Radical epoxyalkyl

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-... 

The Carbon-oxygen bond fragmentation of a,)9-epoxyalkyl radicals provides another way to generate alkoxy radicals for ring expansions (Scheme 13) [21]. 

Small-ring radical opening is a facile process. This process has been incorporated in the zero bridge cleavage of strained bicycles for one- and two-carbon ring expansions (Scheme 14). Ring expansions of cyclopropyloxy and cyclobutyloxy radicals have been discussed in the previous section. This section focuses on cyclopropyl-carbinyl, ot,j8-epoxyalkyl and cyclobutylcarbinyl radicals. 

The ring opening of epoxyalkyl radicals, unlike that of cyclopropanecarbinyl radicals, usually proceeds with cleavage of the cxo-cyclic carbon-oxygen bond of the epoxides. However, a substituent group that stabilizes the radical produced by... 

The protocol has been applied to the addition of epoxyalkyl groups (Eq. 68), which would give rise to severe difficulties with the usual organometallic methods. Additions proceed in good yields with preservation of the epoxide ring,304 when at least 3 carbons separate the functionalities. This result provides further evidence of a radical mechanism a carbanionic species would probably open the 3-membered ring intra- or intermolecularly. 

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