The a- bromomethyl dimethylsilyl ether in radical cyclizations

However, with other systems, the outcome of the final H-abstraction is less stereoselective. Jenkins and Wood have investigated the intramolecular radical cyclization onto cyclopentenols derived from glucose [52]. Two epimeric alcohols were prepared by reduction of enone 127 with either NaBH4/CeCl3 (a/p ratio 1 8) or L-selectride (a/p 

Two diastereoisomeric allylic silyl ethers 133 and 134 were first investigated and in both cases, the desired radical cyclization products were not observed. In the case of 133, cyclization proceeded exclusively in a 6-endo mode, affording diol 135 after oxidative cleavage of the silyl linker. The preference for 6-ring formation was attributed to the conformational rigidity of the allylic system and the less hindered nature of the distal sp - 

The regio- and stereoselectivity may be rationalized in terms of the relative stability of the three possible T. S. conformations, which permit a 107-109° approach trajectory of the radical onto the sp center. Of the two T. S.s leading to 6-exo cyclization products, the one with a pseudoequatorially disposed olefinic group, is favored, leading to the observed 1,3-cw-disubstituted siloxanes. The regioselectivity of the cyclization is also noteworthy. The authors suggest that the preference of terminally unsubstituted olefins to undergo 1-endo cyclization reflects the increased stability of the secondary radical intermediate over the primary radical which would result from an exo cyclization. 

In a competition study, Malacria and co-workers have shown that a 5-exo-dig cyclization is appreciably faster than the 5-exo-trig process [62]. The same group has exploited this 

When the triple bond is terminally substituted, 5-ejco-dig cyclization and subsequent H-abstraction generate a trisubstituted double bond. Malacria and co-workers have shown that the configuration of the double bond can be controlled by varying the nature of the terminal substituent, thus providing a novel entry into stereodefined trisubstituted double bonds [65], which are not always easy to access via more conventional protocols. 

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