Lanthanide Triflates in Action: Efficient Pathways to Coumarins

Coumarins are an important class of oxygen-containing heterocyclic secondary metabolites which exhibits wide range of applications in synthetic, therapeutic, and material chemistry. They are well known for their varied bioactive properties, including anticancer, anticoagulant, antiinflammatory, antimicrobial, antiviral, and antioxidant activities. In addition to their medicinal uses, coumarin derivatives are broadly useful as fluorescent probes, laser dyes, and functional materials. Despite their importance, many classical synthetic routes to coumarin, such as the Pechmann, Knoevenagel, Perkin, and Wittig reactions, often require toxic reagents, acids, or moisture-sensitive catalysts under harsh conditions. These disadvantages limit their compatibility with the environment and industries. In recent years, [Ln(OTf)₃] have emerged as efficient and versatile Lewis acid catalysts for the preparation of coumarins. The distinctive catalytic features of Ln(OTf)₃, such as strong Lewis acidity, high oxophilicity, water tolerance, and thermal and chemical stability, enable their use in aqueous media and under solvent-free conditions. These properties facilitated a range of coumarinforming transformations, including the Pechmann condensation, the Knoevenagel condensation, and multicomponent reactions. Ln(OTf)₃-mediated approaches have enabled the provision of structurally diverse and biologically significant coumarin derivatives. These include natural product analogues and highly emissive fluorescent probes for sensing and bioimaging applications. This review summarizes recent advances in lanthanide triflate-mediated strategies for coumarin synthesis, with highlighting on reaction scope, mechanistic insights, and emerging applications in medicinal and materials chemistry