Peroxy radical, cyclization

Fig. 21 Qualitative depiction of favorable cyclization pathways for representative peroxy radicals of methyl heteroaromatics (top, pyridine and bottom, furan). Cyclization for the alkylated six-membered heteroaromatics is driven by the thermodynamic stability of the resulting ring, while cyclization for the alkylated five-membered heteroaromatics is dictated by which pathway allows the generation of a stable allylic radical system.

A. Peroxidation with Triplet Molecular Oxygen and Peroxy-radical Cyclization... 

Theoretical studies also suggest that some of the peroxy radicals formed by addition of 02 to the OH-aromatic adducts may react with NO in competition with cyclization (Andino et al., 1996), generating N02 and an aromatic peroxy radical that may subsequently form phenolic and unsaturated derivatives through reactions with 02 ... 

The arachidonic acid cascade is a biological free radical oxidation of unsaturated fatty acids leading to formation of the prostaglandins (equation 102). Cyclization of a peroxy radical intermediate 66 leading to endoperoxide 67 was proposed as a pathway for this process, and this was demonstrated in chemical model systems, in which the peroxyl radical 66 was generated by hydrogen abstraction from the hydroperoxide corresponding to 66. 

The asymmetric rearrangement of peroxy radical (5) has recently been used as the key step in the asymmetric synthesis of Plakorin (Scheme l).17 The thermal isomerization of buta-1,2- to buta-1,3-diene has been studied using ab initio calculations and the mechanism concluded to proceed stepwise via radical intermediates.18 The competition between cyclopropyl formation and the homoallyl-homoallyl radical rearrangement has been studied in the radical (6) and found to give the 3-exo cyclization product (7) and the rearranged product (8) in a 1 5 ratio, respectively, under the conditions shown (Scheme 2).19... 

Porter NA, Lehman LS, Weber BA, Smith KJ (1981) Unified mechanism for polyunsaturated fatty acid autoxidation. Competition of peroxy radical hydrogen atom abstraction, p-scission, and cyclization. J Am Chem Soc 103 6447-6455... 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

Porter NA, Funk MO. Letter peroxy radical cyclization as a model for prostaglandin biosynthesis. J. Org. Chem. 1975 40 3614. 

The overall reaction and product stoichiometries for the degradation of chloroalkene substrates by O2 - in DMF are summarized in Table 7-1.20 Within the limits of a reaction time of 10 min or less, chloroethene, frflws-1,2-dichloroethene, Aldrin, and Dieldrin are not oxidized by O2 - in DMF. A reasonable mechanism for these oxidations is an initial nucleophilic addition of superoxide to the chloroalkenes [e.g., tetrachloroethene (Scheme (7-7)]. Subsequent loss of chloride ion would give a vinyl peroxy radical, which can cyclize and decompose to a chloroacyl radical and phosgene. t These would undergo subsequent facile reactions with O2 - to give bicarbonate and chloride ions. 

Nishino and Kurosawa have shown that radicals such as 24 that are formed by the addition of dicarbonyl compounds to alkenes can be trapped by oxygen when the reaction is run under a stream of dry air [23]. The peroxy radical 25 that is formed abstracts a hydrogen atom to give a peroxide that cyclizes to give peroxy hemiketal 26 (Scheme 7). 

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