Radical cyclization ring size preference

The regioselectivity in diene addition reactions can also be influenced by ring strain effects in cyclization reactions. The regioselectivity is highly predictable in those cases, in which addition to the preferred diene center forms the preferred ring size. Thus, the cyclization of radical 15 proceeds readily to form the ct s-disubstituted cyclopentyhnethyl radical 16 with high selectivity. Similarly, cyclization of 17 affords exclusively bicyclic radical 18, in which the additional cyclopentane ring has been formed by addition to the terminal position of the butadiene subunit. This preference for 5-exo cyclizations onto dienes is not even dismpted by substiments at the C1 or C4 positions of the diene system, as seen for radical 19, which cyclizes to 20 (equation lO). This is in contrast to alkyl radical cyclizations to alkenes, in which major amounts of 6-endo cyclization is observed for 5-substituted systems. ... 

The stereoelectronic factor in this reaction is defined by the preferred mode of approach of a radical as it forms bond to the ir-electrons system of an alkene function. As shown below, this is at an angle nearly 20° off the perpendicular to the plane of the double bond. Because of this requirement, many cyclizations to moderately sized rings proceed by radical attack at the nearest carbon of the double bond, regardless of substitution. 

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