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.