model is the fact that steric parameters such as 1,3-diaxial and 1,2-diequatorial repulsions are easy to visualize, which allows for accurate predictions regarding the diastereoselectivity of many reactions. Radical mechanism When a concerted mechanism is geometrically unfavorable, a thermal ene reaction can occur through a stepwise biradical pathway. Another possibility is a free-radical process, if radical initiators are present in the reaction mixture. For example, the ene reaction of cyclopentene and cyclohexene with diethyl azodicarboxylate can be catalyzed by free-radical initiators. As seen in Figure 5, the stepwise nature of the process is favored by the stability of the cyclopentenyl or cyclohexenyl radicals, as well as the difficulty of cyclopentene and cyclohexene in achieving the optimum geometry for a concerted process. Regioselection Just as in the case of any cycloaddition, the success of an ene reaction is largely determined by the steric accessibility of the ene allylic hydrogen. In general, methyl and methylene H atoms are abstracted much more easily than methine hydrogens. In thermal ene reactions, the order of reactivity for the abstracted H atom is primary> secondary> tertiary, irrespective of the thermodynamic stability of the internal olefin product. In Lewis-acid promoted reactions, the pair enophile/Lewis acid employed determines largely the relative ease of abstraction of methyl vs. methylene hydrogens. The orientation of ene addition can be predicted from the relative stabilization of the developing partial charges in an unsymmetrical transition state with early formation of the σ bond. The major regioisomer will come from the transition state in which transient charges are best stabilized by the orientation of the ene and enophile. Internal asymmetric induction In terms of the diastereoselection with respect to the newly created chiral centers, an endo preference has been qualitatively observed, but steric effects can easily modify this preference (Figure 6). Intramolecular ene reactions Intramolecular ene reactions benefit from less negative entropies of activation than their intermolecular counterparts, so are usually more facile, occurring even in the case of simple enophiles, such as unactivated alkenes and alkynes. The high regio- and stereoselectivities that can be obtained in these reactions can offer considerable control in the synthesis of intricate ring systems. Considering the position of attachment of the tether connecting the ene and enophile, Oppolzer has classified both thermal and Lewis acid-catalyzed intramolecular ene reactions as types I, II and III, and Snider has added a type IV reaction (Figure 7). In these reactions, the