Interaction of Maleic Anhydride with 1,2-phenylenediamine under Different Conditions
The reaction of maleic anhydride with 1,2-phenylenediamine when heated in an aprotic solvent, regardless of temperature, proceeds with the initial formation of the primary addition product, namely, 4-[(2-aminophenyl)amino)-4-oxobut-2-enoic acid, which further undergoes heterocyclization within two ways, due to the presence of several electrophilic centers in the molecule, depending on the reaction conditions. Under mild conditions, the formation of 3-(2-hydroxy-2, 3-dihydro-1H-benzo[d]imidazol-2-yl)acrylic acid is observed, the formation of which is explained by the nucleophilic attack of free amino acid on the amide carbon atom, while in more severe conditions, with azeotropic removal of water, the Michael reaction takes place, with the participation of an electron-deficient carbon atom of an exocyclic multiple bond with the formation of isomeric 2-(3-oxo-1, 2, 3, 4-tetrahydroquinoxalin-2-yl)acetic acid. The structures of the compounds obtained were proved using a combination of data of IR, 1H, 13C NMR spectroscopy, as well as two-dimensional NMR experiments of heteronuclear correlation, HSQC and HMBC.
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