Reaction of 5H-1,2,4-Triazines with 2-Amino-4-aryl-Substituted Oxa- and Thiazoles

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Abstract The current research focuses on studying the interaction of 2-amino-4-aryloxazoles with 5-unsubstituted 3,6-di(hetero)aryl-1,2,4-triazines capable of undergoing nucleophilic substitution of hydrogen. Previously, our research group has demonstrated that 2-amino-4-aryloxazoles act as dienophiles in [4+2] cycloaddition reactions with 1,2,4-triazines. In the present study, it has been found that, under high-temperature conditions, reactions with 5-unsubstituted 1,2,4-triazines proceed in a fundamentally different manner: 2-amino-4-aryloxazoles act as C-nucleophiles. These reactions afford two types of products: (Z)-2-(1,2,4-triazin-5-yl)ethen-1-amines, formed via elimination of isocyanic acid from intermediate σH-adducts, and 5-(1,2,4-triazin-5-yl)-1,3-oxazol-2-amines resulting from direct C–H/C–H functionalization. When 2-amino-4-arylthiazoles are employed, no thiazole ring opening occurs and exclusively C–H/C–H coupling products are obtained. The σH-adducts have been synthesized and fully characterized, and for some of them, their oxidative aromatization has been demonstrated. However, when the dihydrotriazine core simultaneously bears an aminooxazole moiety at the C5 position and a 2-pyridyl substituent at the C3 position, such aromatization does not take place. Overall, the results show that the presence of a free electron-deficient C5 position in the 1,2,4-triazine ring switches the reaction pathway with 2-aminooxazoles from inverse electron-demand Diels–Alder cycloaddition to nucleophilic substitution of hydrogen.

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