1986 Volume 59 Issue 2 Pages 433-437
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°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-nitrobenzonitrile (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.
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