more reactive and less selective than the peroxy radical (16)... 

Vreugdenhil AD, Reit H. Liquid-phase co-oxidation of aldehydes and olefins— radical versus non-radical epoxidation. Rec Trav Chim 1972 91 237-245. 

Radical Epoxide Reduction Borane-free Two-step Synthesis of anti-Markownikow Alcohols from Alkenes... 

A study of the synthesis of chromans from allylic carbonates involving Pd-catalysed asymmetric allylic alkylation has established that the addition of acetic acid results in a pronounced increase in enantioselectivity. Furthermore, (E) allylic carbonates afford (R) chromans and the (Z) substrates the (S) heterocycle (Scheme 13) <03JA9276>. This approach to chromans has been combined with a radical epoxide cyclisation in a total synthesis of (-)-siccanin <03AG(E)3943>. 

Reactions Based on Cp2TiCl-Promoted Radical Epoxide Openings. 65... 

These torsional interactions are relatively unimportant in radical epoxidations where the diastereoselectivities for norbornene epoxidation are considerably lower, but strongly dependent on the reagent system employed (vide supra). 

Oxiranes may also be prepared by the cooxidation of aldehydes and olefins. There are two assumptions as regards the mechanism the oxidation occurs via either an acylperoxy radical or a peracid. The peracid oxidation is stereospecific. Experiments carried out with a view to assessing the radical versus nonradical mechanism indicate that the extent of the radical epoxidation depends on the structure of the olefin and the olefin/aldehyde ratio. Cooxidation in the presence of oxygen was achieved by irradiating the aldehyde and carrying out the reaction with the alkene after a suitable quantity of peracid had been obtained. Enantioselective epoxidation has been described in the reaction of (1-phenyl-alkylidene)malonitriles 63 catalyzed by optically active tertiary amines. ... 

F. Minisci, C. Gambarotti, M. Pierini, O. Porta, C. Punta, F. Recupero, M. Lucarini, V. Mugnaini, Molecule-induced homolysis of N-hydroxyphthalimide (NHPI) by peracids and dioxirane. A new, simple, selective aerobic radical epoxidation of alkanes. Tetrahedron Lett. (2006) 1421. 

Radical epoxidation. In the presence of this radical cation, iodosylbcnzcnc can effect epoxidation of stilbene, but only in low yield because of decomposition of the catalyst. Selenium dioxide (Se02) is a suitable oxidant for this catalyzed epoxidation, but suffers from low solubility in CH2CI2 at 0°. The most useful... 

The crucial aspect for catalytic turnover is the stability of radicals, epoxides, and the titanium(III) reagent under buffered protic conditions. C-C bond-forming reactions have also been realized using the same concept. In these cases, both titanium-oxygen bonds (addition to a,)5-unsaturated esters) or a titanium-oxygen... 

It is also assumed that PAH in their original form are not carcinogenic. Through their activation, particularly during their metabolism, free radicals, epoxides and diolepoxides are formed, which are then bound, for example, to DNA (Fig. 9.6). A concept currently prevalent is that PAH are metabolized through several intermediate steps. Among the various metabolites, epoxides and diolepoxides are considered to be most dan-... 

We emphasize that the efficiency of this radical epoxidation mechanism depends on the substrate (i.e., the ease at which the chain-carrying peroxyl radicals abstract aH-atoms or add to the C=C bond) [25], and sometimes even on the precise geometric conformer [26, 27]. However, the maximum theoretical... 

The use of titanocene(III)-mediated radical epoxide opening in syntheses of natural products 06MRO23. 

Tris(4-bromophenyl)aininiuni bcuck Radical epoxidation. In the pi... 

Lipoxygenase (LOX) converts polyunsaturated fatty acids, such as linoleic and linolenic acids, to lipid hydroperoxides (Figure 2)(52,73,74). The lipid hydroperoxides then form hydroperoxide radicals, epoxides, and/or are degraded to form malondialdehyde. These products are also strongly electrophilic, and can destroy individual amino acids by decarboxylative deamination (e.g., lysine, cysteine, histidine, tyrosine, and tryptophan) cause free radical mediated cross-linking of protein at thiol, histidinyl, and tyrosinyl groups and cause Schiff base formation (e.g., malondialdehyde and lysine aldehyde) (39,49,50,74-78). 

LOX destroys linoleic and linolenic acids via their oxidation to lipid hydroperoxides. These lipid hydroperoxides subsequently form hydroperoxides, hydroperoxide free radicals, epoxides and malondialdehyde which can impair the nutritive quality of protein via mechanisms similar to those mediated by POD and PPO. These unsaturated fatty acids are essential for normal larval growth and maturation. 

The (-)-menthyl/cobalt system is selective for epoxide formation but the products are racemic in line with radical epoxidation in solution rather than... 

The electrode converts MbFe into MbFe, which reacts with oxygen to give MbFe -02. Electrochemical reduction of MbFe F02 produces H2O2, which converts MbFe to the radical MbFe = 0, the active oxidant. This oxyferryl radical epoxidizes styrene by oxygen transfer to the double bond. In this process, a catalytic electrochemical reduction drives a catalytic enzymelike oxidation in a doubly catalytic process. 

Oxidation products of Hpids (hydroperoxides, free alkoxyl and peroxyl radicals, epoxides and aldehydes) react with a number of food constituents during the processing and storage of food. These reactions often lead to a reduction in the nutritional value of foods (such as reactions with proteins and vitamins) and a deterioration of their organoleptic properties (e.g. reactions with flavour active substances). 

Reactions of proteins with Hpid hydroperoxides and other oxidised Hpids give rise to different types of Hpoproteins, in which Hpids and proteins are bound by physical bonds (as in natural Hpoproteins), but also by covalent bonds. Reactions with hydroperoxides, free radicals, epoxides and aldehydes lead to the fission of some protein bonds, formation of protein radicals and oligomers, cross links between protein chains and some sensitive functional groups of amino acids are oxidised (see Section 2.5.1.1). 

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