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An exo:endo ratio of 98:2! Thermochemistry would have predicted the opposite, but in the rearrangements of 5-hexenyl radicals the exo pathway is preferred due to kinetic reaction control. Selected ab initio methods are able to properly describe cyclizations of the 4-penten-1-oxyl radical (see sketch).
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Hartung et al. (1996) studied this question.
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