Ring-chain tautomerism in the products of the reaction between 5-substituted furfurylamines and anhydrides of α,β-unsaturated carboxylic acids

The reactions of 5-substituted furfurylamines with anhydrides of α,β-unsaturated carboxylic acids (acryloyl chloride and maleic anhydride) were studied. The first step of the reaction mechanism involved acylation of furfurylamine nitrogen atom, followed by a stereospecific, spontaneous intramolecular Diels–Alder reaction at the furan ring of the N -furfurylamide intermediates. When the starting materials were 5-alkyl-substituted furfurylamines, the expected 1-oxo-2,3,7,7a - hexahydro-1 H -3a,6-epoxyisoindoles or the corresponding 7-carboxylic acids were obtained in up to 98% yields. The acylation of 5-aryl-substituted furfurylamines with maleic anhydride led to N -furfurylmaleic amides, which formed a dynamic equilibrium in solutions with adducts formed by intramolecular [4+2] cycloaddition, 3a,6-epoxyisoindole-7-carboxylic acids, as proved by NMR spectroscopy. X-ray structural analysis results show that these mixtures crystallized in the form of the cyclic tautomer.