Radicals alkoxyl, generation from nitrites

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

Cyclizations of 5-alkyl-substituted 4-penten-l-oxyl radicals are faster and frequently more selective than those of terminal unsubstituted derivatives [48, 53]. This finding is in accord with the electrophilic nature of alkoxyl radicals in addition reaction to C-C double bonds. 5-Alkyl- or 5-phenyl-substituted 4-pentenoxyl radicals, such as intermediates 25 or 26, were generated from a number of different sources. For example, alkyl nitrites [39], A-alkoxypyridine-2(l//)-thiones [46], and A-alkoxy-(/ -chlorophenyl)thiazole-2(3/f)-thiones [54] in photochemically induced reactions, and A-alkoxyphthalimides [55] or type IV radical precursors [53] in thermally initiated reactions have been applied for this purpose (Scheme 6). 

When a molecule is irradiated, particularly with ultraviolet light, some bonds within the molecule can absorb this energy and undergo homolyt-ic cleavage to generate free radicals. These reactions include the formation of alkoxyl (RO ) radicals from alkyl hypoiodites (RO) or nitrites (RONO) or the generation of bromine atoms from bromine or A-bro-mosuccinimide. 

Followed by C-C bond formation. Cekovic reported sequential reactions involving generation of alkoxyl radicals from hypochlorites and nitrite esters followed by... 

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