Highly Regioselective Synthesis of 3,6-Diaryl-2-(het)arylpyridines by aza-Diels-Alder Reactions between 6-H-1,2,4-Triazine Dienes and 2-Amino-4-aryloxazole-Based Dienophiles

A solvent-free reaction between C6-unsubstituted 5-aryl-1,2,4-triazines and 2-amino-4-aryloxazoles at high temperature was studied. The reaction proceeds via inverse electron demand aza-Diels-Alder reaction between the oxazole moiety as dienophile and the 1,2,4-triazine moiety as diene with the highly regioselective formation of 3,6-diarylpyridines in up to 60 % isolated yield. The structure of one of the key-products was proven by the X-ray diffraction analysis, and the mechanism of the transformation was proposed. The reaction proceeds well for both 6-aryl and 6-hetaryl-1,2,4-triazines in the presence of the electron acceptor substituent at the position 3. In the case of the donor substituent in position 3, the pyridine yields are significantly reduced. In the case of 5,6-nonsubstituted 1,2,4-triazines, the triazine cycle is degraded without product formation. The high regioselectivity of the reaction as well as solvent-free conditions are valuable advantages of the present synthetic approach.