Stereochemistry of hexenyl radical cyclizations with tert-butyl and related large groups: substituent and temperature effects.
The long held notion that hexenyl radicals bearing large substituents on the radical carbon cyclize to give 1,2-trans-substituted cyclopentanes is experimentally disproved by study of the radical cyclization of an assortment of simple and complex substrates coupled with careful product analysis and rigorous assignment of configurations. X-ray studies and syntheses of authentic samples establish that the published assignments for cis- and trans-1-tert-butyl-2-methylcyclopentane must be reversed. The original assignment based on catalytic hydrogenation of 1-tert-butyl-2-methylenecyclopentane was compromised by migration of the double bond prior to hydrogenation. The cyclization of 1-tert-butylhexenyl radical is moderately cis selective, and the selectivity is increased by geminal substitution on carbon 3. This selectivity trend is general and extends to relatively complex substrates. It has allowed Ihara to reduce the complexity of an important class of round trip radical cyclizations to make linear triquinanes to the point where two tricyclic products-cis-syn-cis and cis-anti-cis-account for about 80% of the products. However, the further increase in selectivity that was proposed by lowering the temperature is shown to be an artifact of the analysis methods and is not correct. This work solidifies "1,2-cis selectivity" in cyclizations of 1-subsituted hexenyl radicals as one of the most general stereochemical trends in radical cyclizations.
PubMed Abstracts
PubMed Abstracts