The Stereochemistry of Ring Closure of Some Monosubstituted Ortho-(but-3-enyl)phenyl Radicals

The rates and diastereoselectivity of cyclization of o-(but-3-eny1)phenyl radicals bearing substituents on the 1- or 2-position of the side chain have been examined. 1-Substituted radicals afford only 1,5-cyclization products, but 2-substituted radicals also give small amounts of endo products by direct 1,6-cyclization. In conformity with established guidelines, the 2-substituted radicals undergo preferential formation of trans- disubstituted products. However, 1-substituted radicals cyclize with relatively low diastereoselectivity , and the preferred stereochemistry of the product depends on the nature of the substituent . Transition structure strain energies calculated by the MNDO/MM2 method provide qualitative indices of reactivity and diastereoselectivity.