Diels-Alder exo selectivity in terminal-substituted dienes and dienophiles: experimental discoveries and computational explanations.

The Diels-Alder reactions of a series of silyloxydienes and silylated dienes with acyclic alpha,beta-unsaturated ketones and N-acyloxazolidinones have been investigated. The endo/exo stereochemical outcome is strongly influenced by the substitution pattern of the reactants. High exo selectivity was observed when the termini of the diene and the dienophile involved in the shorter of the forming bonds were both substituted, while the normal endo preference was found otherwise. The exo-selective asymmetric Diels-Alder reactions using Evans' oxazolidinone chiral auxiliary furnished a high level of pi-facial selectivity in the same sense as their well-documented endo-selective counterparts. Computational results for these Diels-Alder reactions were consistent with the experimental endo/exo selectivity in most cases. A twist-asynchronous model accounts for the geometries and energies of the computed transition structures.