posted on 2005-04-20, 00:00authored byJonathan C. Tripp, Carl H. Schiesser, Dennis P. Curran
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
productscis-syn-cis and cis-anti-cisaccount 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.