The diastereoselectivity of radical cyclizations is often high. In most all-carbon cases, selectivity can be rationalized according to Beckwiths guidelines, which invoke the reactant-like, exo transition state shown above. Placing substituents in pseudoequatorial positions in the transition state leads to cis products from simple secondary radicals. Introducing polar substituents can favor trans' products due to steric or electronic repulsion between the polar groups. In more complex systems, the development of transition state models requires consideration of factors such as allylic strain and boat-like transition states
Chiral auxiliaries have been used in enantioselective radical cyclizations with limited success. Small energy differences between early transition states constitute a profound barrier to success in this arena. In the example shown, diastereoselectivity (for both configurations of the left-hand stereocenter) is low and enantioselectivity is only moderate.
Substrates with stereocenters between the radical and multiple bond are often highly stereoselective. Radical cyclizations to form polycyclic products often take advantage of this property.