Sulfated Polyborate Catalyzed One-Pot, Two-Step, Four-Component Synthesis of Functionalized Pyridines From (Het.)aryl Methyl Ketones

A novel one-pot, two-step, four-component green synthetic route has been developed for the synthesis of substituted pyridine derivatives through intramolecular cyclisation-aromatisation of (Het.)aryl methyl ketones and mono and/or 1,3-dicarbonyl compounds using sulfated polyborate as a catalyst. The catalyst demonstrated rapid and efficient synthesis of a series of substituted pyridine carboxylates, achieving excellent 82%–96% yields within a reaction time of 50–70 min. The reaction mechanism was investigated by studying the order of addition of reactants, revealing the formation of the desired pyridine carboxylate by nucleophilic attack of active methylene on the β-unsaturated carbon of enaminone, followed by intramolecular cyclization and aromatization. A total of 23 structurally diverse substrates were synthesized under moderate reaction conditions, demonstrating a broad substrate scope. The methodology offers several advantages, including high product yields, moderate reaction conditions, absence of hazardous and volatile organic solvents, simple workup process, and environmental friendliness. The synthesized compounds were characterized using melting point, ¹H NMR, and novel compounds were confirmed by ¹³C NMR, direct mass, and elemental analysis.