Acyloxy radicals cyclization

In contrast to aminyl radicals, alkoxyl (and acyloxy) radicals are highly reactive. As illustrated in equation (7), their cyclization reactions are extremely rapid and irreversible. However, the rapidity of such cyclizations does not guarantee success because alkoxyl radicals are also reactive in inter- and intramolecular hydrogen abstractions, and -fragmentations (see Section 4.2.S.2). This lack of selectivity may limit the use of alkoxyl radicals in cyclizations, but S-exo cyclizations are so rapid that they should succeed in many cases, and other types of cyclizations may also be possible. 

Figure 12 - Trapping of intermediate acyloxy radical by cyclization reactions...

Several useful synthetic methodologies are based on the generation of the oxygen-centered radicals from carboxylic acids and the (diacetoxyiodo)benzene-iodine system [613-617]. In particular, a direct conversion of 2-substituted benzoic acids 566 into lactones 567 via oxidative cyclization induced by [bis(acyloxy)iodo]arene/iodine has been reported (Scheme 3.224) [613,614]. 

Useful synthetic methodologies are based on the cyclization, rearrangement, or fragmentation of the alkoxy radicals generated in the reaction of alcohols with [bis(acyloxy)iodo]arenes in the presence of iodine under photochemical conditions or in the absence of irradiation. Photolysis of PhI(OAc)2 with cyclic alcohols in the presence of iodine leads to the generation of the respective alkoxy radical, which can undergo various... 

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