The Diels-Alder Reactions of 2-Alkyl-5-methoxy-4-(p-nitrophenyl)oxazoles with Ethylenic, Acetylenic, and Azo- Type Dienophiles

Article abstract of DOI:10.1246/bcsj.59.433

The Diels-Alder reactions of 2-methyl- and 2-ethyl-5-methoxy-4-(p-nitrophenyl)oxazoles with N-methyl-, N-ethyl, and N-phenylmaleimides gave endo- and exo-adducts.When the reactions were carried out in an acetonitrile solution containing a small amount of water as an impurity, N-substituted 3-hydroxy-2-(p-nitrophenyl)-4,5-pyridinecarboximides were obtained via the decomposition of the Diels-Alder adducts, while the methanol was eliminated. the decomposition might be catalyzed by contaminated water at the initial stage of the reaction.The adducts of N-phenylmaleimide and N-ethylmaleimide were shown to undergo the retro-Diels-Alder reaction on heating at 80 deg C to give the starting maleimides and oxazole.Diethyl azodicarboxylate and 4-phenyl-3H-1,2,4 triazole-3,5(4H)-dione gave the corresponding single adducts.A reaction with dimethyl acetylenedicarboxylate gave p-nitobenzonitrile (10), dimethyl 2-methyl-5-methoxy-3,4-furandicarboxylate (11), and tetramethyl 3,5-epoxy-3-methoxy-5-methyl-1,4-cyclohexadiene-1,2,4,5-tetracarboxylate (12), although the expected adduct was not isolated.These products were explained on the basis of the Diels-Alder reaction, followed by the retro-Diels-Alder reaction, thus affording 10 and 11, which gave 12 upon cycloaddition with the second molecule of DMAD.