Acyclic radical cyclization stereoselectivities

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. 

In sharp contrast to the acyclic radicals, cyclic radicals exhibited much better stereoselectivities. Cyclization of radical 24a or24b prepared from 4,6-0-benzylidene-2,3-di-0-benzyl-D-glucopyranose 23 gave a single trans product (25a or 25b) in each case (Fig. 7.7) [11]. Likewise, a radical with the 3-benzyloxy substituent removed (26) also cychzes to give almost exclusively the, 5-trans product 27. 

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