PLANT DERIVED CATALYSTS MEDIATED GREEN SYNTHESIS OF BIOACTIVE HETEROCYCLIC COMPOUNDS

Heterocyclic compounds are indispensable in medicinal chemistry because nitrogen-, oxygen-, and sulfur-containing rings not only play a crucial role in determining molecular geometry, polarity, hydrogen bonding capacity, and basicity, but also modulating interaction with biological targets. Conventional synthesis of heterocycle compounds, however, require hazardous corrosive acids or bases, toxic metal catalysts, volatile organic solvents, prolonged heating, and complex purification. Plant-derived catalysts provide a renewable and sustainable alternative, utilizing acidic fruit juices, aqueous botanical extracts, agricultural-waste ashes, natural biopolymers, and plant-mediated metal or metal-oxide nanoparticles. This review explains the utilizing of this sustainable catalytic system in the construction of different bioactive heterocycles such as pyrans, chromenes, pyrazoles, isoxazolones, quinazolinones, pyrimidines, imidazoles, indoles, and benzimidazoles. Organic acids such as Citric acid (C₆H₈O₇), Malic acid (C₄H₆O₅), and Ascorbic acid (C6H8O6) promote carbonyl activation and cyclocondensation. Plant-ash extracts contain alkaline species including potassium carbonate (K2CO3), calcium oxide (CaO), calcium carbonate (CaCO3), and magnesium oxide (MgO), which facilitate Knoevenagel Condensation, Michael addition, and ring cyclization. Plant phytochemicals can also reduce or stabilize CuO, ZnO, Fe₃O₄, and Ag⁰ nano-catalysts used in different coupling and multicomponent reactions. These systems simplify isolation and lower the need for hazardous reagents, as shown in by representative balanced equations and structural schemes. However, there are still significant challenges, including catalyst variability, unclear active-site chemistry, metal leaching, inadequate green-metric reporting, scale-up restrictions, and limited biological validation. Prior to broad industrial use, standardized catalyst preparation, mechanistic controls, pharmaceutical impurity analysis, and quantitative sustainability assessment are necessary.