A novel one-pot synthesis of annulated 2,2′-bipyridine ligands by inverse electron demand Diels–Alder reaction of 5,5′-bi-1,2,4-triazines

The Diels–Alder reaction of 5,5′-bi-1,2,4-triazines with cyclic enamines in the absence of a solvent leads to a range of symmetrical, annulated 2,2′-bipyridines in good yield. When the reaction is carried out in boiling dioxane only 5-(heteroaryl)1,2,4-triazines are formed. The latter, bearing methylsulfanyl substituents, are oxidized with potassium permanganate to the corresponding methylsulfonyl derivatives, which easily undergo Diels–Alder reactions with different enamines to give unsymmetrical, annulated 2,2′-bipyridines.