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352 Organic Chemistry Solutions Manual Ring opening of cyclopropanes is The last example also produces a radical next to a cyclopropane ring but this time it now a standard way of detecting decompose very easily to give a stable secondary benzylic radical. This captures a hydrogen radicals (J. W. Wilt in J. K. Free radicals, Wiley, 1973, from PhSH releasing and maintaining an efficient radical chain. pp. 333-502). PhS PhS H PhS Ph Ph CN PhS PhS Ph Ph Ph H SPh Problem 9 The last few stages of Corey's epibatidine synthesis are shown here. Give mechanisms for the first two reactions and suggest a reagent for the last step. Br F₃COC CI t-BuOK, THF, -78 °C N Br N 75% yield Br N CI F₃COC CI CI H Bu₃SnH, AIBN N ? N N N benzene, reflux 95% yield Purpose of the problem Application of radical reactions in an important sequence plus revision of conformation and stereochemistry. Suggested solution The first step involves deprotonation of the rather acidic amide (the CF₃ group helps) and the displacement of the only possible bromide the one on the opposite face of the six-membered as the SN2 must take place with inversion. 0 CF₃ 0 CF₃ F₃COC N N N H Ot-Bu SN2 N with Br Br Br inversion N CI N CI The second step is a standard dehalogenation by Bu₃SnH. AIBN generates by hydroger abstraction from the reagent and this removes the bromine. The only tricky thing is to make sure you complete the chain and don't use at any point. F₃COC CI F₃COC CI N N N N product Bu₃Sn Br Bu₃Sn H Finally, we need to hydrolyse the amide. This normally requires strong acid or alkali but the trifluoroacetyl group is significantly more electrophilic than most acyl groups and milder conditions