Synthesis of Diels-Alder adducts of N-arylmaleimides by a multicomponent reaction between maleic anhydride, dienes, and anilines

We have carried out the synthesis and characterization of some hexahydroisoindolyl benzoic acids and their corresponding ethyl esters by a multicomponent reaction (MCR) between aminobenzoic acids or aminobenzoates, maleic anhydride, and isoprene in the absence of catalysts. According to additional experiments, the MCR takes place by sequential formation of N-arylmaleamic acids from the aminobenzoic acids or aminobenzoates and maleic anhydride, Diels-Alder adducts of the acids and isoprene, and finally the imides. The (1)H NMR data (coupling constants) of the adducts suggested that the preferred conformation of the corresponding cyclohexene rings is a syn-boat, a fact supported by a density functional theory (DFT) conformational analysis and DFT calculations of the spin-spin coupling constants of the corresponding conformers. Our MCR synthetic methodology was tested successfully in the synthesis of other adducts, for which cyclopentadiene and other anilines were employed.