Synthesis of exo-Imidazolidin-2-one Dienes, Their Isomerization, and Selectivity in Diels–Alder Cycloadditions

An efficient and alternative synthesis of exo-imidazolidin-2-one dienes is described. A condensation reaction was carried out with bis-imino derivatives, diacetyl, and triphosgene, affording symmetrically N,N-disubstituted dienes. The use of alkyl methyl α-diketones led to the formation of nonsymmetrical dienes, which underwent isomerization to provide more stable inner-outer-ring dienes under Lewis acid conditions. Evaluation was made of the reactivity as well as regio- and stereoselectivity of these dienes in Diels–Alder reactions. They proved to be highly reactive and selective. DFT calculations of the transition states accounted for their behavior.