Reaction of Aromatic N-Oxides with Dipolarophiles. XII. Stereoselective exo Cycloaddition of 3, 5-Lutidine N-Oxide with N-Substituted Maleimides and a Frontier Molecular Orbital and Mechanistic Study

Abstract

Pericyclic reactions of 3, 5-lutidine N-oxide with N-phenylmaleimides were investigated. The primary cycloadducts are thermally labile and undergo 1, 5-sigmatropic rearrangement to give the 2, 3-dihydropyridine derivatives. The proton nuclear magnetic resonance structural assignment of the 1, 5-sigmatropy products implies that the primary cycloaddition proceeds through an exo transition state. The reaction behavior is discussed in terms of frontier molecular orbital theory, based on MINDO/3 and CNDO/2 calculations and kinetic data. It was concluded that the reaction falls into the category of a 'normal-type' cycloaddition and the exo cycloaddition is brought about by the unfavorable secondary orbital interaction.