Stereoselectivity of Intermolecular Reaction Acyclic Systems

Until the end of the 1980s it was believed that the high reactivity and flexibility of acyclic radicals prevent stereoselective reactions. This opinion changed in 1991 when the review of Porter, Giese, and Curran appeared [1], In the middle of the 1990s, it became obvious that in most cases acyclic radicals follow the same rules of stereoselectivity as non-radicals [2]. This chapter describes diastereoselective, substrate-controlled reactions of acyclic radicals. The chemistry of cyclic radicals, the influence of chiral auxiliaries and of Lewis acids as well as enantioselective radical reactions are reviewed in Chapters 4.2-4.5. Actually, radicals are suitable intermediates for an understanding of stereoselectivity because (a) their conformation can be determined by ESR spectroscopy, and (b) the transition states of synthetically relevant radical reactions are very early on the reaction coordinate. The present ehapter makes use of these features. 

The concept of allylic strain (A-strain) is based on conformations of Z-alkenes (1) where allylic alkyl groups adopt preferred conformations in which the smallest substituent (in most cases an H atom) points in the direction of the vicinal alkene substituent R- [3]. 

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Annulated ring systems have as /1,7-substituents, when compared to annulated cyclopentyl radical systems, a stronger effect on the stereoselectivity than the corresponding combination of acyclic substituents. In all cases, attack tram to the /J.y-m-annulated ring is preferred. The stereoselectivity depends, furthermore, on additional substituents at the radical and the alkene, but it appears that the reactions of cyclohexyl radicals proceed less selectively than their cyclopentyl analogs. One frequently used route to these systems is sequential cyclization/ addi-tion reactions, in which the primary radical cyclizes to form the bicyclic ring system, followed by intermolecular addition to an alkene45,47 74. 

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