Two decades of progress in C4-unsubstituted3-acetylcoumarins: synthesis, reactivity, heterocyclic diversity, and applications.

Abstract

C4-unsubstituted 3-acetylcoumarins represent a versatile class of coumarin-based heterocycles that combine a reactive acetyl group at the C-3 position with an accessible C4-H site. This structural arrangement makes them valuable synthons for functionalization, annulation and the construction of diverse heterocyclic frameworks. This review summarizes the progress reported from 2006 to 2026 in the synthesis, reactivity and applications of C4-unsubstituted 3-acetylcoumarin derivatives. The main synthetic strategies include Knoevenagel condensation, diketene-based routes, alkynoate-mediated synthesis, hydrodehalogenation, cross-dehydrogenative coupling and carbonyl-alkyne metathesis. Their reactivity covers halogenation, reduction, thionation, phosphorylation, Mannich reactions, condensation with nitrogen nucleophiles, C-H functionalization and cycloaddition reactions. These transformations provide access to a broad range of coumarin-containing heterocycles, including pyrazoles, thiazoles, oxazoles, isoxazoles, pyridines, pyrimidines, thiophenes, triazoles, quinoxalines, diazepines and fused polycyclic systems. Beyond synthetic chemistry, many derivatives show promising biological activities, including antibacterial, antioxidant, anti-inflammatory, anti-Alzheimer, antitumor, antidiabetic and enzyme-inhibitory effects. Their photophysical properties have also enabled their application as fluorescent chemosensors for ions, reactive species and biologically relevant molecules. Finally, this review highlights future opportunities in greener synthesis, structure-activity relationship analysis and advanced biological evaluation.

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